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SubjectRe: gpiolib and sleeping gpios
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@bluewatersys.com PO Box 13 889, Christchurch 8013
http://www.bluewatersys.com New Zealand
Phone: +64 3 3779127 Freecall: Australia 1800 148 751
Fax: +64 3 3779135 USA 1800 261 2934


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