RE: [RFC PATCH] Rework gpio cansleep (was Re: gpiolib andsleepinggpios)

From: Jani Nikula
Date: Thu Jun 24 2010 - 04:30:43 EST

On Thu, 24 Jun 2010, ext Jon Povey wrote:

Ryan Mallon wrote:

If we strip my patch back to just introducing gpio_request_cansleep, which would be used in any driver where all of the calls are gpio_(set/get)_cansleep, and make gpio_request only allow non-sleeping gpios then incorrect use of gpios would be caught at request time and returned to the caller as an error.

It seems like a good idea to catch these at request time. There is support in the API for this already (gpio_cansleep), but driver writers are not steered towards checking and thinking in these ways by the current API or documentation. Perhaps a documentation change with some cut and paste boilerplate would be enough, but I think some API enforcement/encouragement would be helpful.

I think this agrees with you, Ryan:

gpio_request_cansleep would be the same as current gpio_request
gpio_request changes to error if this is a sleepy gpio.

Imagine a situation where GPIOs are being assigned and passed around between drivers in some dynamic way, or some way unpredictable to the driver writer. In development only non-sleeping GPIOs have been seen and everything is fine. One day someone feeds it a GPIO on an I2C expander and the driver crashes. If gpio_request had this built-in check the driver could gracefuly fail to load instead with an appropriate error message.

Hi -

There's no need to imagine such situations. It's not at all uncommon to request GPIOs in board files, and pass the already requested GPIO numbers to drivers. Replacing gpio_request() with gpio_request_cansleep() (or gpio_request_atomic() as suggested in another mail) in the board files does *nothing* to help such drivers use the correct gpio get/set calls. The driver will need to know what it's doing, in what contexts. Some drivers might not work with "sleepy" GPIOs, and that's fine - they can check using gpio_cansleep() and fail gracefully.

I'd just improve the documentation of the various calls and have gpiolib.c emit more warnings about incorrect use.

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