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47 KiB
======================= |
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A Linux CD-ROM standard |
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======================= |
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:Author: David van Leeuwen <[email protected]> |
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:Date: 12 March 1999 |
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:Updated by: Erik Andersen ([email protected]) |
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:Updated by: Jens Axboe ([email protected]) |
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Introduction |
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============ |
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|
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Linux is probably the Unix-like operating system that supports |
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the widest variety of hardware devices. The reasons for this are |
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presumably |
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|
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- The large list of hardware devices available for the many platforms |
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that Linux now supports (i.e., i386-PCs, Sparc Suns, etc.) |
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- The open design of the operating system, such that anybody can write a |
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driver for Linux. |
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- There is plenty of source code around as examples of how to write a driver. |
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The openness of Linux, and the many different types of available |
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hardware has allowed Linux to support many different hardware devices. |
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Unfortunately, the very openness that has allowed Linux to support |
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all these different devices has also allowed the behavior of each |
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device driver to differ significantly from one device to another. |
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This divergence of behavior has been very significant for CD-ROM |
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devices; the way a particular drive reacts to a `standard` *ioctl()* |
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call varies greatly from one device driver to another. To avoid making |
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their drivers totally inconsistent, the writers of Linux CD-ROM |
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drivers generally created new device drivers by understanding, copying, |
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and then changing an existing one. Unfortunately, this practice did not |
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maintain uniform behavior across all the Linux CD-ROM drivers. |
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This document describes an effort to establish Uniform behavior across |
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all the different CD-ROM device drivers for Linux. This document also |
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defines the various *ioctl()'s*, and how the low-level CD-ROM device |
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drivers should implement them. Currently (as of the Linux 2.1.\ *x* |
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development kernels) several low-level CD-ROM device drivers, including |
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both IDE/ATAPI and SCSI, now use this Uniform interface. |
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When the CD-ROM was developed, the interface between the CD-ROM drive |
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and the computer was not specified in the standards. As a result, many |
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different CD-ROM interfaces were developed. Some of them had their |
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own proprietary design (Sony, Mitsumi, Panasonic, Philips), other |
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manufacturers adopted an existing electrical interface and changed |
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the functionality (CreativeLabs/SoundBlaster, Teac, Funai) or simply |
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adapted their drives to one or more of the already existing electrical |
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interfaces (Aztech, Sanyo, Funai, Vertos, Longshine, Optics Storage and |
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most of the `NoName` manufacturers). In cases where a new drive really |
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brought its own interface or used its own command set and flow control |
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scheme, either a separate driver had to be written, or an existing |
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driver had to be enhanced. History has delivered us CD-ROM support for |
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many of these different interfaces. Nowadays, almost all new CD-ROM |
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drives are either IDE/ATAPI or SCSI, and it is very unlikely that any |
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manufacturer will create a new interface. Even finding drives for the |
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old proprietary interfaces is getting difficult. |
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When (in the 1.3.70's) I looked at the existing software interface, |
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which was expressed through `cdrom.h`, it appeared to be a rather wild |
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set of commands and data formats [#f1]_. It seemed that many |
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features of the software interface had been added to accommodate the |
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capabilities of a particular drive, in an *ad hoc* manner. More |
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importantly, it appeared that the behavior of the `standard` commands |
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was different for most of the different drivers: e. g., some drivers |
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close the tray if an *open()* call occurs when the tray is open, while |
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others do not. Some drivers lock the door upon opening the device, to |
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prevent an incoherent file system, but others don't, to allow software |
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ejection. Undoubtedly, the capabilities of the different drives vary, |
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but even when two drives have the same capability their drivers' |
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behavior was usually different. |
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|
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.. [#f1] |
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I cannot recollect what kernel version I looked at, then, |
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presumably 1.2.13 and 1.3.34 --- the latest kernel that I was |
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indirectly involved in. |
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I decided to start a discussion on how to make all the Linux CD-ROM |
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drivers behave more uniformly. I began by contacting the developers of |
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the many CD-ROM drivers found in the Linux kernel. Their reactions |
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encouraged me to write the Uniform CD-ROM Driver which this document is |
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intended to describe. The implementation of the Uniform CD-ROM Driver is |
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in the file `cdrom.c`. This driver is intended to be an additional software |
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layer that sits on top of the low-level device drivers for each CD-ROM drive. |
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By adding this additional layer, it is possible to have all the different |
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CD-ROM devices behave **exactly** the same (insofar as the underlying |
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hardware will allow). |
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The goal of the Uniform CD-ROM Driver is **not** to alienate driver developers |
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whohave not yet taken steps to support this effort. The goal of Uniform CD-ROM |
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Driver is simply to give people writing application programs for CD-ROM drives |
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**one** Linux CD-ROM interface with consistent behavior for all |
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CD-ROM devices. In addition, this also provides a consistent interface |
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between the low-level device driver code and the Linux kernel. Care |
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is taken that 100% compatibility exists with the data structures and |
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programmer's interface defined in `cdrom.h`. This guide was written to |
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help CD-ROM driver developers adapt their code to use the Uniform CD-ROM |
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Driver code defined in `cdrom.c`. |
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Personally, I think that the most important hardware interfaces are |
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the IDE/ATAPI drives and, of course, the SCSI drives, but as prices |
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of hardware drop continuously, it is also likely that people may have |
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more than one CD-ROM drive, possibly of mixed types. It is important |
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that these drives behave in the same way. In December 1994, one of the |
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cheapest CD-ROM drives was a Philips cm206, a double-speed proprietary |
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drive. In the months that I was busy writing a Linux driver for it, |
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proprietary drives became obsolete and IDE/ATAPI drives became the |
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standard. At the time of the last update to this document (November |
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1997) it is becoming difficult to even **find** anything less than a |
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16 speed CD-ROM drive, and 24 speed drives are common. |
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.. _cdrom_api: |
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Standardizing through another software level |
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============================================ |
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At the time this document was conceived, all drivers directly |
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implemented the CD-ROM *ioctl()* calls through their own routines. This |
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led to the danger of different drivers forgetting to do important things |
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like checking that the user was giving the driver valid data. More |
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importantly, this led to the divergence of behavior, which has already |
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been discussed. |
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For this reason, the Uniform CD-ROM Driver was created to enforce consistent |
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CD-ROM drive behavior, and to provide a common set of services to the various |
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low-level CD-ROM device drivers. The Uniform CD-ROM Driver now provides another |
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software-level, that separates the *ioctl()* and *open()* implementation |
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from the actual hardware implementation. Note that this effort has |
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made few changes which will affect a user's application programs. The |
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greatest change involved moving the contents of the various low-level |
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CD-ROM drivers\' header files to the kernel's cdrom directory. This was |
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done to help ensure that the user is only presented with only one cdrom |
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interface, the interface defined in `cdrom.h`. |
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CD-ROM drives are specific enough (i. e., different from other |
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block-devices such as floppy or hard disc drives), to define a set |
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of common **CD-ROM device operations**, *<cdrom-device>_dops*. |
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These operations are different from the classical block-device file |
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operations, *<block-device>_fops*. |
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The routines for the Uniform CD-ROM Driver interface level are implemented |
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in the file `cdrom.c`. In this file, the Uniform CD-ROM Driver interfaces |
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with the kernel as a block device by registering the following general |
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*struct file_operations*:: |
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struct file_operations cdrom_fops = { |
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NULL, /* lseek */ |
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block _read , /* read--general block-dev read */ |
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block _write, /* write--general block-dev write */ |
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NULL, /* readdir */ |
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NULL, /* select */ |
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cdrom_ioctl, /* ioctl */ |
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NULL, /* mmap */ |
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cdrom_open, /* open */ |
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cdrom_release, /* release */ |
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NULL, /* fsync */ |
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NULL, /* fasync */ |
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NULL /* revalidate */ |
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}; |
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Every active CD-ROM device shares this *struct*. The routines |
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declared above are all implemented in `cdrom.c`, since this file is the |
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place where the behavior of all CD-ROM-devices is defined and |
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standardized. The actual interface to the various types of CD-ROM |
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hardware is still performed by various low-level CD-ROM-device |
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drivers. These routines simply implement certain **capabilities** |
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that are common to all CD-ROM (and really, all removable-media |
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devices). |
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Registration of a low-level CD-ROM device driver is now done through |
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the general routines in `cdrom.c`, not through the Virtual File System |
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(VFS) any more. The interface implemented in `cdrom.c` is carried out |
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through two general structures that contain information about the |
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capabilities of the driver, and the specific drives on which the |
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driver operates. The structures are: |
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cdrom_device_ops |
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This structure contains information about the low-level driver for a |
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CD-ROM device. This structure is conceptually connected to the major |
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number of the device (although some drivers may have different |
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major numbers, as is the case for the IDE driver). |
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cdrom_device_info |
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This structure contains information about a particular CD-ROM drive, |
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such as its device name, speed, etc. This structure is conceptually |
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connected to the minor number of the device. |
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Registering a particular CD-ROM drive with the Uniform CD-ROM Driver |
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is done by the low-level device driver though a call to:: |
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register_cdrom(struct cdrom_device_info * <device>_info) |
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The device information structure, *<device>_info*, contains all the |
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information needed for the kernel to interface with the low-level |
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CD-ROM device driver. One of the most important entries in this |
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structure is a pointer to the *cdrom_device_ops* structure of the |
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low-level driver. |
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The device operations structure, *cdrom_device_ops*, contains a list |
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of pointers to the functions which are implemented in the low-level |
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device driver. When `cdrom.c` accesses a CD-ROM device, it does it |
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through the functions in this structure. It is impossible to know all |
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the capabilities of future CD-ROM drives, so it is expected that this |
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list may need to be expanded from time to time as new technologies are |
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developed. For example, CD-R and CD-R/W drives are beginning to become |
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popular, and support will soon need to be added for them. For now, the |
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current *struct* is:: |
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struct cdrom_device_ops { |
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int (*open)(struct cdrom_device_info *, int) |
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void (*release)(struct cdrom_device_info *); |
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int (*drive_status)(struct cdrom_device_info *, int); |
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unsigned int (*check_events)(struct cdrom_device_info *, |
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unsigned int, int); |
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int (*media_changed)(struct cdrom_device_info *, int); |
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int (*tray_move)(struct cdrom_device_info *, int); |
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int (*lock_door)(struct cdrom_device_info *, int); |
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int (*select_speed)(struct cdrom_device_info *, int); |
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int (*select_disc)(struct cdrom_device_info *, int); |
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int (*get_last_session) (struct cdrom_device_info *, |
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struct cdrom_multisession *); |
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int (*get_mcn)(struct cdrom_device_info *, struct cdrom_mcn *); |
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int (*reset)(struct cdrom_device_info *); |
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int (*audio_ioctl)(struct cdrom_device_info *, |
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unsigned int, void *); |
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const int capability; /* capability flags */ |
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int (*generic_packet)(struct cdrom_device_info *, |
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struct packet_command *); |
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}; |
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When a low-level device driver implements one of these capabilities, |
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it should add a function pointer to this *struct*. When a particular |
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function is not implemented, however, this *struct* should contain a |
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NULL instead. The *capability* flags specify the capabilities of the |
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CD-ROM hardware and/or low-level CD-ROM driver when a CD-ROM drive |
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is registered with the Uniform CD-ROM Driver. |
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Note that most functions have fewer parameters than their |
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*blkdev_fops* counterparts. This is because very little of the |
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information in the structures *inode* and *file* is used. For most |
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drivers, the main parameter is the *struct* *cdrom_device_info*, from |
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which the major and minor number can be extracted. (Most low-level |
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CD-ROM drivers don't even look at the major and minor number though, |
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since many of them only support one device.) This will be available |
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through *dev* in *cdrom_device_info* described below. |
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The drive-specific, minor-like information that is registered with |
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`cdrom.c`, currently contains the following fields:: |
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struct cdrom_device_info { |
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const struct cdrom_device_ops * ops; /* device operations for this major */ |
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struct list_head list; /* linked list of all device_info */ |
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struct gendisk * disk; /* matching block layer disk */ |
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void * handle; /* driver-dependent data */ |
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int mask; /* mask of capability: disables them */ |
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int speed; /* maximum speed for reading data */ |
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int capacity; /* number of discs in a jukebox */ |
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unsigned int options:30; /* options flags */ |
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unsigned mc_flags:2; /* media-change buffer flags */ |
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unsigned int vfs_events; /* cached events for vfs path */ |
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unsigned int ioctl_events; /* cached events for ioctl path */ |
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int use_count; /* number of times device is opened */ |
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char name[20]; /* name of the device type */ |
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__u8 sanyo_slot : 2; /* Sanyo 3-CD changer support */ |
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__u8 keeplocked : 1; /* CDROM_LOCKDOOR status */ |
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__u8 reserved : 5; /* not used yet */ |
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int cdda_method; /* see CDDA_* flags */ |
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__u8 last_sense; /* saves last sense key */ |
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__u8 media_written; /* dirty flag, DVD+RW bookkeeping */ |
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unsigned short mmc3_profile; /* current MMC3 profile */ |
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int for_data; /* unknown:TBD */ |
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int (*exit)(struct cdrom_device_info *);/* unknown:TBD */ |
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int mrw_mode_page; /* which MRW mode page is in use */ |
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}; |
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Using this *struct*, a linked list of the registered minor devices is |
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built, using the *next* field. The device number, the device operations |
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struct and specifications of properties of the drive are stored in this |
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structure. |
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The *mask* flags can be used to mask out some of the capabilities listed |
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in *ops->capability*, if a specific drive doesn't support a feature |
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of the driver. The value *speed* specifies the maximum head-rate of the |
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drive, measured in units of normal audio speed (176kB/sec raw data or |
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150kB/sec file system data). The parameters are declared *const* |
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because they describe properties of the drive, which don't change after |
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registration. |
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A few registers contain variables local to the CD-ROM drive. The |
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flags *options* are used to specify how the general CD-ROM routines |
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should behave. These various flags registers should provide enough |
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flexibility to adapt to the different users' wishes (and **not** the |
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`arbitrary` wishes of the author of the low-level device driver, as is |
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the case in the old scheme). The register *mc_flags* is used to buffer |
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the information from *media_changed()* to two separate queues. Other |
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data that is specific to a minor drive, can be accessed through *handle*, |
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which can point to a data structure specific to the low-level driver. |
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The fields *use_count*, *next*, *options* and *mc_flags* need not be |
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initialized. |
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The intermediate software layer that `cdrom.c` forms will perform some |
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additional bookkeeping. The use count of the device (the number of |
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processes that have the device opened) is registered in *use_count*. The |
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function *cdrom_ioctl()* will verify the appropriate user-memory regions |
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for read and write, and in case a location on the CD is transferred, |
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it will `sanitize` the format by making requests to the low-level |
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drivers in a standard format, and translating all formats between the |
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user-software and low level drivers. This relieves much of the drivers' |
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memory checking and format checking and translation. Also, the necessary |
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structures will be declared on the program stack. |
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The implementation of the functions should be as defined in the |
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following sections. Two functions **must** be implemented, namely |
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*open()* and *release()*. Other functions may be omitted, their |
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corresponding capability flags will be cleared upon registration. |
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Generally, a function returns zero on success and negative on error. A |
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function call should return only after the command has completed, but of |
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course waiting for the device should not use processor time. |
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:: |
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int open(struct cdrom_device_info *cdi, int purpose) |
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*Open()* should try to open the device for a specific *purpose*, which |
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can be either: |
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- Open for reading data, as done by `mount()` (2), or the |
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user commands `dd` or `cat`. |
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- Open for *ioctl* commands, as done by audio-CD playing programs. |
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Notice that any strategic code (closing tray upon *open()*, etc.) is |
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done by the calling routine in `cdrom.c`, so the low-level routine |
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should only be concerned with proper initialization, such as spinning |
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up the disc, etc. |
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:: |
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void release(struct cdrom_device_info *cdi) |
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Device-specific actions should be taken such as spinning down the device. |
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However, strategic actions such as ejection of the tray, or unlocking |
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the door, should be left over to the general routine *cdrom_release()*. |
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This is the only function returning type *void*. |
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.. _cdrom_drive_status: |
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:: |
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int drive_status(struct cdrom_device_info *cdi, int slot_nr) |
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The function *drive_status*, if implemented, should provide |
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information on the status of the drive (not the status of the disc, |
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which may or may not be in the drive). If the drive is not a changer, |
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*slot_nr* should be ignored. In `cdrom.h` the possibilities are listed:: |
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CDS_NO_INFO /* no information available */ |
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CDS_NO_DISC /* no disc is inserted, tray is closed */ |
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CDS_TRAY_OPEN /* tray is opened */ |
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CDS_DRIVE_NOT_READY /* something is wrong, tray is moving? */ |
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CDS_DISC_OK /* a disc is loaded and everything is fine */ |
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:: |
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int tray_move(struct cdrom_device_info *cdi, int position) |
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This function, if implemented, should control the tray movement. (No |
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other function should control this.) The parameter *position* controls |
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the desired direction of movement: |
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- 0 Close tray |
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- 1 Open tray |
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This function returns 0 upon success, and a non-zero value upon |
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error. Note that if the tray is already in the desired position, no |
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action need be taken, and the return value should be 0. |
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|
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:: |
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int lock_door(struct cdrom_device_info *cdi, int lock) |
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|
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This function (and no other code) controls locking of the door, if the |
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drive allows this. The value of *lock* controls the desired locking |
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state: |
|
|
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- 0 Unlock door, manual opening is allowed |
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- 1 Lock door, tray cannot be ejected manually |
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|
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This function returns 0 upon success, and a non-zero value upon |
|
error. Note that if the door is already in the requested state, no |
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action need be taken, and the return value should be 0. |
|
|
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:: |
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|
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int select_speed(struct cdrom_device_info *cdi, int speed) |
|
|
|
Some CD-ROM drives are capable of changing their head-speed. There |
|
are several reasons for changing the speed of a CD-ROM drive. Badly |
|
pressed CD-ROM s may benefit from less-than-maximum head rate. Modern |
|
CD-ROM drives can obtain very high head rates (up to *24x* is |
|
common). It has been reported that these drives can make reading |
|
errors at these high speeds, reducing the speed can prevent data loss |
|
in these circumstances. Finally, some of these drives can |
|
make an annoyingly loud noise, which a lower speed may reduce. |
|
|
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This function specifies the speed at which data is read or audio is |
|
played back. The value of *speed* specifies the head-speed of the |
|
drive, measured in units of standard cdrom speed (176kB/sec raw data |
|
or 150kB/sec file system data). So to request that a CD-ROM drive |
|
operate at 300kB/sec you would call the CDROM_SELECT_SPEED *ioctl* |
|
with *speed=2*. The special value `0` means `auto-selection`, i. e., |
|
maximum data-rate or real-time audio rate. If the drive doesn't have |
|
this `auto-selection` capability, the decision should be made on the |
|
current disc loaded and the return value should be positive. A negative |
|
return value indicates an error. |
|
|
|
:: |
|
|
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int select_disc(struct cdrom_device_info *cdi, int number) |
|
|
|
If the drive can store multiple discs (a juke-box) this function |
|
will perform disc selection. It should return the number of the |
|
selected disc on success, a negative value on error. Currently, only |
|
the ide-cd driver supports this functionality. |
|
|
|
:: |
|
|
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int get_last_session(struct cdrom_device_info *cdi, |
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struct cdrom_multisession *ms_info) |
|
|
|
This function should implement the old corresponding *ioctl()*. For |
|
device *cdi->dev*, the start of the last session of the current disc |
|
should be returned in the pointer argument *ms_info*. Note that |
|
routines in `cdrom.c` have sanitized this argument: its requested |
|
format will **always** be of the type *CDROM_LBA* (linear block |
|
addressing mode), whatever the calling software requested. But |
|
sanitization goes even further: the low-level implementation may |
|
return the requested information in *CDROM_MSF* format if it wishes so |
|
(setting the *ms_info->addr_format* field appropriately, of |
|
course) and the routines in `cdrom.c` will make the transformation if |
|
necessary. The return value is 0 upon success. |
|
|
|
:: |
|
|
|
int get_mcn(struct cdrom_device_info *cdi, |
|
struct cdrom_mcn *mcn) |
|
|
|
Some discs carry a `Media Catalog Number` (MCN), also called |
|
`Universal Product Code` (UPC). This number should reflect the number |
|
that is generally found in the bar-code on the product. Unfortunately, |
|
the few discs that carry such a number on the disc don't even use the |
|
same format. The return argument to this function is a pointer to a |
|
pre-declared memory region of type *struct cdrom_mcn*. The MCN is |
|
expected as a 13-character string, terminated by a null-character. |
|
|
|
:: |
|
|
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int reset(struct cdrom_device_info *cdi) |
|
|
|
This call should perform a hard-reset on the drive (although in |
|
circumstances that a hard-reset is necessary, a drive may very well not |
|
listen to commands anymore). Preferably, control is returned to the |
|
caller only after the drive has finished resetting. If the drive is no |
|
longer listening, it may be wise for the underlying low-level cdrom |
|
driver to time out. |
|
|
|
:: |
|
|
|
int audio_ioctl(struct cdrom_device_info *cdi, |
|
unsigned int cmd, void *arg) |
|
|
|
Some of the CD-ROM-\ *ioctl()*\ 's defined in `cdrom.h` can be |
|
implemented by the routines described above, and hence the function |
|
*cdrom_ioctl* will use those. However, most *ioctl()*\ 's deal with |
|
audio-control. We have decided to leave these to be accessed through a |
|
single function, repeating the arguments *cmd* and *arg*. Note that |
|
the latter is of type *void*, rather than *unsigned long int*. |
|
The routine *cdrom_ioctl()* does do some useful things, |
|
though. It sanitizes the address format type to *CDROM_MSF* (Minutes, |
|
Seconds, Frames) for all audio calls. It also verifies the memory |
|
location of *arg*, and reserves stack-memory for the argument. This |
|
makes implementation of the *audio_ioctl()* much simpler than in the |
|
old driver scheme. For example, you may look up the function |
|
*cm206_audio_ioctl()* `cm206.c` that should be updated with |
|
this documentation. |
|
|
|
An unimplemented ioctl should return *-ENOSYS*, but a harmless request |
|
(e. g., *CDROMSTART*) may be ignored by returning 0 (success). Other |
|
errors should be according to the standards, whatever they are. When |
|
an error is returned by the low-level driver, the Uniform CD-ROM Driver |
|
tries whenever possible to return the error code to the calling program. |
|
(We may decide to sanitize the return value in *cdrom_ioctl()* though, in |
|
order to guarantee a uniform interface to the audio-player software.) |
|
|
|
:: |
|
|
|
int dev_ioctl(struct cdrom_device_info *cdi, |
|
unsigned int cmd, unsigned long arg) |
|
|
|
Some *ioctl()'s* seem to be specific to certain CD-ROM drives. That is, |
|
they are introduced to service some capabilities of certain drives. In |
|
fact, there are 6 different *ioctl()'s* for reading data, either in some |
|
particular kind of format, or audio data. Not many drives support |
|
reading audio tracks as data, I believe this is because of protection |
|
of copyrights of artists. Moreover, I think that if audio-tracks are |
|
supported, it should be done through the VFS and not via *ioctl()'s*. A |
|
problem here could be the fact that audio-frames are 2352 bytes long, |
|
so either the audio-file-system should ask for 75264 bytes at once |
|
(the least common multiple of 512 and 2352), or the drivers should |
|
bend their backs to cope with this incoherence (to which I would be |
|
opposed). Furthermore, it is very difficult for the hardware to find |
|
the exact frame boundaries, since there are no synchronization headers |
|
in audio frames. Once these issues are resolved, this code should be |
|
standardized in `cdrom.c`. |
|
|
|
Because there are so many *ioctl()'s* that seem to be introduced to |
|
satisfy certain drivers [#f2]_, any non-standard *ioctl()*\ s |
|
are routed through the call *dev_ioctl()*. In principle, `private` |
|
*ioctl()*\ 's should be numbered after the device's major number, and not |
|
the general CD-ROM *ioctl* number, `0x53`. Currently the |
|
non-supported *ioctl()'s* are: |
|
|
|
CDROMREADMODE1, CDROMREADMODE2, CDROMREADAUDIO, CDROMREADRAW, |
|
CDROMREADCOOKED, CDROMSEEK, CDROMPLAY-BLK and CDROM-READALL |
|
|
|
.. [#f2] |
|
|
|
Is there software around that actually uses these? I'd be interested! |
|
|
|
.. _cdrom_capabilities: |
|
|
|
CD-ROM capabilities |
|
------------------- |
|
|
|
Instead of just implementing some *ioctl* calls, the interface in |
|
`cdrom.c` supplies the possibility to indicate the **capabilities** |
|
of a CD-ROM drive. This can be done by ORing any number of |
|
capability-constants that are defined in `cdrom.h` at the registration |
|
phase. Currently, the capabilities are any of:: |
|
|
|
CDC_CLOSE_TRAY /* can close tray by software control */ |
|
CDC_OPEN_TRAY /* can open tray */ |
|
CDC_LOCK /* can lock and unlock the door */ |
|
CDC_SELECT_SPEED /* can select speed, in units of * sim*150 ,kB/s */ |
|
CDC_SELECT_DISC /* drive is juke-box */ |
|
CDC_MULTI_SESSION /* can read sessions *> rm1* */ |
|
CDC_MCN /* can read Media Catalog Number */ |
|
CDC_MEDIA_CHANGED /* can report if disc has changed */ |
|
CDC_PLAY_AUDIO /* can perform audio-functions (play, pause, etc) */ |
|
CDC_RESET /* hard reset device */ |
|
CDC_IOCTLS /* driver has non-standard ioctls */ |
|
CDC_DRIVE_STATUS /* driver implements drive status */ |
|
|
|
The capability flag is declared *const*, to prevent drivers from |
|
accidentally tampering with the contents. The capability flags actually |
|
inform `cdrom.c` of what the driver can do. If the drive found |
|
by the driver does not have the capability, is can be masked out by |
|
the *cdrom_device_info* variable *mask*. For instance, the SCSI CD-ROM |
|
driver has implemented the code for loading and ejecting CD-ROM's, and |
|
hence its corresponding flags in *capability* will be set. But a SCSI |
|
CD-ROM drive might be a caddy system, which can't load the tray, and |
|
hence for this drive the *cdrom_device_info* struct will have set |
|
the *CDC_CLOSE_TRAY* bit in *mask*. |
|
|
|
In the file `cdrom.c` you will encounter many constructions of the type:: |
|
|
|
if (cdo->capability & ~cdi->mask & CDC _<capability>) ... |
|
|
|
There is no *ioctl* to set the mask... The reason is that |
|
I think it is better to control the **behavior** rather than the |
|
**capabilities**. |
|
|
|
Options |
|
------- |
|
|
|
A final flag register controls the **behavior** of the CD-ROM |
|
drives, in order to satisfy different users' wishes, hopefully |
|
independently of the ideas of the respective author who happened to |
|
have made the drive's support available to the Linux community. The |
|
current behavior options are:: |
|
|
|
CDO_AUTO_CLOSE /* try to close tray upon device open() */ |
|
CDO_AUTO_EJECT /* try to open tray on last device close() */ |
|
CDO_USE_FFLAGS /* use file_pointer->f_flags to indicate purpose for open() */ |
|
CDO_LOCK /* try to lock door if device is opened */ |
|
CDO_CHECK_TYPE /* ensure disc type is data if opened for data */ |
|
|
|
The initial value of this register is |
|
`CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK`, reflecting my own view on user |
|
interface and software standards. Before you protest, there are two |
|
new *ioctl()'s* implemented in `cdrom.c`, that allow you to control the |
|
behavior by software. These are:: |
|
|
|
CDROM_SET_OPTIONS /* set options specified in (int)arg */ |
|
CDROM_CLEAR_OPTIONS /* clear options specified in (int)arg */ |
|
|
|
One option needs some more explanation: *CDO_USE_FFLAGS*. In the next |
|
newsection we explain what the need for this option is. |
|
|
|
A software package `setcd`, available from the Debian distribution |
|
and `sunsite.unc.edu`, allows user level control of these flags. |
|
|
|
|
|
The need to know the purpose of opening the CD-ROM device |
|
========================================================= |
|
|
|
Traditionally, Unix devices can be used in two different `modes`, |
|
either by reading/writing to the device file, or by issuing |
|
controlling commands to the device, by the device's *ioctl()* |
|
call. The problem with CD-ROM drives, is that they can be used for |
|
two entirely different purposes. One is to mount removable |
|
file systems, CD-ROM's, the other is to play audio CD's. Audio commands |
|
are implemented entirely through *ioctl()\'s*, presumably because the |
|
first implementation (SUN?) has been such. In principle there is |
|
nothing wrong with this, but a good control of the `CD player` demands |
|
that the device can **always** be opened in order to give the |
|
*ioctl* commands, regardless of the state the drive is in. |
|
|
|
On the other hand, when used as a removable-media disc drive (what the |
|
original purpose of CD-ROM s is) we would like to make sure that the |
|
disc drive is ready for operation upon opening the device. In the old |
|
scheme, some CD-ROM drivers don't do any integrity checking, resulting |
|
in a number of i/o errors reported by the VFS to the kernel when an |
|
attempt for mounting a CD-ROM on an empty drive occurs. This is not a |
|
particularly elegant way to find out that there is no CD-ROM inserted; |
|
it more-or-less looks like the old IBM-PC trying to read an empty floppy |
|
drive for a couple of seconds, after which the system complains it |
|
can't read from it. Nowadays we can **sense** the existence of a |
|
removable medium in a drive, and we believe we should exploit that |
|
fact. An integrity check on opening of the device, that verifies the |
|
availability of a CD-ROM and its correct type (data), would be |
|
desirable. |
|
|
|
These two ways of using a CD-ROM drive, principally for data and |
|
secondarily for playing audio discs, have different demands for the |
|
behavior of the *open()* call. Audio use simply wants to open the |
|
device in order to get a file handle which is needed for issuing |
|
*ioctl* commands, while data use wants to open for correct and |
|
reliable data transfer. The only way user programs can indicate what |
|
their *purpose* of opening the device is, is through the *flags* |
|
parameter (see `open(2)`). For CD-ROM devices, these flags aren't |
|
implemented (some drivers implement checking for write-related flags, |
|
but this is not strictly necessary if the device file has correct |
|
permission flags). Most option flags simply don't make sense to |
|
CD-ROM devices: *O_CREAT*, *O_NOCTTY*, *O_TRUNC*, *O_APPEND*, and |
|
*O_SYNC* have no meaning to a CD-ROM. |
|
|
|
We therefore propose to use the flag *O_NONBLOCK* to indicate |
|
that the device is opened just for issuing *ioctl* |
|
commands. Strictly, the meaning of *O_NONBLOCK* is that opening and |
|
subsequent calls to the device don't cause the calling process to |
|
wait. We could interpret this as don't wait until someone has |
|
inserted some valid data-CD-ROM. Thus, our proposal of the |
|
implementation for the *open()* call for CD-ROM s is: |
|
|
|
- If no other flags are set than *O_RDONLY*, the device is opened |
|
for data transfer, and the return value will be 0 only upon successful |
|
initialization of the transfer. The call may even induce some actions |
|
on the CD-ROM, such as closing the tray. |
|
- If the option flag *O_NONBLOCK* is set, opening will always be |
|
successful, unless the whole device doesn't exist. The drive will take |
|
no actions whatsoever. |
|
|
|
And what about standards? |
|
------------------------- |
|
|
|
You might hesitate to accept this proposal as it comes from the |
|
Linux community, and not from some standardizing institute. What |
|
about SUN, SGI, HP and all those other Unix and hardware vendors? |
|
Well, these companies are in the lucky position that they generally |
|
control both the hardware and software of their supported products, |
|
and are large enough to set their own standard. They do not have to |
|
deal with a dozen or more different, competing hardware |
|
configurations\ [#f3]_. |
|
|
|
.. [#f3] |
|
|
|
Incidentally, I think that SUN's approach to mounting CD-ROM s is very |
|
good in origin: under Solaris a volume-daemon automatically mounts a |
|
newly inserted CD-ROM under `/cdrom/*<volume-name>*`. |
|
|
|
In my opinion they should have pushed this |
|
further and have **every** CD-ROM on the local area network be |
|
mounted at the similar location, i. e., no matter in which particular |
|
machine you insert a CD-ROM, it will always appear at the same |
|
position in the directory tree, on every system. When I wanted to |
|
implement such a user-program for Linux, I came across the |
|
differences in behavior of the various drivers, and the need for an |
|
*ioctl* informing about media changes. |
|
|
|
We believe that using *O_NONBLOCK* to indicate that a device is being opened |
|
for *ioctl* commands only can be easily introduced in the Linux |
|
community. All the CD-player authors will have to be informed, we can |
|
even send in our own patches to the programs. The use of *O_NONBLOCK* |
|
has most likely no influence on the behavior of the CD-players on |
|
other operating systems than Linux. Finally, a user can always revert |
|
to old behavior by a call to |
|
*ioctl(file_descriptor, CDROM_CLEAR_OPTIONS, CDO_USE_FFLAGS)*. |
|
|
|
The preferred strategy of *open()* |
|
---------------------------------- |
|
|
|
The routines in `cdrom.c` are designed in such a way that run-time |
|
configuration of the behavior of CD-ROM devices (of **any** type) |
|
can be carried out, by the *CDROM_SET/CLEAR_OPTIONS* *ioctls*. Thus, various |
|
modes of operation can be set: |
|
|
|
`CDO_AUTO_CLOSE | CDO_USE_FFLAGS | CDO_LOCK` |
|
This is the default setting. (With *CDO_CHECK_TYPE* it will be better, in |
|
the future.) If the device is not yet opened by any other process, and if |
|
the device is being opened for data (*O_NONBLOCK* is not set) and the |
|
tray is found to be open, an attempt to close the tray is made. Then, |
|
it is verified that a disc is in the drive and, if *CDO_CHECK_TYPE* is |
|
set, that it contains tracks of type `data mode 1`. Only if all tests |
|
are passed is the return value zero. The door is locked to prevent file |
|
system corruption. If the drive is opened for audio (*O_NONBLOCK* is |
|
set), no actions are taken and a value of 0 will be returned. |
|
|
|
`CDO_AUTO_CLOSE | CDO_AUTO_EJECT | CDO_LOCK` |
|
This mimics the behavior of the current sbpcd-driver. The option flags are |
|
ignored, the tray is closed on the first open, if necessary. Similarly, |
|
the tray is opened on the last release, i. e., if a CD-ROM is unmounted, |
|
it is automatically ejected, such that the user can replace it. |
|
|
|
We hope that these option can convince everybody (both driver |
|
maintainers and user program developers) to adopt the new CD-ROM |
|
driver scheme and option flag interpretation. |
|
|
|
Description of routines in `cdrom.c` |
|
==================================== |
|
|
|
Only a few routines in `cdrom.c` are exported to the drivers. In this |
|
new section we will discuss these, as well as the functions that `take |
|
over` the CD-ROM interface to the kernel. The header file belonging |
|
to `cdrom.c` is called `cdrom.h`. Formerly, some of the contents of this |
|
file were placed in the file `ucdrom.h`, but this file has now been |
|
merged back into `cdrom.h`. |
|
|
|
:: |
|
|
|
struct file_operations cdrom_fops |
|
|
|
The contents of this structure were described in cdrom_api_. |
|
A pointer to this structure is assigned to the *fops* field |
|
of the *struct gendisk*. |
|
|
|
:: |
|
|
|
int register_cdrom(struct cdrom_device_info *cdi) |
|
|
|
This function is used in about the same way one registers *cdrom_fops* |
|
with the kernel, the device operations and information structures, |
|
as described in cdrom_api_, should be registered with the |
|
Uniform CD-ROM Driver:: |
|
|
|
register_cdrom(&<device>_info); |
|
|
|
|
|
This function returns zero upon success, and non-zero upon |
|
failure. The structure *<device>_info* should have a pointer to the |
|
driver's *<device>_dops*, as in:: |
|
|
|
struct cdrom_device_info <device>_info = { |
|
<device>_dops; |
|
... |
|
} |
|
|
|
Note that a driver must have one static structure, *<device>_dops*, while |
|
it may have as many structures *<device>_info* as there are minor devices |
|
active. *Register_cdrom()* builds a linked list from these. |
|
|
|
|
|
:: |
|
|
|
void unregister_cdrom(struct cdrom_device_info *cdi) |
|
|
|
Unregistering device *cdi* with minor number *MINOR(cdi->dev)* removes |
|
the minor device from the list. If it was the last registered minor for |
|
the low-level driver, this disconnects the registered device-operation |
|
routines from the CD-ROM interface. This function returns zero upon |
|
success, and non-zero upon failure. |
|
|
|
:: |
|
|
|
int cdrom_open(struct inode * ip, struct file * fp) |
|
|
|
This function is not called directly by the low-level drivers, it is |
|
listed in the standard *cdrom_fops*. If the VFS opens a file, this |
|
function becomes active. A strategy is implemented in this routine, |
|
taking care of all capabilities and options that are set in the |
|
*cdrom_device_ops* connected to the device. Then, the program flow is |
|
transferred to the device_dependent *open()* call. |
|
|
|
:: |
|
|
|
void cdrom_release(struct inode *ip, struct file *fp) |
|
|
|
This function implements the reverse-logic of *cdrom_open()*, and then |
|
calls the device-dependent *release()* routine. When the use-count has |
|
reached 0, the allocated buffers are flushed by calls to *sync_dev(dev)* |
|
and *invalidate_buffers(dev)*. |
|
|
|
|
|
.. _cdrom_ioctl: |
|
|
|
:: |
|
|
|
int cdrom_ioctl(struct inode *ip, struct file *fp, |
|
unsigned int cmd, unsigned long arg) |
|
|
|
This function handles all the standard *ioctl* requests for CD-ROM |
|
devices in a uniform way. The different calls fall into three |
|
categories: *ioctl()'s* that can be directly implemented by device |
|
operations, ones that are routed through the call *audio_ioctl()*, and |
|
the remaining ones, that are presumable device-dependent. Generally, a |
|
negative return value indicates an error. |
|
|
|
Directly implemented *ioctl()'s* |
|
-------------------------------- |
|
|
|
The following `old` CD-ROM *ioctl()*\ 's are implemented by directly |
|
calling device-operations in *cdrom_device_ops*, if implemented and |
|
not masked: |
|
|
|
`CDROMMULTISESSION` |
|
Requests the last session on a CD-ROM. |
|
`CDROMEJECT` |
|
Open tray. |
|
`CDROMCLOSETRAY` |
|
Close tray. |
|
`CDROMEJECT_SW` |
|
If *arg\not=0*, set behavior to auto-close (close |
|
tray on first open) and auto-eject (eject on last release), otherwise |
|
set behavior to non-moving on *open()* and *release()* calls. |
|
`CDROM_GET_MCN` |
|
Get the Media Catalog Number from a CD. |
|
|
|
*Ioctl*s routed through *audio_ioctl()* |
|
--------------------------------------- |
|
|
|
The following set of *ioctl()'s* are all implemented through a call to |
|
the *cdrom_fops* function *audio_ioctl()*. Memory checks and |
|
allocation are performed in *cdrom_ioctl()*, and also sanitization of |
|
address format (*CDROM_LBA*/*CDROM_MSF*) is done. |
|
|
|
`CDROMSUBCHNL` |
|
Get sub-channel data in argument *arg* of type |
|
`struct cdrom_subchnl *`. |
|
`CDROMREADTOCHDR` |
|
Read Table of Contents header, in *arg* of type |
|
`struct cdrom_tochdr *`. |
|
`CDROMREADTOCENTRY` |
|
Read a Table of Contents entry in *arg* and specified by *arg* |
|
of type `struct cdrom_tocentry *`. |
|
`CDROMPLAYMSF` |
|
Play audio fragment specified in Minute, Second, Frame format, |
|
delimited by *arg* of type `struct cdrom_msf *`. |
|
`CDROMPLAYTRKIND` |
|
Play audio fragment in track-index format delimited by *arg* |
|
of type `struct cdrom_ti *`. |
|
`CDROMVOLCTRL` |
|
Set volume specified by *arg* of type `struct cdrom_volctrl *`. |
|
`CDROMVOLREAD` |
|
Read volume into by *arg* of type `struct cdrom_volctrl *`. |
|
`CDROMSTART` |
|
Spin up disc. |
|
`CDROMSTOP` |
|
Stop playback of audio fragment. |
|
`CDROMPAUSE` |
|
Pause playback of audio fragment. |
|
`CDROMRESUME` |
|
Resume playing. |
|
|
|
New *ioctl()'s* in `cdrom.c` |
|
---------------------------- |
|
|
|
The following *ioctl()'s* have been introduced to allow user programs to |
|
control the behavior of individual CD-ROM devices. New *ioctl* |
|
commands can be identified by the underscores in their names. |
|
|
|
`CDROM_SET_OPTIONS` |
|
Set options specified by *arg*. Returns the option flag register |
|
after modification. Use *arg = \rm0* for reading the current flags. |
|
`CDROM_CLEAR_OPTIONS` |
|
Clear options specified by *arg*. Returns the option flag register |
|
after modification. |
|
`CDROM_SELECT_SPEED` |
|
Select head-rate speed of disc specified as by *arg* in units |
|
of standard cdrom speed (176\,kB/sec raw data or |
|
150kB/sec file system data). The value 0 means `auto-select`, |
|
i. e., play audio discs at real time and data discs at maximum speed. |
|
The value *arg* is checked against the maximum head rate of the |
|
drive found in the *cdrom_dops*. |
|
`CDROM_SELECT_DISC` |
|
Select disc numbered *arg* from a juke-box. |
|
|
|
First disc is numbered 0. The number *arg* is checked against the |
|
maximum number of discs in the juke-box found in the *cdrom_dops*. |
|
`CDROM_MEDIA_CHANGED` |
|
Returns 1 if a disc has been changed since the last call. |
|
For juke-boxes, an extra argument *arg* |
|
specifies the slot for which the information is given. The special |
|
value *CDSL_CURRENT* requests that information about the currently |
|
selected slot be returned. |
|
`CDROM_DRIVE_STATUS` |
|
Returns the status of the drive by a call to |
|
*drive_status()*. Return values are defined in cdrom_drive_status_. |
|
Note that this call doesn't return information on the |
|
current playing activity of the drive; this can be polled through |
|
an *ioctl* call to *CDROMSUBCHNL*. For juke-boxes, an extra argument |
|
*arg* specifies the slot for which (possibly limited) information is |
|
given. The special value *CDSL_CURRENT* requests that information |
|
about the currently selected slot be returned. |
|
`CDROM_DISC_STATUS` |
|
Returns the type of the disc currently in the drive. |
|
It should be viewed as a complement to *CDROM_DRIVE_STATUS*. |
|
This *ioctl* can provide *some* information about the current |
|
disc that is inserted in the drive. This functionality used to be |
|
implemented in the low level drivers, but is now carried out |
|
entirely in Uniform CD-ROM Driver. |
|
|
|
The history of development of the CD's use as a carrier medium for |
|
various digital information has lead to many different disc types. |
|
This *ioctl* is useful only in the case that CDs have \emph {only |
|
one} type of data on them. While this is often the case, it is |
|
also very common for CDs to have some tracks with data, and some |
|
tracks with audio. Because this is an existing interface, rather |
|
than fixing this interface by changing the assumptions it was made |
|
under, thereby breaking all user applications that use this |
|
function, the Uniform CD-ROM Driver implements this *ioctl* as |
|
follows: If the CD in question has audio tracks on it, and it has |
|
absolutely no CD-I, XA, or data tracks on it, it will be reported |
|
as *CDS_AUDIO*. If it has both audio and data tracks, it will |
|
return *CDS_MIXED*. If there are no audio tracks on the disc, and |
|
if the CD in question has any CD-I tracks on it, it will be |
|
reported as *CDS_XA_2_2*. Failing that, if the CD in question |
|
has any XA tracks on it, it will be reported as *CDS_XA_2_1*. |
|
Finally, if the CD in question has any data tracks on it, |
|
it will be reported as a data CD (*CDS_DATA_1*). |
|
|
|
This *ioctl* can return:: |
|
|
|
CDS_NO_INFO /* no information available */ |
|
CDS_NO_DISC /* no disc is inserted, or tray is opened */ |
|
CDS_AUDIO /* Audio disc (2352 audio bytes/frame) */ |
|
CDS_DATA_1 /* data disc, mode 1 (2048 user bytes/frame) */ |
|
CDS_XA_2_1 /* mixed data (XA), mode 2, form 1 (2048 user bytes) */ |
|
CDS_XA_2_2 /* mixed data (XA), mode 2, form 1 (2324 user bytes) */ |
|
CDS_MIXED /* mixed audio/data disc */ |
|
|
|
For some information concerning frame layout of the various disc |
|
types, see a recent version of `cdrom.h`. |
|
|
|
`CDROM_CHANGER_NSLOTS` |
|
Returns the number of slots in a juke-box. |
|
`CDROMRESET` |
|
Reset the drive. |
|
`CDROM_GET_CAPABILITY` |
|
Returns the *capability* flags for the drive. Refer to section |
|
cdrom_capabilities_ for more information on these flags. |
|
`CDROM_LOCKDOOR` |
|
Locks the door of the drive. `arg == 0` unlocks the door, |
|
any other value locks it. |
|
`CDROM_DEBUG` |
|
Turns on debugging info. Only root is allowed to do this. |
|
Same semantics as CDROM_LOCKDOOR. |
|
|
|
|
|
Device dependent *ioctl()'s* |
|
---------------------------- |
|
|
|
Finally, all other *ioctl()'s* are passed to the function *dev_ioctl()*, |
|
if implemented. No memory allocation or verification is carried out. |
|
|
|
How to update your driver |
|
========================= |
|
|
|
- Make a backup of your current driver. |
|
- Get hold of the files `cdrom.c` and `cdrom.h`, they should be in |
|
the directory tree that came with this documentation. |
|
- Make sure you include `cdrom.h`. |
|
- Change the 3rd argument of *register_blkdev* from `&<your-drive>_fops` |
|
to `&cdrom_fops`. |
|
- Just after that line, add the following to register with the Uniform |
|
CD-ROM Driver:: |
|
|
|
register_cdrom(&<your-drive>_info);* |
|
|
|
Similarly, add a call to *unregister_cdrom()* at the appropriate place. |
|
- Copy an example of the device-operations *struct* to your |
|
source, e. g., from `cm206.c` *cm206_dops*, and change all |
|
entries to names corresponding to your driver, or names you just |
|
happen to like. If your driver doesn't support a certain function, |
|
make the entry *NULL*. At the entry *capability* you should list all |
|
capabilities your driver currently supports. If your driver |
|
has a capability that is not listed, please send me a message. |
|
- Copy the *cdrom_device_info* declaration from the same example |
|
driver, and modify the entries according to your needs. If your |
|
driver dynamically determines the capabilities of the hardware, this |
|
structure should also be declared dynamically. |
|
- Implement all functions in your `<device>_dops` structure, |
|
according to prototypes listed in `cdrom.h`, and specifications given |
|
in cdrom_api_. Most likely you have already implemented |
|
the code in a large part, and you will almost certainly need to adapt the |
|
prototype and return values. |
|
- Rename your `<device>_ioctl()` function to *audio_ioctl* and |
|
change the prototype a little. Remove entries listed in the first |
|
part in cdrom_ioctl_, if your code was OK, these are |
|
just calls to the routines you adapted in the previous step. |
|
- You may remove all remaining memory checking code in the |
|
*audio_ioctl()* function that deals with audio commands (these are |
|
listed in the second part of cdrom_ioctl_. There is no |
|
need for memory allocation either, so most *case*s in the *switch* |
|
statement look similar to:: |
|
|
|
case CDROMREADTOCENTRY: |
|
get_toc_entry\bigl((struct cdrom_tocentry *) arg); |
|
|
|
- All remaining *ioctl* cases must be moved to a separate |
|
function, *<device>_ioctl*, the device-dependent *ioctl()'s*. Note that |
|
memory checking and allocation must be kept in this code! |
|
- Change the prototypes of *<device>_open()* and |
|
*<device>_release()*, and remove any strategic code (i. e., tray |
|
movement, door locking, etc.). |
|
- Try to recompile the drivers. We advise you to use modules, both |
|
for `cdrom.o` and your driver, as debugging is much easier this |
|
way. |
|
|
|
Thanks |
|
====== |
|
|
|
Thanks to all the people involved. First, Erik Andersen, who has |
|
taken over the torch in maintaining `cdrom.c` and integrating much |
|
CD-ROM-related code in the 2.1-kernel. Thanks to Scott Snyder and |
|
Gerd Knorr, who were the first to implement this interface for SCSI |
|
and IDE-CD drivers and added many ideas for extension of the data |
|
structures relative to kernel~2.0. Further thanks to Heiko Eißfeldt, |
|
Thomas Quinot, Jon Tombs, Ken Pizzini, Eberhard Mönkeberg and Andrew Kroll, |
|
the Linux CD-ROM device driver developers who were kind |
|
enough to give suggestions and criticisms during the writing. Finally |
|
of course, I want to thank Linus Torvalds for making this possible in |
|
the first place.
|
|
|