Re: [PATCH 2/2] drm/msm/a6xx: Use CX AO Counter register for timestamp on a750 GPUs

[Neil Armstrong]

On 9/9/26 15:24, Connor Abbott wrote:
> On Wed, Sep 9, 2026 at 5:32 AM Neil Armstrong <neil.armstrong@xxxxxxxxxx> wrote:
> > On 9/9/26 11:06, Konrad Dybcio wrote:
> > > On 9/8/26 3:24 PM, Neil Armstrong wrote:
> > > > The a750 uses the CX AO Counters instead of the GMU_ALWAYS_ON_COUNTER
> > > > register on A6xx and other A7xx GPUs, use it when running a A750 GPU.
> > > >
> > > > Signed-off-by: Neil Armstrong <neil.armstrong@xxxxxxxxxx>
> > > > ---
> > > >    drivers/gpu/drm/msm/adreno/a6xx_gpu.c | 12 +++++++++---
> > > >    1 file changed, 9 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > > > index f9de9329dee3..081e79ea4652 100644
> > > > --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > > > +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> > > > @@ -23,9 +23,15 @@ static u64 a6xx_gmu_get_timestamp(struct msm_gpu *gpu)
> > > >       u64 count_hi, count_lo, temp;
> > > >
> > > >       do {
> > > > -            count_hi = gmu_read(&a6xx_gpu->gmu, REG_A6XX_GMU_ALWAYS_ON_COUNTER_H);
> > > > -            count_lo = gmu_read(&a6xx_gpu->gmu, REG_A6XX_GMU_ALWAYS_ON_COUNTER_L);
> > > > -            temp = gmu_read(&a6xx_gpu->gmu, REG_A6XX_GMU_ALWAYS_ON_COUNTER_H);
> > > > +            if (adreno_is_a750_family(adreno_gpu)) {
> > > > +                    count_hi = gmu_read(&a6xx_gpu->gmu, REG_A7XX_GMU_CX_AO_COUNTER_H);
> > > > +                    count_lo = gmu_read(&a6xx_gpu->gmu, REG_A7XX_GMU_CX_AO_COUNTER_L);
> > > > +                    temp = gmu_read(&a6xx_gpu->gmu, REG_A7XX_GMU_CX_AO_COUNTER_H);
> > >
> > > KGSL apparently *writes* the system time to these registers, is that
> > > an idiosyncrasy regarding how the vendor userland drivers then use
> > > this counter?
> >
> > This is a GMU counter, no sure why & how KGSL or freedreno would write anything to it...
>
> It is writeable. This is part of implementing
> VK_KHR_calibrated_timestamps. The idea is that when the GPU suspends
> and resumes, we keep track of how many ticks it would've taken and
> then pretend to advance the counter by writing the CX counter (after
> powering on CX but before powering on GX which initializes the normal
> GMU counter).
>
> Note that VK_KHR_calibrated_timestamps worked initially on turnip but
> the tests started failing with v6.19. I haven't bisected yet but my
> guess is CX was never actually collapsing for whatever reason and we
> made it collapse hence breaking the tests. So we probably do need
> this.

OK thanks for the detail! Today it's only used for perfetto, but still
on A750 a different register is used for this purpose.

Neil

> Connor
>
> > > Konrad
> > >
> > > P.S. looks like a8xx also has this counter