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. >>> >>> 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
