Re: [PATCH v4 1/4] perf/arm_cspmu: Add Arm system PMU

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Medium] perf/arm_cspmu: arm_cspmu_init_impl_ops() defers probe when
  ARM_SYSTEM_PMU=n
--

commit ae4570e49b496427e88dd55c46f9506b33a8b68e
Author: Robin Murphy <robin.murphy@arm.com>

perf/arm_cspmu: Add Arm system PMU

This commit adds the arm_syspmu backend module to support an additional
64-bit event filter register (PMEVFILT3R) on Arm system PMUs with product
ID 0x9c3.

> diff --git a/drivers/perf/arm_cspmu/arm_cspmu.c b/drivers/perf/arm_cspmu/arm_cspmu.c
> index 912f03ce3ef5f..9f298ea1a9745 100644
> --- a/drivers/perf/arm_cspmu/arm_cspmu.c
> +++ b/drivers/perf/arm_cspmu/arm_cspmu.c
> @@ -358,6 +358,13 @@ static struct arm_cspmu_impl_match impl_match[] = {
>  		.module		= NULL,
>  		.impl_init_ops	= NULL,
>  	},
> +	{
> +		.module_name	= "arm_syspmu",
> +		.pmiidr_val	= ARM_CSPMU_IMPL_ID_ARM,
> +		.pmiidr_mask	= PMIIDR_IMPLEMENTER,
> +		.module		= NULL,
> +		.impl_init_ops	= NULL,
> +	},
>  	{
>  		.module_name	= "ampere_cspmu",
>  		.pmiidr_val	= ARM_CSPMU_IMPL_ID_AMPERE,

[Severity: Medium]
Does adding an unconditional impl_match[] entry for ARM_CSPMU_IMPL_ID_ARM
cause arm_cspmu_init_impl_ops() to defer probing indefinitely when
CONFIG_ARM_SYSTEM_PMU is disabled?

Previously, CoreSight PMUs with Arm's implementer ID did not match any
entry in impl_match[] and probed using the generic ops.  With this entry
present, arm_cspmu_init_impl_ops() requires the arm_syspmu module to
register match->impl_init_ops:

drivers/perf/arm_cspmu/arm_cspmu.c:arm_cspmu_init_impl_ops() {
    ...
		if (match->impl_init_ops) {
    ...
		} else {
			request_module_nowait(match->module_name);
			ret = dev_err_probe(cspmu->dev, -EPROBE_DEFER,
					    "Waiting for module %s to load\n",
					    match->module_name);
		}
    ...
}

Because CONFIG_ARM_SYSTEM_PMU has no default in Kconfig, existing configs
with CONFIG_ARM_CORESIGHT_PMU_ARCH_SYSTEM_PMU enabled will have
CONFIG_ARM_SYSTEM_PMU=n, leaving every Arm-implemented CoreSight PMU
(including non-0x9c3 devices where arm_syspmu_init_ops() is a no-op)
permanently stuck in -EPROBE_DEFER.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1793986672.git.robin.murphy@arm.com?part=1