netdev - Re: [PATCH 1/2] platform: make platform_get_irq

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Date:   Wed, 12 Jan 2022 15:41:38 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Uwe Kleine-König 
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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional

On Wed, Jan 12, 2022 at 2:55 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> Hi Andrew,
>
> On Wed, Jan 12, 2022 at 2:38 PM Andrew Lunn <andrew@...n.ch> wrote:
> > > If an optional IRQ is not present, drivers either just ignore it (e.g.
> > > for devices that can have multiple interrupts or a single muxed IRQ),
> > > or they have to resort to polling. For the latter, fall-back handling
> > > is needed elsewhere in the driver.
> > > 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.
> >
> > The *_optional() functions return an error code if there has been a
> > real error which should be reported up the call stack. This excludes
> > whatever error code indicates the requested resource does not exist,
> > which can be -ENODEV etc. If the device does not exist, a magic cookie
> > is returned which appears to be a valid resources but in fact is
> > not. So the users of these functions just need to check for an error
> > code, and fail the probe if present.
>
> Agreed.
>
> Note that in most (all?) other cases, the return type is a pointer
> (e.g. to struct clk), and NULL is the magic cookie.
>
> > You seems to be suggesting in binary return value: non-zero
> > (available) or zero (not available)
>
> Only in case of success. In case of a real failure, an error code
> must be returned.
>
> > This discards the error code when something goes wrong. That is useful
> > information to have, so we should not be discarding it.
>
> No, the error code must be retained in case of failure.
>
> > IRQ don't currently have a magic cookie value. One option would be to
> > add such a magic cookie to the subsystem. Otherwise, since 0 is
> > invalid, return 0 to indicate the IRQ does not exist.
>
> Exactly. And using 0 means the similar code can be used as for other
> subsystems, where NULL would be returned.
>
> The only remaining difference is the "dummy cookie can be passed
> to other functions" behavior.  Which is IMHO a valid difference,
> as unlike with e.g. clk_prepare_enable(), you do pass extra data to
> request_irq(), and sometimes you do need to handle the absence of
> the interrupt using e.g. polling.
>
> > The request for a script checking this then makes sense. However, i
> > don't know how well coccinelle/sparse can track values across function
> > calls. They probably can check for:
> >
> >    ret = irq_get_optional()
> >    if (ret < 0)
> >       return ret;
> >
> > A missing if < 0 statement somewhere later is very likely to be an
> > error. A comparison of <= 0 is also likely to be an error. A check for
> > > 0 before calling any other IRQ functions would be good. I'm
> > surprised such a check does not already existing in the IRQ API, but
> > there are probably historical reasons for that.
>
> There are still a few platforms where IRQ 0 does exist.

Not just a few even.  This happens on a reasonably recent x86 PC:

rafael@...tch:~/work/linux-pm> head -2 /proc/interrupts
           CPU0       CPU1       CPU2       CPU3       CPU4       CPU5
  0:         10          0          0          0          0          0
 IR-IO-APIC    2-edge
timer