Re: [PATCH v2 1/5] perf: arm_spe: Add format option for discard mode

[James Clark <james.clark@linaro.org>]

On 07/01/2025 5:39 pm, Will Deacon wrote:
> On Tue, Dec 24, 2024 at 10:44:08AM +0000, James Clark wrote:
>> FEAT_SPEv1p2 (optional from Armv8.6) adds a discard mode that allows all
>> SPE data to be discarded rather than written to memory. Add a format
>> bit for this mode.
>>
>> If the mode isn't supported, the format bit isn't published and attempts
>> to use it will result in -EOPNOTSUPP. Allocating an aux buffer is still
>> allowed even though it won't be written to so that old tools continue to
>> work, but updated tools can choose to skip this step.
>>
>> Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
>> Signed-off-by: James Clark <james.clark@linaro.org>
>> ---
>>   drivers/perf/arm_spe_pmu.c | 23 +++++++++++++++++++++++
>>   1 file changed, 23 insertions(+)
>>
>> diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
>> index fd5b78732603..9aaf3f98e6f5 100644
>> --- a/drivers/perf/arm_spe_pmu.c
>> +++ b/drivers/perf/arm_spe_pmu.c
>> @@ -193,6 +193,9 @@ static const struct attribute_group arm_spe_pmu_cap_group = {
>>   #define ATTR_CFG_FLD_store_filter_CFG		config	/* PMSFCR_EL1.ST */
>>   #define ATTR_CFG_FLD_store_filter_LO		34
>>   #define ATTR_CFG_FLD_store_filter_HI		34
>> +#define ATTR_CFG_FLD_discard_CFG		config	/* PMBLIMITR_EL1.FM = DISCARD */
>> +#define ATTR_CFG_FLD_discard_LO			35
>> +#define ATTR_CFG_FLD_discard_HI			35
>>   
>>   #define ATTR_CFG_FLD_event_filter_CFG		config1	/* PMSEVFR_EL1 */
>>   #define ATTR_CFG_FLD_event_filter_LO		0
>> @@ -216,6 +219,7 @@ GEN_PMU_FORMAT_ATTR(store_filter);
>>   GEN_PMU_FORMAT_ATTR(event_filter);
>>   GEN_PMU_FORMAT_ATTR(inv_event_filter);
>>   GEN_PMU_FORMAT_ATTR(min_latency);
>> +GEN_PMU_FORMAT_ATTR(discard);
>>   
>>   static struct attribute *arm_spe_pmu_formats_attr[] = {
>>   	&format_attr_ts_enable.attr,
>> @@ -228,9 +232,15 @@ static struct attribute *arm_spe_pmu_formats_attr[] = {
>>   	&format_attr_event_filter.attr,
>>   	&format_attr_inv_event_filter.attr,
>>   	&format_attr_min_latency.attr,
>> +	&format_attr_discard.attr,
>>   	NULL,
>>   };
>>   
>> +static bool discard_unsupported(struct arm_spe_pmu *spe_pmu)
>> +{
>> +	return spe_pmu->pmsver < ID_AA64DFR0_EL1_PMSVer_V1P2;
>> +}
> 
> Why not add a new SPE_PMU_FEAT_* for this and handle it in a similar
> way to other optional hardware features?
> 
> Will

Hmmm good point, I'm not sure why I didn't do it that way. Possibly 
because it's only based off pmsver which is already saved, whereas the 
other feats are based off reading PMSIDR which is thrown away. But I can 
add SPE_PMU_FEAT_DISCARD.

Thanks
James