Re: [PATCH 1/4] drm/msm/a5xx: disable preemption in submits by default

[Rob Clark]

On Thu, Jul 11, 2024 at 3:02 AM Vladimir Lypak <vladimir.lypak@xxxxxxxxx> wrote:
>
> Fine grain preemption (switching from/to points within submits)
> requires extra handling in command stream of those submits, especially
> when rendering with tiling (using GMEM). However this handling is
> missing at this point in mesa (and always was). For this reason we get
> random GPU faults and hangs if more than one priority level is used
> because local preemption is enabled prior to executing command stream
> from submit.
> With that said it was ahead of time to enable local preemption by
> default considering the fact that even on downstream kernel it is only
> enabled if requested via UAPI.
>
> Fixes: a7a4c19c36de ("drm/msm/a5xx: fix setting of the CP_PREEMPT_ENABLE_LOCAL register")
> Signed-off-by: Vladimir Lypak <vladimir.lypak@xxxxxxxxx>
> ---
>  drivers/gpu/drm/msm/adreno/a5xx_gpu.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/adreno/a5xx_gpu.c b/drivers/gpu/drm/msm/adreno/a5xx_gpu.c
> index c0b5373e90d7..6c80d3003966 100644
> --- a/drivers/gpu/drm/msm/adreno/a5xx_gpu.c
> +++ b/drivers/gpu/drm/msm/adreno/a5xx_gpu.c
> @@ -150,9 +150,13 @@ static void a5xx_submit(struct msm_gpu *gpu, struct msm_gem_submit *submit)
>         OUT_PKT7(ring, CP_SET_PROTECTED_MODE, 1);
>         OUT_RING(ring, 1);
>
> -       /* Enable local preemption for finegrain preemption */
> +       /*
> +        * Disable local preemption by default because it requires
> +        * user-space to be aware of it and provide additional handling
> +        * to restore rendering state or do various flushes on switch.
> +        */
>         OUT_PKT7(ring, CP_PREEMPT_ENABLE_LOCAL, 1);
> -       OUT_RING(ring, 0x1);
> +       OUT_RING(ring, 0x0);

>From a quick look at the a530 pfp fw, it looks like
CP_PREEMPT_ENABLE_LOCAL is allowed in IB1/IB2 (ie. not restricted to
kernel RB).  So we should just disable it in the kernel, and let
userspace send a CP_PREEMPT_ENABLE_LOCAL to enable local preemption.

BR,
-R

>         /* Allow CP_CONTEXT_SWITCH_YIELD packets in the IB2 */
>         OUT_PKT7(ring, CP_YIELD_ENABLE, 1);
> --
> 2.45.2
>