mirror of https://github.com/Qortal/Brooklyn
You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
769 lines
34 KiB
769 lines
34 KiB
====================== |
|
Legacy GPIO Interfaces |
|
====================== |
|
|
|
This provides an overview of GPIO access conventions on Linux. |
|
|
|
These calls use the gpio_* naming prefix. No other calls should use that |
|
prefix, or the related __gpio_* prefix. |
|
|
|
|
|
What is a GPIO? |
|
=============== |
|
A "General Purpose Input/Output" (GPIO) is a flexible software-controlled |
|
digital signal. They are provided from many kinds of chip, and are familiar |
|
to Linux developers working with embedded and custom hardware. Each GPIO |
|
represents a bit connected to a particular pin, or "ball" on Ball Grid Array |
|
(BGA) packages. Board schematics show which external hardware connects to |
|
which GPIOs. Drivers can be written generically, so that board setup code |
|
passes such pin configuration data to drivers. |
|
|
|
System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every |
|
non-dedicated pin can be configured as a GPIO; and most chips have at least |
|
several dozen of them. Programmable logic devices (like FPGAs) can easily |
|
provide GPIOs; multifunction chips like power managers, and audio codecs |
|
often have a few such pins to help with pin scarcity on SOCs; and there are |
|
also "GPIO Expander" chips that connect using the I2C or SPI serial busses. |
|
Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS |
|
firmware knowing how they're used). |
|
|
|
The exact capabilities of GPIOs vary between systems. Common options: |
|
|
|
- Output values are writable (high=1, low=0). Some chips also have |
|
options about how that value is driven, so that for example only one |
|
value might be driven ... supporting "wire-OR" and similar schemes |
|
for the other value (notably, "open drain" signaling). |
|
|
|
- Input values are likewise readable (1, 0). Some chips support readback |
|
of pins configured as "output", which is very useful in such "wire-OR" |
|
cases (to support bidirectional signaling). GPIO controllers may have |
|
input de-glitch/debounce logic, sometimes with software controls. |
|
|
|
- Inputs can often be used as IRQ signals, often edge triggered but |
|
sometimes level triggered. Such IRQs may be configurable as system |
|
wakeup events, to wake the system from a low power state. |
|
|
|
- Usually a GPIO will be configurable as either input or output, as needed |
|
by different product boards; single direction ones exist too. |
|
|
|
- Most GPIOs can be accessed while holding spinlocks, but those accessed |
|
through a serial bus normally can't. Some systems support both types. |
|
|
|
On a given board each GPIO is used for one specific purpose like monitoring |
|
MMC/SD card insertion/removal, detecting card writeprotect status, driving |
|
a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware |
|
watchdog, sensing a switch, and so on. |
|
|
|
|
|
GPIO conventions |
|
================ |
|
Note that this is called a "convention" because you don't need to do it this |
|
way, and it's no crime if you don't. There **are** cases where portability |
|
is not the main issue; GPIOs are often used for the kind of board-specific |
|
glue logic that may even change between board revisions, and can't ever be |
|
used on a board that's wired differently. Only least-common-denominator |
|
functionality can be very portable. Other features are platform-specific, |
|
and that can be critical for glue logic. |
|
|
|
Plus, this doesn't require any implementation framework, just an interface. |
|
One platform might implement it as simple inline functions accessing chip |
|
registers; another might implement it by delegating through abstractions |
|
used for several very different kinds of GPIO controller. (There is some |
|
optional code supporting such an implementation strategy, described later |
|
in this document, but drivers acting as clients to the GPIO interface must |
|
not care how it's implemented.) |
|
|
|
That said, if the convention is supported on their platform, drivers should |
|
use it when possible. Platforms must select GPIOLIB if GPIO functionality |
|
is strictly required. Drivers that can't work without |
|
standard GPIO calls should have Kconfig entries which depend on GPIOLIB. The |
|
GPIO calls are available, either as "real code" or as optimized-away stubs, |
|
when drivers use the include file: |
|
|
|
#include <linux/gpio.h> |
|
|
|
If you stick to this convention then it'll be easier for other developers to |
|
see what your code is doing, and help maintain it. |
|
|
|
Note that these operations include I/O barriers on platforms which need to |
|
use them; drivers don't need to add them explicitly. |
|
|
|
|
|
Identifying GPIOs |
|
----------------- |
|
GPIOs are identified by unsigned integers in the range 0..MAX_INT. That |
|
reserves "negative" numbers for other purposes like marking signals as |
|
"not available on this board", or indicating faults. Code that doesn't |
|
touch the underlying hardware treats these integers as opaque cookies. |
|
|
|
Platforms define how they use those integers, and usually #define symbols |
|
for the GPIO lines so that board-specific setup code directly corresponds |
|
to the relevant schematics. In contrast, drivers should only use GPIO |
|
numbers passed to them from that setup code, using platform_data to hold |
|
board-specific pin configuration data (along with other board specific |
|
data they need). That avoids portability problems. |
|
|
|
So for example one platform uses numbers 32-159 for GPIOs; while another |
|
uses numbers 0..63 with one set of GPIO controllers, 64-79 with another |
|
type of GPIO controller, and on one particular board 80-95 with an FPGA. |
|
The numbers need not be contiguous; either of those platforms could also |
|
use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders. |
|
|
|
If you want to initialize a structure with an invalid GPIO number, use |
|
some negative number (perhaps "-EINVAL"); that will never be valid. To |
|
test if such number from such a structure could reference a GPIO, you |
|
may use this predicate: |
|
|
|
int gpio_is_valid(int number); |
|
|
|
A number that's not valid will be rejected by calls which may request |
|
or free GPIOs (see below). Other numbers may also be rejected; for |
|
example, a number might be valid but temporarily unused on a given board. |
|
|
|
Whether a platform supports multiple GPIO controllers is a platform-specific |
|
implementation issue, as are whether that support can leave "holes" in the space |
|
of GPIO numbers, and whether new controllers can be added at runtime. Such issues |
|
can affect things including whether adjacent GPIO numbers are both valid. |
|
|
|
Using GPIOs |
|
----------- |
|
The first thing a system should do with a GPIO is allocate it, using |
|
the gpio_request() call; see later. |
|
|
|
One of the next things to do with a GPIO, often in board setup code when |
|
setting up a platform_device using the GPIO, is mark its direction:: |
|
|
|
/* set as input or output, returning 0 or negative errno */ |
|
int gpio_direction_input(unsigned gpio); |
|
int gpio_direction_output(unsigned gpio, int value); |
|
|
|
The return value is zero for success, else a negative errno. It should |
|
be checked, since the get/set calls don't have error returns and since |
|
misconfiguration is possible. You should normally issue these calls from |
|
a task context. However, for spinlock-safe GPIOs it's OK to use them |
|
before tasking is enabled, as part of early board setup. |
|
|
|
For output GPIOs, the value provided becomes the initial output value. |
|
This helps avoid signal glitching during system startup. |
|
|
|
For compatibility with legacy interfaces to GPIOs, setting the direction |
|
of a GPIO implicitly requests that GPIO (see below) if it has not been |
|
requested already. That compatibility is being removed from the optional |
|
gpiolib framework. |
|
|
|
Setting the direction can fail if the GPIO number is invalid, or when |
|
that particular GPIO can't be used in that mode. It's generally a bad |
|
idea to rely on boot firmware to have set the direction correctly, since |
|
it probably wasn't validated to do more than boot Linux. (Similarly, |
|
that board setup code probably needs to multiplex that pin as a GPIO, |
|
and configure pullups/pulldowns appropriately.) |
|
|
|
|
|
Spinlock-Safe GPIO access |
|
------------------------- |
|
Most GPIO controllers can be accessed with memory read/write instructions. |
|
Those don't need to sleep, and can safely be done from inside hard |
|
(nonthreaded) IRQ handlers and similar contexts. |
|
|
|
Use the following calls to access such GPIOs, |
|
for which gpio_cansleep() will always return false (see below):: |
|
|
|
/* GPIO INPUT: return zero or nonzero */ |
|
int gpio_get_value(unsigned gpio); |
|
|
|
/* GPIO OUTPUT */ |
|
void gpio_set_value(unsigned gpio, int value); |
|
|
|
The values are boolean, zero for low, nonzero for high. When reading the |
|
value of an output pin, the value returned should be what's seen on the |
|
pin ... that won't always match the specified output value, because of |
|
issues including open-drain signaling and output latencies. |
|
|
|
The get/set calls have no error returns because "invalid GPIO" should have |
|
been reported earlier from gpio_direction_*(). However, note that not all |
|
platforms can read the value of output pins; those that can't should always |
|
return zero. Also, using these calls for GPIOs that can't safely be accessed |
|
without sleeping (see below) is an error. |
|
|
|
Platform-specific implementations are encouraged to optimize the two |
|
calls to access the GPIO value in cases where the GPIO number (and for |
|
output, value) are constant. It's normal for them to need only a couple |
|
of instructions in such cases (reading or writing a hardware register), |
|
and not to need spinlocks. Such optimized calls can make bitbanging |
|
applications a lot more efficient (in both space and time) than spending |
|
dozens of instructions on subroutine calls. |
|
|
|
|
|
GPIO access that may sleep |
|
-------------------------- |
|
Some GPIO controllers must be accessed using message based busses like I2C |
|
or SPI. Commands to read or write those GPIO values require waiting to |
|
get to the head of a queue to transmit a command and get its response. |
|
This requires sleeping, which can't be done from inside IRQ handlers. |
|
|
|
Platforms that support this type of GPIO distinguish them from other GPIOs |
|
by returning nonzero from this call (which requires a valid GPIO number, |
|
which should have been previously allocated with gpio_request):: |
|
|
|
int gpio_cansleep(unsigned gpio); |
|
|
|
To access such GPIOs, a different set of accessors is defined:: |
|
|
|
/* GPIO INPUT: return zero or nonzero, might sleep */ |
|
int gpio_get_value_cansleep(unsigned gpio); |
|
|
|
/* GPIO OUTPUT, might sleep */ |
|
void gpio_set_value_cansleep(unsigned gpio, int value); |
|
|
|
|
|
Accessing such GPIOs requires a context which may sleep, for example |
|
a threaded IRQ handler, and those accessors must be used instead of |
|
spinlock-safe accessors without the cansleep() name suffix. |
|
|
|
Other than the fact that these accessors might sleep, and will work |
|
on GPIOs that can't be accessed from hardIRQ handlers, these calls act |
|
the same as the spinlock-safe calls. |
|
|
|
**IN ADDITION** calls to setup and configure such GPIOs must be made |
|
from contexts which may sleep, since they may need to access the GPIO |
|
controller chip too (These setup calls are usually made from board |
|
setup or driver probe/teardown code, so this is an easy constraint.):: |
|
|
|
gpio_direction_input() |
|
gpio_direction_output() |
|
gpio_request() |
|
|
|
## gpio_request_one() |
|
## gpio_request_array() |
|
## gpio_free_array() |
|
|
|
gpio_free() |
|
gpio_set_debounce() |
|
|
|
|
|
|
|
Claiming and Releasing GPIOs |
|
---------------------------- |
|
To help catch system configuration errors, two calls are defined:: |
|
|
|
/* request GPIO, returning 0 or negative errno. |
|
* non-null labels may be useful for diagnostics. |
|
*/ |
|
int gpio_request(unsigned gpio, const char *label); |
|
|
|
/* release previously-claimed GPIO */ |
|
void gpio_free(unsigned gpio); |
|
|
|
Passing invalid GPIO numbers to gpio_request() will fail, as will requesting |
|
GPIOs that have already been claimed with that call. The return value of |
|
gpio_request() must be checked. You should normally issue these calls from |
|
a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs |
|
before tasking is enabled, as part of early board setup. |
|
|
|
These calls serve two basic purposes. One is marking the signals which |
|
are actually in use as GPIOs, for better diagnostics; systems may have |
|
several hundred potential GPIOs, but often only a dozen are used on any |
|
given board. Another is to catch conflicts, identifying errors when |
|
(a) two or more drivers wrongly think they have exclusive use of that |
|
signal, or (b) something wrongly believes it's safe to remove drivers |
|
needed to manage a signal that's in active use. That is, requesting a |
|
GPIO can serve as a kind of lock. |
|
|
|
Some platforms may also use knowledge about what GPIOs are active for |
|
power management, such as by powering down unused chip sectors and, more |
|
easily, gating off unused clocks. |
|
|
|
For GPIOs that use pins known to the pinctrl subsystem, that subsystem should |
|
be informed of their use; a gpiolib driver's .request() operation may call |
|
pinctrl_gpio_request(), and a gpiolib driver's .free() operation may call |
|
pinctrl_gpio_free(). The pinctrl subsystem allows a pinctrl_gpio_request() |
|
to succeed concurrently with a pin or pingroup being "owned" by a device for |
|
pin multiplexing. |
|
|
|
Any programming of pin multiplexing hardware that is needed to route the |
|
GPIO signal to the appropriate pin should occur within a GPIO driver's |
|
.direction_input() or .direction_output() operations, and occur after any |
|
setup of an output GPIO's value. This allows a glitch-free migration from a |
|
pin's special function to GPIO. This is sometimes required when using a GPIO |
|
to implement a workaround on signals typically driven by a non-GPIO HW block. |
|
|
|
Some platforms allow some or all GPIO signals to be routed to different pins. |
|
Similarly, other aspects of the GPIO or pin may need to be configured, such as |
|
pullup/pulldown. Platform software should arrange that any such details are |
|
configured prior to gpio_request() being called for those GPIOs, e.g. using |
|
the pinctrl subsystem's mapping table, so that GPIO users need not be aware |
|
of these details. |
|
|
|
Also note that it's your responsibility to have stopped using a GPIO |
|
before you free it. |
|
|
|
Considering in most cases GPIOs are actually configured right after they |
|
are claimed, three additional calls are defined:: |
|
|
|
/* request a single GPIO, with initial configuration specified by |
|
* 'flags', identical to gpio_request() wrt other arguments and |
|
* return value |
|
*/ |
|
int gpio_request_one(unsigned gpio, unsigned long flags, const char *label); |
|
|
|
/* request multiple GPIOs in a single call |
|
*/ |
|
int gpio_request_array(struct gpio *array, size_t num); |
|
|
|
/* release multiple GPIOs in a single call |
|
*/ |
|
void gpio_free_array(struct gpio *array, size_t num); |
|
|
|
where 'flags' is currently defined to specify the following properties: |
|
|
|
* GPIOF_DIR_IN - to configure direction as input |
|
* GPIOF_DIR_OUT - to configure direction as output |
|
|
|
* GPIOF_INIT_LOW - as output, set initial level to LOW |
|
* GPIOF_INIT_HIGH - as output, set initial level to HIGH |
|
* GPIOF_OPEN_DRAIN - gpio pin is open drain type. |
|
* GPIOF_OPEN_SOURCE - gpio pin is open source type. |
|
|
|
* GPIOF_EXPORT_DIR_FIXED - export gpio to sysfs, keep direction |
|
* GPIOF_EXPORT_DIR_CHANGEABLE - also export, allow changing direction |
|
|
|
since GPIOF_INIT_* are only valid when configured as output, so group valid |
|
combinations as: |
|
|
|
* GPIOF_IN - configure as input |
|
* GPIOF_OUT_INIT_LOW - configured as output, initial level LOW |
|
* GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH |
|
|
|
When setting the flag as GPIOF_OPEN_DRAIN then it will assume that pins is |
|
open drain type. Such pins will not be driven to 1 in output mode. It is |
|
require to connect pull-up on such pins. By enabling this flag, gpio lib will |
|
make the direction to input when it is asked to set value of 1 in output mode |
|
to make the pin HIGH. The pin is make to LOW by driving value 0 in output mode. |
|
|
|
When setting the flag as GPIOF_OPEN_SOURCE then it will assume that pins is |
|
open source type. Such pins will not be driven to 0 in output mode. It is |
|
require to connect pull-down on such pin. By enabling this flag, gpio lib will |
|
make the direction to input when it is asked to set value of 0 in output mode |
|
to make the pin LOW. The pin is make to HIGH by driving value 1 in output mode. |
|
|
|
In the future, these flags can be extended to support more properties. |
|
|
|
Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is |
|
introduced to encapsulate all three fields as:: |
|
|
|
struct gpio { |
|
unsigned gpio; |
|
unsigned long flags; |
|
const char *label; |
|
}; |
|
|
|
A typical example of usage:: |
|
|
|
static struct gpio leds_gpios[] = { |
|
{ 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */ |
|
{ 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */ |
|
{ 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */ |
|
{ 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */ |
|
{ ... }, |
|
}; |
|
|
|
err = gpio_request_one(31, GPIOF_IN, "Reset Button"); |
|
if (err) |
|
... |
|
|
|
err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios)); |
|
if (err) |
|
... |
|
|
|
gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios)); |
|
|
|
|
|
GPIOs mapped to IRQs |
|
-------------------- |
|
GPIO numbers are unsigned integers; so are IRQ numbers. These make up |
|
two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can |
|
map between them using calls like:: |
|
|
|
/* map GPIO numbers to IRQ numbers */ |
|
int gpio_to_irq(unsigned gpio); |
|
|
|
/* map IRQ numbers to GPIO numbers (avoid using this) */ |
|
int irq_to_gpio(unsigned irq); |
|
|
|
Those return either the corresponding number in the other namespace, or |
|
else a negative errno code if the mapping can't be done. (For example, |
|
some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO |
|
number that wasn't set up as an input using gpio_direction_input(), or |
|
to use an IRQ number that didn't originally come from gpio_to_irq(). |
|
|
|
These two mapping calls are expected to cost on the order of a single |
|
addition or subtraction. They're not allowed to sleep. |
|
|
|
Non-error values returned from gpio_to_irq() can be passed to request_irq() |
|
or free_irq(). They will often be stored into IRQ resources for platform |
|
devices, by the board-specific initialization code. Note that IRQ trigger |
|
options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are |
|
system wakeup capabilities. |
|
|
|
Non-error values returned from irq_to_gpio() would most commonly be used |
|
with gpio_get_value(), for example to initialize or update driver state |
|
when the IRQ is edge-triggered. Note that some platforms don't support |
|
this reverse mapping, so you should avoid using it. |
|
|
|
|
|
Emulating Open Drain Signals |
|
---------------------------- |
|
Sometimes shared signals need to use "open drain" signaling, where only the |
|
low signal level is actually driven. (That term applies to CMOS transistors; |
|
"open collector" is used for TTL.) A pullup resistor causes the high signal |
|
level. This is sometimes called a "wire-AND"; or more practically, from the |
|
negative logic (low=true) perspective this is a "wire-OR". |
|
|
|
One common example of an open drain signal is a shared active-low IRQ line. |
|
Also, bidirectional data bus signals sometimes use open drain signals. |
|
|
|
Some GPIO controllers directly support open drain outputs; many don't. When |
|
you need open drain signaling but your hardware doesn't directly support it, |
|
there's a common idiom you can use to emulate it with any GPIO pin that can |
|
be used as either an input or an output: |
|
|
|
LOW: gpio_direction_output(gpio, 0) ... this drives the signal |
|
and overrides the pullup. |
|
|
|
HIGH: gpio_direction_input(gpio) ... this turns off the output, |
|
so the pullup (or some other device) controls the signal. |
|
|
|
If you are "driving" the signal high but gpio_get_value(gpio) reports a low |
|
value (after the appropriate rise time passes), you know some other component |
|
is driving the shared signal low. That's not necessarily an error. As one |
|
common example, that's how I2C clocks are stretched: a slave that needs a |
|
slower clock delays the rising edge of SCK, and the I2C master adjusts its |
|
signaling rate accordingly. |
|
|
|
|
|
GPIO controllers and the pinctrl subsystem |
|
------------------------------------------ |
|
|
|
A GPIO controller on a SOC might be tightly coupled with the pinctrl |
|
subsystem, in the sense that the pins can be used by other functions |
|
together with an optional gpio feature. We have already covered the |
|
case where e.g. a GPIO controller need to reserve a pin or set the |
|
direction of a pin by calling any of:: |
|
|
|
pinctrl_gpio_request() |
|
pinctrl_gpio_free() |
|
pinctrl_gpio_direction_input() |
|
pinctrl_gpio_direction_output() |
|
|
|
But how does the pin control subsystem cross-correlate the GPIO |
|
numbers (which are a global business) to a certain pin on a certain |
|
pin controller? |
|
|
|
This is done by registering "ranges" of pins, which are essentially |
|
cross-reference tables. These are described in |
|
Documentation/driver-api/pin-control.rst |
|
|
|
While the pin allocation is totally managed by the pinctrl subsystem, |
|
gpio (under gpiolib) is still maintained by gpio drivers. It may happen |
|
that different pin ranges in a SoC is managed by different gpio drivers. |
|
|
|
This makes it logical to let gpio drivers announce their pin ranges to |
|
the pin ctrl subsystem before it will call 'pinctrl_gpio_request' in order |
|
to request the corresponding pin to be prepared by the pinctrl subsystem |
|
before any gpio usage. |
|
|
|
For this, the gpio controller can register its pin range with pinctrl |
|
subsystem. There are two ways of doing it currently: with or without DT. |
|
|
|
For with DT support refer to Documentation/devicetree/bindings/gpio/gpio.txt. |
|
|
|
For non-DT support, user can call gpiochip_add_pin_range() with appropriate |
|
parameters to register a range of gpio pins with a pinctrl driver. For this |
|
exact name string of pinctrl device has to be passed as one of the |
|
argument to this routine. |
|
|
|
|
|
What do these conventions omit? |
|
=============================== |
|
One of the biggest things these conventions omit is pin multiplexing, since |
|
this is highly chip-specific and nonportable. One platform might not need |
|
explicit multiplexing; another might have just two options for use of any |
|
given pin; another might have eight options per pin; another might be able |
|
to route a given GPIO to any one of several pins. (Yes, those examples all |
|
come from systems that run Linux today.) |
|
|
|
Related to multiplexing is configuration and enabling of the pullups or |
|
pulldowns integrated on some platforms. Not all platforms support them, |
|
or support them in the same way; and any given board might use external |
|
pullups (or pulldowns) so that the on-chip ones should not be used. |
|
(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.) |
|
Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a |
|
platform-specific issue, as are models like (not) having a one-to-one |
|
correspondence between configurable pins and GPIOs. |
|
|
|
There are other system-specific mechanisms that are not specified here, |
|
like the aforementioned options for input de-glitching and wire-OR output. |
|
Hardware may support reading or writing GPIOs in gangs, but that's usually |
|
configuration dependent: for GPIOs sharing the same bank. (GPIOs are |
|
commonly grouped in banks of 16 or 32, with a given SOC having several such |
|
banks.) Some systems can trigger IRQs from output GPIOs, or read values |
|
from pins not managed as GPIOs. Code relying on such mechanisms will |
|
necessarily be nonportable. |
|
|
|
Dynamic definition of GPIOs is not currently standard; for example, as |
|
a side effect of configuring an add-on board with some GPIO expanders. |
|
|
|
|
|
GPIO implementor's framework (OPTIONAL) |
|
======================================= |
|
As noted earlier, there is an optional implementation framework making it |
|
easier for platforms to support different kinds of GPIO controller using |
|
the same programming interface. This framework is called "gpiolib". |
|
|
|
As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file |
|
will be found there. That will list all the controllers registered through |
|
this framework, and the state of the GPIOs currently in use. |
|
|
|
|
|
Controller Drivers: gpio_chip |
|
----------------------------- |
|
In this framework each GPIO controller is packaged as a "struct gpio_chip" |
|
with information common to each controller of that type: |
|
|
|
- methods to establish GPIO direction |
|
- methods used to access GPIO values |
|
- flag saying whether calls to its methods may sleep |
|
- optional debugfs dump method (showing extra state like pullup config) |
|
- label for diagnostics |
|
|
|
There is also per-instance data, which may come from device.platform_data: |
|
the number of its first GPIO, and how many GPIOs it exposes. |
|
|
|
The code implementing a gpio_chip should support multiple instances of the |
|
controller, possibly using the driver model. That code will configure each |
|
gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be |
|
rare; use gpiochip_remove() when it is unavoidable. |
|
|
|
Most often a gpio_chip is part of an instance-specific structure with state |
|
not exposed by the GPIO interfaces, such as addressing, power management, |
|
and more. Chips such as codecs will have complex non-GPIO state. |
|
|
|
Any debugfs dump method should normally ignore signals which haven't been |
|
requested as GPIOs. They can use gpiochip_is_requested(), which returns |
|
either NULL or the label associated with that GPIO when it was requested. |
|
|
|
|
|
Platform Support |
|
---------------- |
|
To force-enable this framework, a platform's Kconfig will "select" GPIOLIB, |
|
else it is up to the user to configure support for GPIO. |
|
|
|
It may also provide a custom value for ARCH_NR_GPIOS, so that it better |
|
reflects the number of GPIOs in actual use on that platform, without |
|
wasting static table space. (It should count both built-in/SoC GPIOs and |
|
also ones on GPIO expanders. |
|
|
|
If neither of these options are selected, the platform does not support |
|
GPIOs through GPIO-lib and the code cannot be enabled by the user. |
|
|
|
Trivial implementations of those functions can directly use framework |
|
code, which always dispatches through the gpio_chip:: |
|
|
|
#define gpio_get_value __gpio_get_value |
|
#define gpio_set_value __gpio_set_value |
|
#define gpio_cansleep __gpio_cansleep |
|
|
|
Fancier implementations could instead define those as inline functions with |
|
logic optimizing access to specific SOC-based GPIOs. For example, if the |
|
referenced GPIO is the constant "12", getting or setting its value could |
|
cost as little as two or three instructions, never sleeping. When such an |
|
optimization is not possible those calls must delegate to the framework |
|
code, costing at least a few dozen instructions. For bitbanged I/O, such |
|
instruction savings can be significant. |
|
|
|
For SOCs, platform-specific code defines and registers gpio_chip instances |
|
for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to |
|
match chip vendor documentation, and directly match board schematics. They |
|
may well start at zero and go up to a platform-specific limit. Such GPIOs |
|
are normally integrated into platform initialization to make them always be |
|
available, from arch_initcall() or earlier; they can often serve as IRQs. |
|
|
|
|
|
Board Support |
|
------------- |
|
For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi |
|
function devices, FPGAs or CPLDs -- most often board-specific code handles |
|
registering controller devices and ensures that their drivers know what GPIO |
|
numbers to use with gpiochip_add(). Their numbers often start right after |
|
platform-specific GPIOs. |
|
|
|
For example, board setup code could create structures identifying the range |
|
of GPIOs that chip will expose, and passes them to each GPIO expander chip |
|
using platform_data. Then the chip driver's probe() routine could pass that |
|
data to gpiochip_add(). |
|
|
|
Initialization order can be important. For example, when a device relies on |
|
an I2C-based GPIO, its probe() routine should only be called after that GPIO |
|
becomes available. That may mean the device should not be registered until |
|
calls for that GPIO can work. One way to address such dependencies is for |
|
such gpio_chip controllers to provide setup() and teardown() callbacks to |
|
board specific code; those board specific callbacks would register devices |
|
once all the necessary resources are available, and remove them later when |
|
the GPIO controller device becomes unavailable. |
|
|
|
|
|
Sysfs Interface for Userspace (OPTIONAL) |
|
======================================== |
|
Platforms which use the "gpiolib" implementors framework may choose to |
|
configure a sysfs user interface to GPIOs. This is different from the |
|
debugfs interface, since it provides control over GPIO direction and |
|
value instead of just showing a gpio state summary. Plus, it could be |
|
present on production systems without debugging support. |
|
|
|
Given appropriate hardware documentation for the system, userspace could |
|
know for example that GPIO #23 controls the write protect line used to |
|
protect boot loader segments in flash memory. System upgrade procedures |
|
may need to temporarily remove that protection, first importing a GPIO, |
|
then changing its output state, then updating the code before re-enabling |
|
the write protection. In normal use, GPIO #23 would never be touched, |
|
and the kernel would have no need to know about it. |
|
|
|
Again depending on appropriate hardware documentation, on some systems |
|
userspace GPIO can be used to determine system configuration data that |
|
standard kernels won't know about. And for some tasks, simple userspace |
|
GPIO drivers could be all that the system really needs. |
|
|
|
Note that standard kernel drivers exist for common "LEDs and Buttons" |
|
GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those |
|
instead of talking directly to the GPIOs; they integrate with kernel |
|
frameworks better than your userspace code could. |
|
|
|
|
|
Paths in Sysfs |
|
-------------- |
|
There are three kinds of entry in /sys/class/gpio: |
|
|
|
- Control interfaces used to get userspace control over GPIOs; |
|
|
|
- GPIOs themselves; and |
|
|
|
- GPIO controllers ("gpio_chip" instances). |
|
|
|
That's in addition to standard files including the "device" symlink. |
|
|
|
The control interfaces are write-only: |
|
|
|
/sys/class/gpio/ |
|
|
|
"export" ... Userspace may ask the kernel to export control of |
|
a GPIO to userspace by writing its number to this file. |
|
|
|
Example: "echo 19 > export" will create a "gpio19" node |
|
for GPIO #19, if that's not requested by kernel code. |
|
|
|
"unexport" ... Reverses the effect of exporting to userspace. |
|
|
|
Example: "echo 19 > unexport" will remove a "gpio19" |
|
node exported using the "export" file. |
|
|
|
GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) |
|
and have the following read/write attributes: |
|
|
|
/sys/class/gpio/gpioN/ |
|
|
|
"direction" ... reads as either "in" or "out". This value may |
|
normally be written. Writing as "out" defaults to |
|
initializing the value as low. To ensure glitch free |
|
operation, values "low" and "high" may be written to |
|
configure the GPIO as an output with that initial value. |
|
|
|
Note that this attribute *will not exist* if the kernel |
|
doesn't support changing the direction of a GPIO, or |
|
it was exported by kernel code that didn't explicitly |
|
allow userspace to reconfigure this GPIO's direction. |
|
|
|
"value" ... reads as either 0 (low) or 1 (high). If the GPIO |
|
is configured as an output, this value may be written; |
|
any nonzero value is treated as high. |
|
|
|
If the pin can be configured as interrupt-generating interrupt |
|
and if it has been configured to generate interrupts (see the |
|
description of "edge"), you can poll(2) on that file and |
|
poll(2) will return whenever the interrupt was triggered. If |
|
you use poll(2), set the events POLLPRI. If you use select(2), |
|
set the file descriptor in exceptfds. After poll(2) returns, |
|
either lseek(2) to the beginning of the sysfs file and read the |
|
new value or close the file and re-open it to read the value. |
|
|
|
"edge" ... reads as either "none", "rising", "falling", or |
|
"both". Write these strings to select the signal edge(s) |
|
that will make poll(2) on the "value" file return. |
|
|
|
This file exists only if the pin can be configured as an |
|
interrupt generating input pin. |
|
|
|
"active_low" ... reads as either 0 (false) or 1 (true). Write |
|
any nonzero value to invert the value attribute both |
|
for reading and writing. Existing and subsequent |
|
poll(2) support configuration via the edge attribute |
|
for "rising" and "falling" edges will follow this |
|
setting. |
|
|
|
GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the |
|
controller implementing GPIOs starting at #42) and have the following |
|
read-only attributes: |
|
|
|
/sys/class/gpio/gpiochipN/ |
|
|
|
"base" ... same as N, the first GPIO managed by this chip |
|
|
|
"label" ... provided for diagnostics (not always unique) |
|
|
|
"ngpio" ... how many GPIOs this manges (N to N + ngpio - 1) |
|
|
|
Board documentation should in most cases cover what GPIOs are used for |
|
what purposes. However, those numbers are not always stable; GPIOs on |
|
a daughtercard might be different depending on the base board being used, |
|
or other cards in the stack. In such cases, you may need to use the |
|
gpiochip nodes (possibly in conjunction with schematics) to determine |
|
the correct GPIO number to use for a given signal. |
|
|
|
|
|
Exporting from Kernel code |
|
-------------------------- |
|
Kernel code can explicitly manage exports of GPIOs which have already been |
|
requested using gpio_request():: |
|
|
|
/* export the GPIO to userspace */ |
|
int gpio_export(unsigned gpio, bool direction_may_change); |
|
|
|
/* reverse gpio_export() */ |
|
void gpio_unexport(); |
|
|
|
/* create a sysfs link to an exported GPIO node */ |
|
int gpio_export_link(struct device *dev, const char *name, |
|
unsigned gpio) |
|
|
|
After a kernel driver requests a GPIO, it may only be made available in |
|
the sysfs interface by gpio_export(). The driver can control whether the |
|
signal direction may change. This helps drivers prevent userspace code |
|
from accidentally clobbering important system state. |
|
|
|
This explicit exporting can help with debugging (by making some kinds |
|
of experiments easier), or can provide an always-there interface that's |
|
suitable for documenting as part of a board support package. |
|
|
|
After the GPIO has been exported, gpio_export_link() allows creating |
|
symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can |
|
use this to provide the interface under their own device in sysfs with |
|
a descriptive name. |
|
|
|
|
|
API Reference |
|
============= |
|
|
|
The functions listed in this section are deprecated. The GPIO descriptor based |
|
API should be used in new code. |
|
|
|
.. kernel-doc:: drivers/gpio/gpiolib-legacy.c |
|
:export:
|
|
|