On Wed, Jan 12, 2022 at 4:14 PM Hans de Goede <hdegoede@xxxxxxxxxx> 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.
