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Date: Wed, 12 Jan 2022 16:48:14 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Hans de Goede <hdegoede@...hat.com>
Cc: Sergey Shtylyov <s.shtylyov@....ru>,
"Rafael J. Wysocki" <rafael@...nel.org>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
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Subject: Re: [PATCH 1/2] platform: make platform_get_irq_optional() optional
On Wed, Jan 12, 2022 at 4:14 PM Hans de Goede <hdegoede@...hat.com> wrote:
>
> Hi,
>
> On 1/12/22 16:05, Sergey Shtylyov wrote:
> > On 1/12/22 5:41 PM, Rafael J. Wysocki 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.
> >>>>Hi,
> >>>> 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
> >
> > IIRC Linus has proclaimed that IRQ0 was valid for the i8253 driver (living in
> > arch/x86/); IRQ0 only was frowned upon when returned by platform_get_irq() and its
> > ilk.
> >
> > MBR, Sergey
>
> Right, platform_get_irq() has this:
>
> WARN(ret == 0, "0 is an invalid IRQ number\n");
>
> So given that platform_get_irq() returning 0 is not expected, it seems
> reasonable for platform_get_irq_optional() to use 0 as a special
> "no irq available" return value, matching the NULL returned by
> gpiod_get_optional().
Sounds reasonable to me.
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