Subject: [RFC PATCH] Rework gpio cansleep (was Re: gpiolib and sleeping gpios)

From: [email-masked] (Ryan Mallon)
Date: Thu, 24 Jun 2010 11:06:23 +1200

On 06/24/2010 10:53 AM, Jamie Lokier wrote:
> Ryan Mallon wrote:
>> On 06/23/2010 04:37 PM, David Brownell wrote:
>> I'm not. Some gpios, such as those on io expanders, may sleep in their
>> implementations of the gpio_(set/get) functions.
> 
> I'm having a hard time figuring out where some GPIOs I'm using fit
> into this picture.
> 
> I have some hardware that is currently using a 2.4.26 kernel, but I
> look from time to time at forward-porting all the drivers to 2.6.recent.
> 
> It has an I2C driven GPIO expander, with a watchdog reset chip hanging
> off the expander.
> 
> The watchdog is kept alive off the back end of a timer BH, which means
> the I2C GPIO routines are written to be safe in BH context (which
> isn't sleepable), but they can't be used in IRQ context because the
> necessary spin_lock_irqsave() would turn off interrupts for too long
> for other subsystems to function properly.

Do the implementations of the get/set calls for the io expander gpios
sleep at all?

> How should I flag those GPIO routines in your scheme?  They're safe to
> use in some non-sleeping contexts, but not safe in irq context.

The idea in my proposal is to use gpio_request in a driver if the
requested gpio can never sleep (ie because of the context it is used
in), and gpio_request_cansleep if the gpio is never used from non-sleep
safe context in a driver. I suggested stripping back the patch to just
add the gpio_request_cansleep function.

In the current code, if a driver ever calls gpio_(set/get)_value on a
gpio then you cannot pass a sleeping gpio to that driver. The request
will succeed, but you will get warnings with the get/get calls are made.
My idea is basically to move the denotation of whether a gpio will be
used in non-sleep safe context to the gpio request.

~Ryan

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