linux-kernel - Re: gpiolib and sleeping gpios

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Message-ID: <AANLkTinunR-mrXFh8uLV5YfeArC-_A1NJWz708n7r4pU@mail.gmail.com>
Date:	Tue, 29 Jun 2010 16:29:04 +0800
From:	CoffBeta <coffbeta@...il.com>
To:	Ryan Mallon <ryan@...ewatersys.com>
Cc:	David Brownell <david-b@...bell.net>,
	David Brownell <dbrownell@...rs.sourceforge.net>,
	gregkh@...e.de, linux kernel <linux-kernel@...r.kernel.org>,
	ext-jani.1.nikula@...ia.com,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: gpiolib and sleeping gpios

 non-sleeping

On Mon, Jun 21, 2010 at 05:31, Ryan Mallon <ryan@...ewatersys.com> wrote:
> On 06/19/2010 06:21 PM, David Brownell wrote:
>>
>>>
>>> The runtime warnings will only show instances where the
>>> non-sleeping
>>> versions where called instead of the sleeping versions.
>>
>> ... *AND* the GPIO requires the cansleep() version...
>>
>> Right; such calls are errors.  We issue
>> warnings since fault returns are inapplicable.
>
> A driver which only uses the non-sleeping versions, but _could_ use the
> cansleep variants (ie all calls to gpio_(set/get)_value are made from
> contexts where it is possible to sleep) is not so easy to spot. Passing
> a sleeping to gpio to such a driver will result in spurious warnings.
>
>>> There is no
>>> warning to say that we are calling the spinlock safe
>>> version, where it is possible to sleep.
>>
>> The call context isn't what controls whether
>> gpio_get_value() or gpio_get_value_cansleep()
>> is appropriate ... it's the GPIO itself, and
>> how its implementation works.
>
> No, a driver should not know anything about a gpio which is passed to
> it. If a driver is able to call the cansleep variants, then it should,
> and it will allow any gpio, sleeping or non-sleeping, to be used with
> that driver.
>
> If a driver uses a gpio in such a way that it cannot sleep, ie the
> gpio_(get/set)_value calls are made from spinlock context, then only
> gpios which do not sleep may be used with that driver.
>
> Thats why I think specifying whether the gpio is able to sleep when it
> is requested is a good idea. A driver which cannot use a sleeping gpio
>
>
>> "possible to sleep" is a GPIO attribute,
>> exposed by a predicate.  If spinlock-safe
>> calls are used on GPIOs with that attribute,
>>  a warning *IS* issued.
>
> Possible to sleep is also an attribute of how a driver _uses_ a gpio.
>
>>>
>>> The point I was trying to make is that there are lots of
>>> drivers which
>>> will not work with gpios on sleeping io expanders because
>>> they call the
>>> spinlock safe gpio calls.
>>
>> And they will trigger runtime warnings, and
>> thus eventually get fixed.  The way to do that
>> is to check if the GPIO needs the cansleep()
>> call
>
> Hmm, maybe this then for drivers which cannot accept sleeping gpios:
>
>  if (gpio_cansleep(some_gpio)) {
>          dev_err(&dev, "This driver only supports non-sleeping gpios");
>          return -EINVAL;
>  }
>
>  err = gpio_request(some_gpio, "some_gpio");
>
> I think ideally, gpio_request should specify this via a flags argument, ie:
>
>  #define GPIOF_NO_SLEEP        0x0
>  #define GPIOF_CANSLEEP        0x1
>
>  err = gpio_request(some_gpio, "some_gpio", GPIOF_NO_SLEEP);
>
> ~Ryan
>
> --
> Bluewater Systems Ltd - ARM Technology Solution Centre
>
> Ryan Mallon                     5 Amuri Park, 404 Barbadoes St
> ryan@...ewatersys.com           PO Box 13 889, Christchurch 8013
> http://www.bluewatersys.com     New Zealand
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