Re: [PATCH v2 1/5] perf: arm_cspmu: Support 32-bit accesses to 64-bit registers

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Hi Robin,

On Thu, 1 Jun 2023, Robin Murphy wrote:
On 2023-06-01 04:01, Ilkka Koskinen wrote:
Split the 64-bit register accesses if 64-bit access is not supported
by the PMU.

Signed-off-by: Ilkka Koskinen <ilkka@xxxxxxxxxxxxxxxxxxxxxx>
---
  drivers/perf/arm_cspmu/arm_cspmu.c | 8 ++++++--
  drivers/perf/arm_cspmu/arm_cspmu.h | 1 +
  2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/perf/arm_cspmu/arm_cspmu.c b/drivers/perf/arm_cspmu/arm_cspmu.c
index a3f1c410b417..88547a2b73e6 100644
--- a/drivers/perf/arm_cspmu/arm_cspmu.c
+++ b/drivers/perf/arm_cspmu/arm_cspmu.c
@@ -701,8 +701,12 @@ static void arm_cspmu_write_counter(struct perf_event *event, u64 val)
    	if (use_64b_counter_reg(cspmu)) {
  		offset = counter_offset(sizeof(u64), event->hw.idx);
-
-		writeq(val, cspmu->base1 + offset);
+		if (!cspmu->impl.split_64bit_access) {

Could we not just hang this off the 64-bit atomicity property to match the read path? It doesn't seem like there's much benefit in micro-optimising for whether the interconnect splits 64-bit accesses into 32-bit bursts vs. just not accepting them at all.

Sure, I was actually wondering if I could use it or if there could be a really weird hw implementation. I'll change it.


+			writeq(val, cspmu->base1 + offset);
+		} else {
+			writel(lower_32_bits(val), cspmu->base1 + offset);
+ writel(upper_32_bits(val), cspmu->base1 + offset + 4);

lo_hi_writeq() - the header's already included for 32-bit build coverage, so we may as well put it to use :)

Oh, I have missed that function completely. I fix this too.

Cheers, Ilkka


Thanks,
Robin.

+		}
  	} else {
  		offset = counter_offset(sizeof(u32), event->hw.idx);
diff --git a/drivers/perf/arm_cspmu/arm_cspmu.h b/drivers/perf/arm_cspmu/arm_cspmu.h
index 51323b175a4a..c0412cf2bd97 100644
--- a/drivers/perf/arm_cspmu/arm_cspmu.h
+++ b/drivers/perf/arm_cspmu/arm_cspmu.h
@@ -110,6 +110,7 @@ struct arm_cspmu_impl_ops {
  /* Vendor/implementer descriptor. */
  struct arm_cspmu_impl {
  	u32 pmiidr;
+	bool split_64bit_access;
  	struct arm_cspmu_impl_ops ops;
  	void *ctx;
  };




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