2023-10-13 02:44:42

by Anshuman Khandual

Subject: [PATCH V2] drivers: perf: arm_pmuv3: Read PMMIR_EL1 unconditionally

Currently the PMUv3 driver only reads PMMIR_EL1 if the PMU implements
FEAT_PMUv3p4 and the STALL_SLOT event, but the check for STALL_SLOT event
isn't necessary and can be removed.

The check for STALL_SLOT event was introduced with the read of PMMIR_EL1 in
commit f5be3a61fdb5dd11 ("arm64: perf: Add support caps under sysfs")

When this logic was written, the ARM ARM said:

| If STALL_SLOT is not implemented, it is IMPLEMENTATION DEFINED whether
| the PMMIR System registers are implemented.

... and thus the driver had to check for STALL_SLOT event to verify that
PMMIR_EL1 was implemented and accesses to PMMIR_EL1 would not be UNDEFINED.

Subsequently, the architecture was retrospectively tightened to require
that any FEAT_PMUv3p4 implementation implements PMMIR_EL1. Since the G.b
release of the ARM ARM, the wording regarding STALL_SLOT event has been
removed, and the description of PMMIR_EL1 says:

| This register is present only when FEAT_PMUv3p4 is implemented.

Drop the unnecessary check for STALL_SLOT event when reading PMMIR_EL1.

Cc: Will Deacon <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: [email protected]
Cc: [email protected]
Reviewed-by: James Clark <[email protected]>
Signed-off-by: Anshuman Khandual <[email protected]>
---
This applies on v6.6-rc5

Changes in V2:

- Updated commit message to include previous context about STALL_SLOT check

Changes in V1:

https://lore.kernel.org/all/[email protected]/

drivers/perf/arm_pmuv3.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
index 144593092727..92fefc1bc7f6 100644
--- a/drivers/perf/arm_pmuv3.c
+++ b/drivers/perf/arm_pmuv3.c
@@ -1126,7 +1126,7 @@ static void __armv8pmu_probe_pmu(void *info)
pmceid, ARMV8_PMUV3_MAX_COMMON_EVENTS);

/* store PMMIR register for sysfs */
- if (is_pmuv3p4(pmuver) && (pmceid_raw[1] & BIT(31)))
+ if (is_pmuv3p4(pmuver))
cpu_pmu->reg_pmmir = read_pmmir();
else
cpu_pmu->reg_pmmir = 0;
--
2.25.1

2023-10-19 15:46:59

by Will Deacon

Subject: Re: [PATCH V2] drivers: perf: arm_pmuv3: Read PMMIR_EL1 unconditionally

On Fri, 13 Oct 2023 08:13:54 +0530, Anshuman Khandual wrote:
> Currently the PMUv3 driver only reads PMMIR_EL1 if the PMU implements
> FEAT_PMUv3p4 and the STALL_SLOT event, but the check for STALL_SLOT event
> isn't necessary and can be removed.
>
> The check for STALL_SLOT event was introduced with the read of PMMIR_EL1 in
> commit f5be3a61fdb5dd11 ("arm64: perf: Add support caps under sysfs")
>
> [...]

Applied to will (for-next/perf), thanks!

[1/1] drivers: perf: arm_pmuv3: Read PMMIR_EL1 unconditionally
https://git.kernel.org/will/c/58f8fc57b1d3

Cheers,
--
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev