[PATCH 2/3] drm/xe/guc: Publish the ring tail when replaying a chained ULLS job

[Varun Gupta <varun.gupta@intel.com>]

A chained ULLS job skips the ring tail write in submit_exec_queue()
since its predecessor's postamble has already published it from within
the ring. After a GT reset the context is re-registered with scheduling
disabled and that postamble is gone, so nothing publishes the replayed
job's tail: the context is enabled with head == tail and the job never
starts. On the kernel migration queue the second timeout wedges the
device.

Publish the tail for a chained job when the queue is not enabled. This
is the state in which submit_exec_queue() already sends the scheduling
enable H2G instead of relying on a resident context, and the tail write
precedes that H2G. guc_exec_queue_stop() clears ENABLED on reset and
register_exec_queue() does not set it, so the first replayed ULLS job
always takes this path; its re-emitted postamble then publishes the
next tail as usual. With the queue enabled nothing changes.

job->restore_replay does not identify this case, it is only set on the
SR-IOV VF pause/unpause path.

Fixes: 40c756bdbbee ("drm/xe: Add ULLS migration job support to GuC submission")
Cc: Matthew Brost <matthew.brost@intel.com>
Assisted-by: LLM
Signed-off-by: Varun Gupta <varun.gupta@intel.com>
---
 drivers/gpu/drm/xe/xe_guc_submit.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_guc_submit.c b/drivers/gpu/drm/xe/xe_guc_submit.c
index cd9bb9f9e28c..1d98ec67c5c6 100644
--- a/drivers/gpu/drm/xe/xe_guc_submit.c
+++ b/drivers/gpu/drm/xe/xe_guc_submit.c
@@ -1190,10 +1190,15 @@ static void submit_exec_queue(struct xe_exec_queue *q, struct xe_sched_job *job)
 	xe_gt_assert(guc_to_gt(guc), exec_queue_registered(q));
 
 	if (!job->restore_replay || job->last_replay) {
-		/* A ULLS job past the first publishes its own ring tail */
+		/*
+		 * A chained ULLS job relies on its predecessor's postamble to
+		 * publish its ring tail. After a reset the context comes back
+		 * disabled with that postamble gone, so publish it here.
+		 */
 		if (xe_exec_queue_is_parallel(q))
 			wq_item_append(q);
-		else if (!xe_sched_job_ulls_is_chained(job))
+		else if (!xe_sched_job_ulls_is_chained(job) ||
+			 !exec_queue_enabled(q))
 			xe_lrc_set_ring_tail(lrc, lrc->ring.tail);
 		job->last_replay = false;
 	}
-- 
2.43.0