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SubjectRe: [PATCH v5 6/7] drivers/perf: Add support for ARMv8.2 Statistical Profiling Extension
On Thu, 28 Sep 2017 15:09:50 +0100
Will Deacon <will.deacon@arm.com> wrote:

> +/* Perf callbacks */
> +static int arm_spe_pmu_event_init(struct perf_event *event)
> +{
> + u64 reg;
> + struct perf_event_attr *attr = &event->attr;
> + struct arm_spe_pmu *spe_pmu = to_spe_pmu(event->pmu);
> +
> + /* This is, of course, deeply driver-specific */
> + if (attr->type != event->pmu->type)
> + return -ENOENT;
> +
> + if (event->cpu >= 0 &&
> + !cpumask_test_cpu(event->cpu, &spe_pmu->supported_cpus))
> + return -ENOENT;

So -ENOENT will make tools/perf/util/evsel.c tell the user "The %s event is not
supported." whereas returning -ENODEV will say "No such device - did
you specify an out-of-range profile CPU?" which may or may not be more
appropriate for this check.

> + if (arm_spe_event_to_pmsevfr(event) & SYS_PMSEVFR_EL1_RES0)
> + return -EOPNOTSUPP;
> + if (attr->exclude_idle)
> + return -EOPNOTSUPP;

"PMU Hardware doesn't support sampling/overflow-interrupts." will be
printed if the user didn't specify a sample period. Otherwise, a
string with "/bin/dmesg may provide additional information." will be
printed.

I was hoping for a response from acme by now for this:

https://www.spinics.net/lists/linux-perf-users/msg04066.html

Alas, nothing. Looking at the #ifdef x86 in evsel.c, I'm guessing
it'll be ok, although I'm still not sure how PMU-specific we can get in
evsel.c, nor whether it's ok to communicate lists of h/w supported
sample periods through /sys/bus/event_source/devices/...

acme? OK to refactor evsel messaging for Arm, including parsing for
which PMUs are being used, so customize the message?

Kim

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