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authorStephen Warren <swarren@nvidia.com>2012-03-06 04:22:14 +0400
committerGrant Likely <grant.likely@secretlab.ca>2012-03-12 21:27:07 +0400
commit0dc665d426691fd75fe9b6b16295ad0c02677d21 (patch)
treee45f9a48aeab4d1dd3ad20ef175a9d4cedda4ba2 /Documentation/gpio.txt
parent46158aad96b0a90b52fd345f89951a50b3d1a81f (diff)
downloadlinux-0dc665d426691fd75fe9b6b16295ad0c02677d21.tar.xz
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>
Diffstat (limited to 'Documentation/gpio.txt')
-rw-r--r--Documentation/gpio.txt23
1 files changed, 20 insertions, 3 deletions
diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
index f783e7fed764..620a07844e8c 100644
--- a/Documentation/gpio.txt
+++ b/Documentation/gpio.txt
@@ -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.