Re: [PATCH] arm64: watchdog_hld: provide arm_pmu_irq_is_nmi stub

[Doug Anderson]

Hi,

On Mon, May 22, 2023 at 4:49 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote:
>
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> The newly added arch_perf_nmi_is_available() function fails to build
> when CONFIG_ARM_PMU is disabled:
>
> arch/arm64/kernel/watchdog_hld.c: In function 'arch_perf_nmi_is_available':
> arch/arm64/kernel/watchdog_hld.c:35:16: error: implicit declaration of function 'arm_pmu_irq_is_nmi' [-Werror=implicit-function-declaration]
>    35 |         return arm_pmu_irq_is_nmi();
>
> As it turns out, there is only one caller for that function anyway,
> in the same file as the __weak definition, and this can only be called
> if CONFIG_ARM_PMU is also enabled.
>
> I tried a number of variants, but everything ended up with more
> complexity from having both the __weak function and one or more
> added #ifdef. Keeping it in watchdog_perf.c is a small layering
> violation but otherwise the most robust.

Sorry for the breakage!

The intention here is that turning on CONFIG_HARDLOCKUP_DETECTOR_PERF
doesn't make any sense if the PMU is not enabled. In that sense, maybe
a better option would be to just fix this in Kconfig? What about going
into `arch/arm64/Kconfig` and instead of:

select HAVE_HARDLOCKUP_DETECTOR_PERF if PERF_EVENTS && HAVE_PERF_EVENTS_NMI

We do:

select HAVE_HARDLOCKUP_DETECTOR_PERF if PERF_EVENTS &&
HAVE_PERF_EVENTS_NMI && HW_PERF_EVENTS

It looks like HW_PERF_EVENTS is a synonym for ARM_PMU and that's the
same symbol used to include the needed definition.

Making that change seems to fix the compile error and has the added
benefit that enabling CONFIG_HARDLOCKUP_DETECTOR will automatically
choose the "buddy" backend when CONFIG_ARM_PMU isn't turned on.

-Doug