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230 lines
9.9 KiB
230 lines
9.9 KiB
=========== |
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EHCI driver |
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=========== |
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27-Dec-2002 |
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The EHCI driver is used to talk to high speed USB 2.0 devices using |
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USB 2.0-capable host controller hardware. The USB 2.0 standard is |
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compatible with the USB 1.1 standard. It defines three transfer speeds: |
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- "High Speed" 480 Mbit/sec (60 MByte/sec) |
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- "Full Speed" 12 Mbit/sec (1.5 MByte/sec) |
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- "Low Speed" 1.5 Mbit/sec |
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USB 1.1 only addressed full speed and low speed. High speed devices |
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can be used on USB 1.1 systems, but they slow down to USB 1.1 speeds. |
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USB 1.1 devices may also be used on USB 2.0 systems. When plugged |
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into an EHCI controller, they are given to a USB 1.1 "companion" |
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controller, which is a OHCI or UHCI controller as normally used with |
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such devices. When USB 1.1 devices plug into USB 2.0 hubs, they |
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interact with the EHCI controller through a "Transaction Translator" |
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(TT) in the hub, which turns low or full speed transactions into |
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high speed "split transactions" that don't waste transfer bandwidth. |
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At this writing, this driver has been seen to work with implementations |
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of EHCI from (in alphabetical order): Intel, NEC, Philips, and VIA. |
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Other EHCI implementations are becoming available from other vendors; |
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you should expect this driver to work with them too. |
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While usb-storage devices have been available since mid-2001 (working |
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quite speedily on the 2.4 version of this driver), hubs have only |
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been available since late 2001, and other kinds of high speed devices |
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appear to be on hold until more systems come with USB 2.0 built-in. |
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Such new systems have been available since early 2002, and became much |
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more typical in the second half of 2002. |
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Note that USB 2.0 support involves more than just EHCI. It requires |
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other changes to the Linux-USB core APIs, including the hub driver, |
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but those changes haven't needed to really change the basic "usbcore" |
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APIs exposed to USB device drivers. |
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- David Brownell |
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<[email protected]> |
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Functionality |
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============= |
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This driver is regularly tested on x86 hardware, and has also been |
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used on PPC hardware so big/little endianness issues should be gone. |
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It's believed to do all the right PCI magic so that I/O works even on |
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systems with interesting DMA mapping issues. |
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Transfer Types |
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-------------- |
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At this writing the driver should comfortably handle all control, bulk, |
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and interrupt transfers, including requests to USB 1.1 devices through |
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transaction translators (TTs) in USB 2.0 hubs. But you may find bugs. |
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High Speed Isochronous (ISO) transfer support is also functional, but |
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at this writing no Linux drivers have been using that support. |
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Full Speed Isochronous transfer support, through transaction translators, |
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is not yet available. Note that split transaction support for ISO |
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transfers can't share much code with the code for high speed ISO transfers, |
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since EHCI represents these with a different data structure. So for now, |
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most USB audio and video devices can't be connected to high speed buses. |
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Driver Behavior |
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--------------- |
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Transfers of all types can be queued. This means that control transfers |
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from a driver on one interface (or through usbfs) won't interfere with |
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ones from another driver, and that interrupt transfers can use periods |
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of one frame without risking data loss due to interrupt processing costs. |
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The EHCI root hub code hands off USB 1.1 devices to its companion |
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controller. This driver doesn't need to know anything about those |
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drivers; a OHCI or UHCI driver that works already doesn't need to change |
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just because the EHCI driver is also present. |
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There are some issues with power management; suspend/resume doesn't |
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behave quite right at the moment. |
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Also, some shortcuts have been taken with the scheduling periodic |
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transactions (interrupt and isochronous transfers). These place some |
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limits on the number of periodic transactions that can be scheduled, |
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and prevent use of polling intervals of less than one frame. |
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Use by |
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====== |
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Assuming you have an EHCI controller (on a PCI card or motherboard) |
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and have compiled this driver as a module, load this like:: |
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# modprobe ehci-hcd |
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and remove it by:: |
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# rmmod ehci-hcd |
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You should also have a driver for a "companion controller", such as |
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"ohci-hcd" or "uhci-hcd". In case of any trouble with the EHCI driver, |
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remove its module and then the driver for that companion controller will |
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take over (at lower speed) all the devices that were previously handled |
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by the EHCI driver. |
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Module parameters (pass to "modprobe") include: |
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log2_irq_thresh (default 0): |
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Log2 of default interrupt delay, in microframes. The default |
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value is 0, indicating 1 microframe (125 usec). Maximum value |
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is 6, indicating 2^6 = 64 microframes. This controls how often |
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the EHCI controller can issue interrupts. |
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If you're using this driver on a 2.5 kernel, and you've enabled USB |
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debugging support, you'll see three files in the "sysfs" directory for |
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any EHCI controller: |
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"async" |
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dumps the asynchronous schedule, used for control |
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and bulk transfers. Shows each active qh and the qtds |
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pending, usually one qtd per urb. (Look at it with |
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usb-storage doing disk I/O; watch the request queues!) |
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"periodic" |
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dumps the periodic schedule, used for interrupt |
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and isochronous transfers. Doesn't show qtds. |
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"registers" |
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show controller register state, and |
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The contents of those files can help identify driver problems. |
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Device drivers shouldn't care whether they're running over EHCI or not, |
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but they may want to check for "usb_device->speed == USB_SPEED_HIGH". |
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High speed devices can do things that full speed (or low speed) ones |
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can't, such as "high bandwidth" periodic (interrupt or ISO) transfers. |
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Also, some values in device descriptors (such as polling intervals for |
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periodic transfers) use different encodings when operating at high speed. |
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However, do make a point of testing device drivers through USB 2.0 hubs. |
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Those hubs report some failures, such as disconnections, differently when |
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transaction translators are in use; some drivers have been seen to behave |
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badly when they see different faults than OHCI or UHCI report. |
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Performance |
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=========== |
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USB 2.0 throughput is gated by two main factors: how fast the host |
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controller can process requests, and how fast devices can respond to |
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them. The 480 Mbit/sec "raw transfer rate" is obeyed by all devices, |
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but aggregate throughput is also affected by issues like delays between |
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individual high speed packets, driver intelligence, and of course the |
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overall system load. Latency is also a performance concern. |
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Bulk transfers are most often used where throughput is an issue. It's |
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good to keep in mind that bulk transfers are always in 512 byte packets, |
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and at most 13 of those fit into one USB 2.0 microframe. Eight USB 2.0 |
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microframes fit in a USB 1.1 frame; a microframe is 1 msec/8 = 125 usec. |
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So more than 50 MByte/sec is available for bulk transfers, when both |
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hardware and device driver software allow it. Periodic transfer modes |
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(isochronous and interrupt) allow the larger packet sizes which let you |
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approach the quoted 480 MBit/sec transfer rate. |
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Hardware Performance |
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-------------------- |
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At this writing, individual USB 2.0 devices tend to max out at around |
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20 MByte/sec transfer rates. This is of course subject to change; |
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and some devices now go faster, while others go slower. |
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The first NEC implementation of EHCI seems to have a hardware bottleneck |
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at around 28 MByte/sec aggregate transfer rate. While this is clearly |
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enough for a single device at 20 MByte/sec, putting three such devices |
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onto one bus does not get you 60 MByte/sec. The issue appears to be |
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that the controller hardware won't do concurrent USB and PCI access, |
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so that it's only trying six (or maybe seven) USB transactions each |
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microframe rather than thirteen. (Seems like a reasonable trade off |
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for a product that beat all the others to market by over a year!) |
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It's expected that newer implementations will better this, throwing |
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more silicon real estate at the problem so that new motherboard chip |
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sets will get closer to that 60 MByte/sec target. That includes an |
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updated implementation from NEC, as well as other vendors' silicon. |
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There's a minimum latency of one microframe (125 usec) for the host |
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to receive interrupts from the EHCI controller indicating completion |
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of requests. That latency is tunable; there's a module option. By |
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default ehci-hcd driver uses the minimum latency, which means that if |
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you issue a control or bulk request you can often expect to learn that |
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it completed in less than 250 usec (depending on transfer size). |
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Software Performance |
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-------------------- |
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To get even 20 MByte/sec transfer rates, Linux-USB device drivers will |
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need to keep the EHCI queue full. That means issuing large requests, |
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or using bulk queuing if a series of small requests needs to be issued. |
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When drivers don't do that, their performance results will show it. |
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In typical situations, a usb_bulk_msg() loop writing out 4 KB chunks is |
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going to waste more than half the USB 2.0 bandwidth. Delays between the |
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I/O completion and the driver issuing the next request will take longer |
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than the I/O. If that same loop used 16 KB chunks, it'd be better; a |
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sequence of 128 KB chunks would waste a lot less. |
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But rather than depending on such large I/O buffers to make synchronous |
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I/O be efficient, it's better to just queue up several (bulk) requests |
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to the HC, and wait for them all to complete (or be canceled on error). |
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Such URB queuing should work with all the USB 1.1 HC drivers too. |
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In the Linux 2.5 kernels, new usb_sg_*() api calls have been defined; they |
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queue all the buffers from a scatterlist. They also use scatterlist DMA |
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mapping (which might apply an IOMMU) and IRQ reduction, all of which will |
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help make high speed transfers run as fast as they can. |
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TBD: |
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Interrupt and ISO transfer performance issues. Those periodic |
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transfers are fully scheduled, so the main issue is likely to be how |
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to trigger "high bandwidth" modes. |
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TBD: |
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More than standard 80% periodic bandwidth allocation is possible |
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through sysfs uframe_periodic_max parameter. Describe that.
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