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

[Maíra Canal <mcanal@igalia.com>]

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]);
 

-- 
2.54.0