Subject: Re: [PATCH] perf: Change PMCR write to read-modify-write

From: Sodagudi Prasad <[email-masked]>
Date: Thu, 21 Mar 2019 13:02:27 -0700

On 2019-03-21 06:34, Julien Thierry wrote:
> Hi Prasad,
> 
> On 21/03/2019 02:07, Prasad Sodagudi wrote:
>> Preserves the bitfields of PMCR_EL0(AArch64) during PMU reset.
>> Reset routine should write a 1 to PMCR.C and PMCR.P fields only
>> to reset the counters. Other fields should not be changed
>> as they could be set before PMU initialization and their
>> value must be preserved even after reset.
>> 
> 
> Are there any particular bit you are concerned about? Apart from the RO
> ones and the Res0 ones (to which we are already writing 0), I see:
> 
> DP -> irrelevant for non-secure
> X -> This one is the only potentially interesting, however it resets to
> an architecturally unknown value, so unless we know for a fact it was
> set before hand, we probably want to clear it
> D -> ignored when we have LC set (and we do)
> E -> Since this is the function we use to reset the PMU on the current
> CPU, we probably want to set this bit to 0 regardless of its previous 
> value
> 
> So, is there any issue this patch is solving?

Hi Julien,

Thanks for taking a look into this patch. Yes. On our Qualcomm 
platforms, observed that X bit is getting cleared by kernel.
This bit is getting set by firmware for Qualcomm use cases and 
non-secure world is resetting without this patch.
I think, changing that register this register modifications to 
read-modify-write style make sense.

-Thanks, Prasad

> 
> Thanks,
> 
>> Signed-off-by: Prasad Sodagudi <[email-masked]>
>> ---
>>  arch/arm64/kernel/perf_event.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/arm64/kernel/perf_event.c 
>> b/arch/arm64/kernel/perf_event.c
>> index 4addb38..0c1afdd 100644
>> --- a/arch/arm64/kernel/perf_event.c
>> +++ b/arch/arm64/kernel/perf_event.c
>> @@ -868,8 +868,8 @@ static void armv8pmu_reset(void *info)
>>  	 * Initialize & Reset PMNC. Request overflow interrupt for
>>  	 * 64 bit cycle counter but cheat in armv8pmu_write_counter().
>>  	 */
>> -	armv8pmu_pmcr_write(ARMV8_PMU_PMCR_P | ARMV8_PMU_PMCR_C |
>> -			    ARMV8_PMU_PMCR_LC);
>> +	armv8pmu_pmcr_write(armv8pmu_pmcr_read() | ARMV8_PMU_PMCR_P |
>> +			ARMV8_PMU_PMCR_C | ARMV8_PMU_PMCR_LC);
>>  }
>> 
>>  static int __armv8_pmuv3_map_event(struct perf_event *event,
>> 

-- 
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