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

[Ilkka Koskinen]

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@xxxxxxxxxxxxxxxxxxxxxx>
> > ---
> >   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;
> >   };