Date:   Fri, 14 Jan 2022 10:39:07 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Sergey Shtylyov <s.shtylyov@....ru>,
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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

Hi Uwe,

On Fri, Jan 14, 2022 at 10:26 AM Uwe Kleine-König
<u.kleine-koenig@...gutronix.de> wrote:
> On Thu, Jan 13, 2022 at 11:35:34PM +0300, Sergey Shtylyov wrote:
> > On 1/13/22 12:45 AM, Mark Brown 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(),
>         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:
>                         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) {
>                 ... 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".

Please don't make that change. If platform_get_irq() would return 0 on
"not found", all existing users have to be changed to:

        irq = platform_get_irq(...);
        if (irq < 0) {
                return irq;
        } else if (!irq) {
                return -ENOENT;
        } else {
                ... setup irq operation ...
        }

If the IRQ is not optional, there should be an error code when it is
not present. This keeps error handling simple.

The _optional() difference lies in the zero/NULL vs. error code in
case of not present.

> Note the win is considerably less compared to gpiod_get_optional vs
> gpiod_get however. And then it still lacks the semantic of a dummy irq
> which IMHO forfeits the right to call it ..._optional().
>
> 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".

Usually drivers do have to check if the interrupt was present or
not, because they may have to change the operation of the driver,
depending on interrupt-based or timer/polling-based processing.

Clocks, regulators, and resets are different, as their absence is
really a no-op.  The absence of an interrupt is not a no-op (except
for the separate interrupts vs. a single muxed interrupt case).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

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