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SubjectRe: [PATCH 1/2] platform: make platform_get_irq_optional() optional
Date
On 1/14/22 12:25 PM, Uwe Kleine-König wrote:

>>>>> To me it sounds much more logical for the driver to check if an
>>>>> optional irq is non-zero (available) or zero (not available), than to
>>>>> sprinkle around checks for -ENXIO. In addition, you have to remember
>>>>> that this one returns -ENXIO, while other APIs use -ENOENT or -ENOSYS
>>>>> (or some other error code) to indicate absence. I thought not having
>>>>> to care about the actual error code was the main reason behind the
>>>>> introduction of the *_optional() APIs.
>>>
>>>> No, the main benefit of gpiod_get_optional() (and clk_get_optional()) is
>>>> that you can handle an absent GPIO (or clk) as if it were available.
>>
>> Hm, I've just looked at these and must note that they match 1:1 with
>> platform_get_irq_optional(). Unfortunately, we can't however behave the
>> same way in request_irq() -- because it has to support IRQ0 for the sake
>> of i8253 drivers in arch/...
>
> Let me reformulate your statement to the IMHO equivalent:
>
> If you set aside the differences between
> platform_get_irq_optional() and gpiod_get_optional(),

Sorry, I should make it clear this is actually the diff between a would-be
platform_get_irq_optional() after my patch, not the current code...

> platform_get_irq_optional() is like gpiod_get_optional().
>
> The introduction of gpiod_get_optional() made it possible to simplify
> the following code:
>
> reset_gpio = gpiod_get(...)
> if IS_ERR(reset_gpio):
> error = PTR_ERR(reset_gpio)
> if error != -ENDEV:

ENODEV?

> return error
> else:
> gpiod_set_direction(reset_gpiod, INACTIVE)
>
> to
>
> reset_gpio = gpiod_get_optional(....)
> if IS_ERR(reset_gpio):
> return reset_gpio
> gpiod_set_direction(reset_gpiod, INACTIVE)
>
> and I never need to actually know if the reset_gpio actually exists.
> Either the line is put into its inactive state, or it doesn't exist and
> then gpiod_set_direction is a noop. For a regulator or a clk this works
> in a similar way.
>
> However for an interupt this cannot work. You will always have to check
> if the irq is actually there or not because if it's not you cannot just
> ignore that. So there is no benefit of an optional irq.
>
> Leaving error message reporting aside, the introduction of
> platform_get_irq_optional() allows to change
>
> irq = platform_get_irq(...);
> if (irq < 0 && irq != -ENXIO) {
> return irq;
> } else if (irq >= 0) {

Rather (irq > 0) actually, IRQ0 is considered invalid (but still returned).

> ... setup irq operation ...
> } else { /* irq == -ENXIO */
> ... setup polling ...
> }
>
> to
>
> irq = platform_get_irq_optional(...);
> if (irq < 0 && irq != -ENXIO) {
> return irq;
> } else if (irq >= 0) {
> ... setup irq operation ...
> } else { /* irq == -ENXIO */
> ... setup polling ...
> }
>
> which isn't a win. When changing the return value as you suggest, it can
> be changed instead to:
>
> irq = platform_get_irq_optional(...);
> if (irq < 0) {
> return irq;
> } else if (irq > 0) {
> ... setup irq operation ...
> } else { /* irq == 0 */
> ... setup polling ...
> }
>
> which is a tad nicer. If that is your goal however I ask you to also
> change the semantic of platform_get_irq() to return 0 on "not found".

Well, I'm not totally opposed to that... but would there be a considerable win?
Anyway, we should 1st stop returning 0 for "valid" IRQs -- this is done by my patch
the discussed patch series are atop of.

> Note the win is considerably less compared to gpiod_get_optional vs

If there's any at all... We'd basically have to touch /all/ platform_get_irq()
calls (and get an even larger CC list ;-)).

> gpiod_get however. And then it still lacks the semantic of a dummy irq
> which IMHO forfeits the right to call it ..._optional().

Not quite grasping it... Why e.g. clk_get() doesn't return 0 for a not found clock?

> Now I'm unwilling to continue the discussion unless there pops up a
> suggestion that results in a considerable part (say > 10%) of the
> drivers using platform_get_irq_optional not having to check if the
> return value corresponds to "not found".

Note that many actual drivers don't follow the pattern prescribed in the comment to
platform_get_irq_optional() and their check for an optional IRQ look like irq < 0
(and, after my patches, irq <= 0). Maybe we shouldn't even bother returning the error
codes and just return 0 for all kinds of misfortunes instead? :-)

> Best regards
> Uwe

MBR, Sergey

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