Re: [PATCH] drm/panthor: Don't add write fences to the shared BOs

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu,  5 Sep 2024 09:01:54 +0200
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> The only user (the mesa gallium driver) is already assuming explicit
> synchronization and doing the export/import dance on shared BOs. The
> only reason we were registering ourselves as writers on external BOs
> is because Xe, which was the reference back when we developed Panthor,
> was doing so. Turns out Xe was wrong, and we really want bookkeep on
> all registered fences, so userspace can explicitly upgrade those to
> read/write when needed.
> 
> Fixes: 4bdca1150792 ("drm/panthor: Add the driver frontend block")
> Cc: Matthew Brost <matthew.brost@intel.com>
> Cc: Simona Vetter <simona.vetter@ffwll.ch>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

Queued to drm-misc-fixes.

> ---
>  drivers/gpu/drm/panthor/panthor_sched.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_sched.c b/drivers/gpu/drm/panthor/panthor_sched.c
> index 9a0ff48f7061..41260cf4beb8 100644
> --- a/drivers/gpu/drm/panthor/panthor_sched.c
> +++ b/drivers/gpu/drm/panthor/panthor_sched.c
> @@ -3423,13 +3423,8 @@ void panthor_job_update_resvs(struct drm_exec *exec, struct drm_sched_job *sched
>  {
>  	struct panthor_job *job = container_of(sched_job, struct panthor_job, base);
>  
> -	/* Still not sure why we want USAGE_WRITE for external objects, since I
> -	 * was assuming this would be handled through explicit syncs being imported
> -	 * to external BOs with DMA_BUF_IOCTL_IMPORT_SYNC_FILE, but other drivers
> -	 * seem to pass DMA_RESV_USAGE_WRITE, so there must be a good reason.
> -	 */
>  	panthor_vm_update_resvs(job->group->vm, exec, &sched_job->s_fence->finished,
> -				DMA_RESV_USAGE_BOOKKEEP, DMA_RESV_USAGE_WRITE);
> +				DMA_RESV_USAGE_BOOKKEEP, DMA_RESV_USAGE_BOOKKEEP);
>  }
>  
>  void panthor_sched_unplug(struct panthor_device *ptdev)