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

[Anshuman Khandual]

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 <will at kernel.org>
Cc: Mark Rutland <mark.rutland at arm.com>
Cc: linux-arm-kernel at lists.infradead.org
Cc: linux-kernel at vger.kernel.org
Reviewed-by: James Clark <james.clark at arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual at arm.com>
---
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/20231009075631.193208-1-anshuman.khandual@arm.com/

 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