Re: [PATCH v7 13/17] drm/panfrost: Skip cache flush/invalidate when enabling perfcnt

[Adrián Larumbe]

On 01.09.2026 15:32, Boris Brezillon wrote:
> On Fri, 28 Aug 2026 21:56:53 +0100
> Adrián Larumbe <adrian.larumbe@xxxxxxxxxxxxx> wrote:
> 
> > The GPU cache flush/invalidate operation is unnecessary, because at present
> > all counters are always enabled for all sessions, and there are no plans to
> > expand the uAPI so as to let UM select specific counter groups.
> > 
> > If the above was ever implemented, then different counter groups being
> > selected between sessions might leave stale data in the GPU caches, so
> > the flush/invalidate would have to be reintroduced.
> 
> This explanation confuses me. To me, the reason we don't need a
> cache-flush has nothing to do with the fact we always enable all
> counters, it's just that, either the perfcnt buffer has been freshly
> allocated/mapped and the cache has no entry pointing to it, or it's
> been written already, and the flush should have happened after the
> dump. For the case where the enable happens after a RESET (which you
> introduce in one of the remaining commits, I guess), there's no flush
> needed either, because the RESET clears all the caches.

This is something I wrote after a chat with Steven about whether removing this flush/invalidate was fine.
If memory serves me right, he expressed his concern that flushing GPU caches after a sample might write
disabled counter data into system memory, but in that case UM should know better, and I guess reading off
disabled counter data is seen as undefined behaviour in the TRM.

> > 
> > Signed-off-by: Adrián Larumbe <adrian.larumbe@xxxxxxxxxxxxx>
> > ---
> >  drivers/gpu/drm/panfrost/panfrost_perfcnt.c | 15 ++-------------
> >  1 file changed, 2 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_perfcnt.c b/drivers/gpu/drm/panfrost/panfrost_perfcnt.c
> > index f71534e741b6..ffc77121070e 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_perfcnt.c
> > +++ b/drivers/gpu/drm/panfrost/panfrost_perfcnt.c
> > @@ -124,21 +124,10 @@ static int panfrost_perfcnt_enable_locked(struct panfrost_device *pfdev,
> >  	panfrost_gem_internal_set_label(&bo->base, "Perfcnt sample buffer");
> >  
> >  	/*
> > -	 * Invalidate the cache and clear the counters to start from a fresh
> > -	 * state.
> > +	 * Clear the counters to start from a fresh state.
> >  	 */
> > -	reinit_completion(&pfdev->perfcnt->dump_comp);
> > -	gpu_write(pfdev, GPU_INT_CLEAR,
> > -		  GPU_IRQ_CLEAN_CACHES_COMPLETED |
> > -		  GPU_IRQ_PERFCNT_SAMPLE_COMPLETED);
> > +	gpu_write(pfdev, GPU_INT_CLEAR, GPU_IRQ_PERFCNT_SAMPLE_COMPLETED);
> >  	gpu_write(pfdev, GPU_CMD, GPU_CMD_PERFCNT_CLEAR);
> > -	gpu_write(pfdev, GPU_CMD, GPU_CMD_CLEAN_INV_CACHES);
> > -	ret = wait_for_completion_timeout(&pfdev->perfcnt->dump_comp,
> > -					  msecs_to_jiffies(1000));
> > -	if (!ret) {
> > -		ret = -ETIMEDOUT;
> > -		goto err_vunmap;
> > -	}
> >  
> >  	ret = panfrost_mmu_as_get(pfdev, perfcnt->mapping->mmu);
> >  	if (ret < 0)
> > 

Adrian Larumbe