Re: [PATCH v4 2/2] drm/v3d: Skip CSD when it has zeroed workgroups

[Iago Toral <itoral@igalia.com>]

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

El mar, 02-06-2026 a las 14:50 -0300, Maíra Canal escribió:
> A compute shader dispatch encodes its workgroup counts in the
> CFG0..CFG2
> registers. Kicking off a dispatch with a zero count in any of the
> three
> dimensions is invalid. First, the hardware will process 0 as 65536,
> while the user-space driver exposes a maximum of 65535. Over that, a
> submission with a zeroed workgroup dimension should be a no-op.
> 
> These zeroed counts can reach the dispatch path through an indirect
> CSD
> job, whose workgroup counts are only known once the indirect buffer
> is
> read and may legitimately be zero, but such scenario should only
> result in
> a no-op.
> 
> Overwrite the indirect CSD job workgroup counts with the indirect BO
> ones, even if they are zeroed, and don't submit the job to the
> hardware
> when any of the workgroup counts is zero, so the job completes
> immediately
> instead of running the shader.
> 
> Cc: stable@vger.kernel.org
> Fixes: d223f98f0209 ("drm/v3d: Add support for compute shader
> dispatch.")
> Suggested-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
> Signed-off-by: Maíra Canal <mcanal@igalia.com>
> ---
>  drivers/gpu/drm/v3d/v3d_sched.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/v3d/v3d_sched.c
> b/drivers/gpu/drm/v3d/v3d_sched.c
> index 47f83936cd73..8a635a9ec046 100644
> --- a/drivers/gpu/drm/v3d/v3d_sched.c
> +++ b/drivers/gpu/drm/v3d/v3d_sched.c
> @@ -352,6 +352,16 @@ v3d_csd_job_run(struct drm_sched_job *sched_job)
>  		return NULL;
>  	}
>  
> +	/* The HW interprets a workgroup size of 0 as 65536;
> however, the
> +	 * user-space driver exposes a maximum of 65535. Therefore,
> a 0 in
> +	 * any dimension means that we have no workgroups and the
> compute
> +	 * shader should not be dispatched.
> +	 */
> +	if (!V3D_GET_FIELD(job->args.cfg[0],
> V3D_CSD_QUEUED_CFG0_NUM_WGS_X) ||
> +	    !V3D_GET_FIELD(job->args.cfg[1],
> V3D_CSD_QUEUED_CFG1_NUM_WGS_Y) ||
> +	    !V3D_GET_FIELD(job->args.cfg[2],
> V3D_CSD_QUEUED_CFG2_NUM_WGS_Z))
> +		return NULL;
> +
>  	v3d->queue[V3D_CSD].active_job = &job->base;
>  
>  	v3d_invalidate_caches(v3d);
> @@ -402,13 +412,13 @@
> v3d_rewrite_csd_job_wg_counts_from_indirect(struct v3d_cpu_job *job)
>  
>  	wg_counts = (uint32_t *)(bo->vaddr + indirect_csd->offset);
>  
> -	if (wg_counts[0] == 0 || wg_counts[1] == 0 || wg_counts[2]
> == 0)
> -		goto unmap_bo;
> -
>  	args->cfg[0] = wg_counts[0] <<
> V3D_CSD_CFG012_WG_COUNT_SHIFT;
>  	args->cfg[1] = wg_counts[1] <<
> V3D_CSD_CFG012_WG_COUNT_SHIFT;
>  	args->cfg[2] = wg_counts[2] <<
> V3D_CSD_CFG012_WG_COUNT_SHIFT;
>  
> +	if (wg_counts[0] == 0 || wg_counts[1] == 0 || wg_counts[2]
> == 0)
> +		goto unmap_bo;
> +
>  	num_batches = DIV_ROUND_UP(indirect_csd->wg_size, 16) *
>  		      (wg_counts[0] * wg_counts[1] * wg_counts[2]);
>  
>