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    SubjectRe: [PATCH 1/2] platform: make platform_get_irq_optional() optional
    On Wed, Jan 12, 2022 at 4:14 PM Hans de Goede <hdegoede@redhat.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@gratch:~/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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