drm/panthor: Fix tiler OOM handling to allow incremental rendering

If the kernel couldn't allocate memory because we reached the maximum
number of chunks but no render passes are in flight
(panthor_heap_grow() returning -ENOMEM), we should defer the OOM
handling to the FW by returning a NULL chunk. The FW will then call
the tiler OOM exception handler, which is supposed to implement
incremental rendering (execute an intermediate fragment job to flush
the pending primitives, release the tiler memory that was used to
store those primitives, and start over from where it stopped).

Instead of checking for both ENOMEM and EBUSY, make panthor_heap_grow()
return ENOMEM no matter the reason of this allocation failure, the FW
doesn't care anyway.

v3:
- Add R-bs

v2:
- Make panthor_heap_grow() return -ENOMEM for all kind of allocation
  failures
- Document the panthor_heap_grow() semantics

Fixes: de854881 ("drm/panthor: Add the scheduler logical block")
Signed-off-by: Antonino Maniscalco <antonino.maniscalco@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240502165158.1458959-2-boris.brezillon@collabora.com

drivers/gpu/drm/panthor/panthor_heap.c

@@ -410,6 +410,13 @@ int panthor_heap_return_chunk(struct panthor_heap_pool *pool,
  * @renderpasses_in_flight: Number of render passes currently in-flight.
  * @pending_frag_count: Number of fragment jobs waiting for execution/completion.
  * @new_chunk_gpu_va: Pointer used to return the chunk VA.
+ *
+ * Return:
+ * - 0 if a new heap was allocated
+ * - -ENOMEM if the tiler context reached the maximum number of chunks
+ *   or if too many render passes are in-flight
+ *   or if the allocation failed
+ * - -EINVAL if any of the arguments passed to panthor_heap_grow() is invalid
  */
 int panthor_heap_grow(struct panthor_heap_pool *pool,
 		      u64 heap_gpu_va,
@@ -439,10 +446,7 @@ int panthor_heap_grow(struct panthor_heap_pool *pool,
 	 * handler provided by the userspace driver, if any).
 	 */
 	if (renderpasses_in_flight > heap->target_in_flight ||
-	    (pending_frag_count > 0 && heap->chunk_count >= heap->max_chunks)) {
-		ret = -EBUSY;
-		goto out_unlock;
-	} else if (heap->chunk_count >= heap->max_chunks) {
+	    heap->chunk_count >= heap->max_chunks) {
 		ret = -ENOMEM;
 		goto out_unlock;
 	}

drivers/gpu/drm/panthor/panthor_sched.c

@@ -1385,7 +1385,12 @@ static int group_process_tiler_oom(struct panthor_group *group, u32 cs_id)
 					pending_frag_count, &new_chunk_va);
 	}
 
-	if (ret && ret != -EBUSY) {
+	/* If the heap context doesn't have memory for us, we want to let the
+	 * FW try to reclaim memory by waiting for fragment jobs to land or by
+	 * executing the tiler OOM exception handler, which is supposed to
+	 * implement incremental rendering.
+	 */
+	if (ret && ret != -ENOMEM) {
 		drm_warn(&ptdev->base, "Failed to extend the tiler heap\n");
 		group->fatal_queues |= BIT(cs_id);
 		sched_queue_delayed_work(sched, tick, 0);