Re: [PATCH] drm/msm/a6xx: Fix recovery vs runpm race

[Akhil P Oommen]

On Mon, Dec 18, 2023 at 07:59:24AM -0800, Rob Clark wrote:
> 
> From: Rob Clark <robdclark@xxxxxxxxxxxx>
> 
> a6xx_recover() is relying on the gpu lock to serialize against incoming
> submits doing a runpm get, as it tries to temporarily balance out the
> runpm gets with puts in order to power off the GPU.  Unfortunately this
> gets worse when we (in a later patch) will move the runpm get out of the
> scheduler thread/work to move it out of the fence signaling path.
> 
> Instead we can just simplify the whole thing by using force_suspend() /
> force_resume() instead of trying to be clever.

At some places, we take a pm_runtime vote and access the gpu
registers assuming it will be powered until we drop the vote.  a6xx_get_timestamp()
is an example. If we do a force suspend, it may cause bus errors from
those threads. Now you have to serialize every place we do runtime_get/put with a
mutex. Or is there a better way to handle the 'later patch' you
mentioned?

-Akhil.

> 
> Reported-by: David Heidelberg <david.heidelberg@xxxxxxxxxxxxx>
> Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/10272
> Fixes: abe2023b4cea ("drm/msm/gpu: Push gpu lock down past runpm")
> Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/msm/adreno/a6xx_gpu.c | 12 ++----------
>  1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> index 268737e59131..a5660d63535b 100644
> --- a/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> +++ b/drivers/gpu/drm/msm/adreno/a6xx_gpu.c
> @@ -1244,12 +1244,7 @@ static void a6xx_recover(struct msm_gpu *gpu)
>  	dev_pm_genpd_add_notifier(gmu->cxpd, &gmu->pd_nb);
>  	dev_pm_genpd_synced_poweroff(gmu->cxpd);
>  
> -	/* Drop the rpm refcount from active submits */
> -	if (active_submits)
> -		pm_runtime_put(&gpu->pdev->dev);
> -
> -	/* And the final one from recover worker */
> -	pm_runtime_put_sync(&gpu->pdev->dev);
> +	pm_runtime_force_suspend(&gpu->pdev->dev);
>  
>  	if (!wait_for_completion_timeout(&gmu->pd_gate, msecs_to_jiffies(1000)))
>  		DRM_DEV_ERROR(&gpu->pdev->dev, "cx gdsc didn't collapse\n");
> @@ -1258,10 +1253,7 @@ static void a6xx_recover(struct msm_gpu *gpu)
>  
>  	pm_runtime_use_autosuspend(&gpu->pdev->dev);
>  
> -	if (active_submits)
> -		pm_runtime_get(&gpu->pdev->dev);
> -
> -	pm_runtime_get_sync(&gpu->pdev->dev);
> +	pm_runtime_force_resume(&gpu->pdev->dev);
>  
>  	gpu->active_submits = active_submits;
>  	mutex_unlock(&gpu->active_lock);
> -- 
> 2.43.0
>