Re: [PATCH v2 08/34] drm/xe: Don't change LRC ring head on job resubmission

[Matthew Brost <matthew.brost@intel.com>]

On Wed, Sep 24, 2025 at 05:14:57PM +0200, Lis, Tomasz wrote:
> 
> On 9/24/2025 3:15 AM, Matthew Brost wrote:
> > Now that we save the job's head during submission, it's no longer
> > necessary to adjust the LRC ring head during resubmission. Instead, a
> > software-based adjustment of the tail will overwrite the old jobs in
> > place. For some odd reason, adjusting the LRC ring head didn't work on
> > parallel queues, which was causing issues in our CI.
> > 
> > v6:
> >   - Also set LRC tail to head so queue is idle coming out of reset
> 
> This is complex.. if the job got preempted, then not touching the head makes
> 
> sense. But what if the queue was removed by reset? Are the problematic
> 
> commands NOP'ed so that we really can leave the head?
> 
> Setting tails looks ok.
> 

So this case is a bit different than VF restore. If a job has started
but not completed across GT reset, we just ban the queue unless it is
KMD migration queue so preempted jobs are a non issue. KMD jobs could
potentially have been preempted but copy jobs are simple copies which
would be fine to run twice.

Also coming out of GT reset the queues are not registered with GuC as
this point so I believe it is fine to move KMD own LRC tail back the
original position of the job which we resubmiting.

TL;DR after some thought I believe this patch is correct.

Matt

> -Tomasz
> 
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >   drivers/gpu/drm/xe/xe_guc_submit.c | 10 ++++++++--
> >   1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/xe/xe_guc_submit.c b/drivers/gpu/drm/xe/xe_guc_submit.c
> > index 3a534d93505f..70306f902ba5 100644
> > --- a/drivers/gpu/drm/xe/xe_guc_submit.c
> > +++ b/drivers/gpu/drm/xe/xe_guc_submit.c
> > @@ -2008,11 +2008,17 @@ static void guc_exec_queue_start(struct xe_exec_queue *q)
> >   	struct xe_gpu_scheduler *sched = &q->guc->sched;
> >   	if (!exec_queue_killed_or_banned_or_wedged(q)) {
> > +		struct xe_sched_job *job = xe_sched_first_pending_job(sched);
> >   		int i;
> >   		trace_xe_exec_queue_resubmit(q);
> > -		for (i = 0; i < q->width; ++i)
> > -			xe_lrc_set_ring_head(q->lrc[i], q->lrc[i]->ring.tail);
> > +		if (job) {
> > +			for (i = 0; i < q->width; ++i) {
> > +				q->lrc[i]->ring.tail = job->ptrs[i].head;
> > +				xe_lrc_set_ring_tail(q->lrc[i],
> > +						     xe_lrc_ring_head(q->lrc[i]));
> > +			}
> > +		}
> >   		xe_sched_resubmit_jobs(sched);
> >   	}