[PATCH v3 1/3] arm64: perf: Add support caps in sysfs

[Will Deacon]

On Mon, Jul 20, 2020 at 11:50:19AM +0100, Mark Rutland wrote:
> On Mon, Jul 20, 2020 at 11:15:19AM +0100, Will Deacon wrote:
> > On Thu, Jun 18, 2020 at 09:35:42PM +0800, Shaokun Zhang wrote:
> 
> > > +static umode_t
> > > +armv8pmu_caps_attr_is_visible(struct kobject *kobj, struct attribute *attr,
> > > +			      int unused)
> > > +{
> > > +	int pmuver = armv8pmu_get_pmu_version();
> > > +
> > > +	if (pmuver >= ID_AA64DFR0_PMUVER_8_4)
> > > +		return attr->mode;
> > 
> > Is this sufficient? I'm a bit confused by the text in the Arm ARM that says:
> > 
> >   | If ARMv8.4-PMU is implemented:
> >   | * If STALL_SLOT is not implemented, it is IMPLEMENTATION DEFINED whether
> >   |   the PMMIR System registers are implemented.
> >   | * If STALL_SLOT is implemented, then the PMMIR System registers are
> >   |   implemented.
> > 
> > whereas the register description for PMMIR_EL1 says:
> > 
> >   | This register is present only when ARMv8.4-PMU is implemented.
> 
> I think this is trying to say that when ARMv8.4-PMU is not implemented,
> PMMIR definitely isn't implemented (i.e. the the presence of PMMIR_EL1
> implies the presence of ARMv8.4-PMU).
> 
> > Mark -- please could you clarify whether or not we need to check STALL_SLOT
> > as well as the PMUVer?
> 
> Given the explciit wording that it's IMP DEF, I suspect that we need to
> check both.
> 
> I'll go chase this up.

Thanks. In the meantime, Shaokun, can you send updated versions of the first
two patches, please? We'll play it safe and check STALL_SLOT as well pending
Mark's findings.

Will