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Documentation/gpio.txt: Explain expected pinctrl interaction 

Update gpio.txt based on recent discussions regarding interaction with the
pinctrl subsystem.

Previously, gpio_request() was described as explicitly not performing any
required mux setup operations etc.

Now, gpio_request() is explicitly as explicitly performing any required mux
setup operations where possible. In the case it isn't, platform code is
required to have set up any required muxing or other configuration prior to
gpio_request() being called, in order to maintain the same semantics.

This is achieved by gpiolib drivers calling e.g. pinctrl_request_gpio() in
their .request() operation.

Cc: Randy Dunlap <rdunlap@xenotime.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
This commit is contained in:
Stephen Warren 2012-03-05 17:22:14 -07:00 committed by Grant Likely
parent 46158aad96
commit 0dc665d426
1 changed files with 20 additions and 3 deletions
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23
Documentation/gpio.txt
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@ -271,9 +271,26 @@ 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.
Note that requesting a GPIO does NOT cause it to be configured in any
way; it just marks that GPIO as in use. Separate code must handle any
pin setup (e.g. controlling which pin the GPIO uses, pullup/pulldown).
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_request_gpio(), and a gpiolib driver's .free() operation may call
pinctrl_free_gpio(). The pinctrl subsystem allows a pinctrl_request_gpio()
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.