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

[Ryan Mallon]

On 06/24/2010 12:04 PM, Jamie Lokier wrote:
> Ryan Mallon wrote:
>> 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?
> 
> No, because sleeping isn't allowed in BH context.  (Note that this is
> 2.4.26 code - things have changed a bit for 2.6, but the hardware is
> the same, and still needs the I2C watchdog to be driven from a BH-like
> context).
> 
>>> 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.
> 
> The reason I'm asking about my scenario is because the GPIO routines
> can't sleep and are used from a non-sleep safe context - but they are
> not safe to call in irq contexts.
> 
> So my watchdog driver would have to call gpio_request (not _cansleep)
> - that's fine.  But if I connected other GPIOs from the same GPIO
> driver (other lines on the same I/O expander chip) to another
> GPIO-using driver which happens to use them from irq context, then
> your changes won't detect the problem - the code will just break at
> runtime.
> 
> Of course if I did that, it would be my fault and my problem.  I get
> to keep both pieces etc.  But it's a scenario which your proposal
> would fail to catch at compile time, that's why I bring it up.

That's true. It wouldn't be caught in the current code either. I'm not
sure how you would go about sensibly writing code to handle that
situation; it's really a problem that needs to be caught during patch
review.

~Ryan

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