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

[Ilkka Koskinen <ilkka@os.amperecomputing.com>]

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@os.amperecomputing.com>
>> ---
>>   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;
>>   };
>