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Commit adbf0299 authored by Linus Walleij's avatar Linus Walleij
Browse files

gpio: Documentation update 



Update a slew of documentation files with the latest changes in the
API/ABI. Again stress that sysfs is deprecated. Add all new flags and
clean up and move some text.

Signed-off-by: default avatarLinus Walleij <linus.walleij@linaro.org>
parent 40a3c9db

Documentation/gpio/board.txt

+10 −4
Original line number Diff line number Diff line
@@ -2,6 +2,7 @@ GPIO Mappings 
=============



This document explains how GPIOs can be assigned to given devices and functions.



Note that it only applies to the new descriptor-based interface. For a

description of the deprecated integer-based GPIO interface please refer to

gpio-legacy.txt (actually, there is no real mapping possible with the old
@@ -49,7 +50,7 @@ This property will make GPIOs 15, 16 and 17 available to the driver under the 


	power = gpiod_get(dev, "power", GPIOD_OUT_HIGH);



The led GPIOs will be active-high, while the power GPIO will be active-low (i.e.

The led GPIOs will be active high, while the power GPIO will be active low (i.e.

gpiod_is_active_low(power) will be true).



The second parameter of the gpiod_get() functions, the con_id string, has to be
@@ -122,9 +123,14 @@ where 
	can be NULL, in which case it will match any function.

  - idx is the index of the GPIO within the function.

  - flags is defined to specify the following properties:

	* GPIOF_ACTIVE_LOW	- to configure the GPIO as active-low

	* GPIOF_OPEN_DRAIN	- GPIO pin is open drain type.

	* GPIOF_OPEN_SOURCE	- GPIO pin is open source type.

	* GPIO_ACTIVE_HIGH	- GPIO line is active high

	* GPIO_ACTIVE_LOW	- GPIO line is active low

	* GPIO_OPEN_DRAIN	- GPIO line ise set up as open drain

	* GPIO_OPEN_SOURCE	- GPIO line is set up as open source

	* GPIO_PERSISTENT	- GPIO line is persistent during

				  suspend/resume and maintains its value

	* GPIO_TRANSITORY	- GPIO line is transitory and may loose its

				  electrical state during suspend/resume



In the future, these flags might be extended to support more properties.

Documentation/gpio/consumer.txt

+63 −42
Original line number Diff line number Diff line
@@ -66,6 +66,15 @@ for the GPIO. Values can be: 
* GPIOD_IN to initialize the GPIO as input.

* GPIOD_OUT_LOW to initialize the GPIO as output with a value of 0.

* GPIOD_OUT_HIGH to initialize the GPIO as output with a value of 1.

* GPIOD_OUT_LOW_OPEN_DRAIN same as GPIOD_OUT_LOW but also enforce the line

  to be electrically used with open drain.

* GPIOD_OUT_HIGH_OPEN_DRAIN same as GPIOD_OUT_HIGH but also enforce the line

  to be electrically used with open drain.



The two last flags are used for use cases where open drain is mandatory, such

as I2C: if the line is not already configured as open drain in the mappings

(see board.txt), then open drain will be enforced anyway and a warning will be

printed that the board configuration needs to be updated to match the use case.



Both functions return either a valid GPIO descriptor, or an error code checkable

with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned
@@ -240,59 +249,71 @@ that can't be accessed from hardIRQ handlers, these calls act the same as the 
spinlock-safe calls.





Active-low State and Raw GPIO Values

------------------------------------

Device drivers like to manage the logical state of a GPIO, i.e. the value their

device will actually receive, no matter what lies between it and the GPIO line.

In some cases, it might make sense to control the actual GPIO line value. The

following set of calls ignore the active-low property of a GPIO and work on the

raw line value:



	int gpiod_get_raw_value(const struct gpio_desc *desc)

	void gpiod_set_raw_value(struct gpio_desc *desc, int value)

	int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)

	void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)

	int gpiod_direction_output_raw(struct gpio_desc *desc, int value)



The active-low state of a GPIO can also be queried using the following call:



	int gpiod_is_active_low(const struct gpio_desc *desc)



Note that these functions should only be used with great moderation ; a driver

should not have to care about the physical line level.





The active-low property

-----------------------



As a driver should not have to care about the physical line level, all of the

The active low and open drain semantics

---------------------------------------

As a consumer should not have to care about the physical line level, all of the

gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with

the *logical* value. With this they take the active-low property into account.

This means that they check whether the GPIO is configured to be active-low,

the *logical* value. With this they take the active low property into account.

This means that they check whether the GPIO is configured to be active low,

and if so, they manipulate the passed value before the physical line level is

driven.



The same is applicable for open drain or open source output lines: those do not

actively drive their output high (open drain) or low (open source), they just

switch their output to a high impedance value. The consumer should not need to

care. (For details read about open drain in driver.txt.)



With this, all the gpiod_set_(array)_value_xxx() functions interpret the

parameter "value" as "active" ("1") or "inactive" ("0"). The physical line

parameter "value" as "asserted" ("1") or "de-asserted" ("0"). The physical line

level will be driven accordingly.



As an example, if the active-low property for a dedicated GPIO is set, and the

gpiod_set_(array)_value_xxx() passes "active" ("1"), the physical line level

As an example, if the active low property for a dedicated GPIO is set, and the

gpiod_set_(array)_value_xxx() passes "asserted" ("1"), the physical line level

will be driven low.



To summarize:



Function (example)               active-low property  physical line

Function (example)                 line property          physical line

gpiod_set_raw_value(desc, 0);      don't care             low

gpiod_set_raw_value(desc, 1);      don't care             high

gpiod_set_value(desc, 0);       default (active-high)     low

gpiod_set_value(desc, 1);       default (active-high)     high

gpiod_set_value(desc, 0);             active-low          high

gpiod_set_value(desc, 1);             active-low          low



Please note again that the set_raw/get_raw functions should be avoided as much

as possible, especially by drivers which should not care about the actual

physical line level and worry about the logical value instead.

gpiod_set_value(desc, 0);          default (active high)  low

gpiod_set_value(desc, 1);          default (active high)  high

gpiod_set_value(desc, 0);          active low             high

gpiod_set_value(desc, 1);          active low             low

gpiod_set_value(desc, 0);          default (active high)  low

gpiod_set_value(desc, 1);          default (active high)  high

gpiod_set_value(desc, 0);          open drain             low

gpiod_set_value(desc, 1);          open drain             high impedance

gpiod_set_value(desc, 0);          open source            high impedance

gpiod_set_value(desc, 1);          open source            high



It is possible to override these semantics using the *set_raw/'get_raw functions

but it should be avoided as much as possible, especially by system-agnostic drivers

which should not need to care about the actual physical line level and worry about

the logical value instead.





Accessing raw GPIO values

-------------------------

Consumers exist that need to manage the logical state of a GPIO line, i.e. the value

their device will actually receive, no matter what lies between it and the GPIO

line.



The following set of calls ignore the active-low or open drain property of a GPIO and

work on the raw line value:



	int gpiod_get_raw_value(const struct gpio_desc *desc)

	void gpiod_set_raw_value(struct gpio_desc *desc, int value)

	int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)

	void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)

	int gpiod_direction_output_raw(struct gpio_desc *desc, int value)



The active low state of a GPIO can also be queried using the following call:



	int gpiod_is_active_low(const struct gpio_desc *desc)



Note that these functions should only be used with great moderation; a driver

should not have to care about the physical line level or open drain semantics.





Access multiple GPIOs with a single function call

Documentation/gpio/driver.txt

+4 −0
Original line number Diff line number Diff line
@@ -88,6 +88,10 @@ ending up in the pin control back-end "behind" the GPIO controller, usually 
closer to the actual pins. This way the pin controller can manage the below

listed GPIO configurations.



If a pin controller back-end is used, the GPIO controller or hardware

description needs to provide "GPIO ranges" mapping the GPIO line offsets to pin

numbers on the pin controller so they can properly cross-reference each other.





GPIOs with debounce support

---------------------------

Documentation/gpio/sysfs.txt

+11 −0
Original line number Diff line number Diff line 
GPIO Sysfs Interface for Userspace

==================================



THIS ABI IS DEPRECATED, THE ABI DOCUMENTATION HAS BEEN MOVED TO

Documentation/ABI/obsolete/sysfs-gpio AND NEW USERSPACE CONSUMERS

ARE SUPPOSED TO USE THE CHARACTER DEVICE ABI. THIS OLD SYSFS ABI WILL

NOT BE DEVELOPED (NO NEW FEATURES), IT WILL JUST BE MAINTAINED.



Refer to the examples in tools/gpio/* for an introduction to the new

character device ABI. Also see the userspace header in

include/uapi/linux/gpio.h



The deprecated sysfs ABI

------------------------

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