drm/i915/gt: Add delay to let engine resumes properly

Sometimes engine reset fails because the engine resumes from an
incorrect RING_HEAD. Engine head failed to set to zero even after
writing into it. This is a timing issue and we experimented
different values and found out that 20ms delay works best based
on testing.

So, add a 20ms delay to let engine resumes from correct RING_HEAD.

Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/13968
Signed-off-by: Nitin Gote <nitin.r.gote@intel.com>
Reviewed-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Link: https://lore.kernel.org/r/20250416103640.212269-1-nitin.r.gote@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

Author: NitinGote

drivers/gpu/drm/i915/gt/intel_ring_submission.c

@@ -365,7 +365,13 @@ static void reset_prepare(struct intel_engine_cs *engine)
 			     ENGINE_READ_FW(engine, RING_HEAD),
 			     ENGINE_READ_FW(engine, RING_TAIL),
 			     ENGINE_READ_FW(engine, RING_START));
-		if (!stop_ring(engine)) {
+		/*
+		 * Sometimes engine head failed to set to zero even after writing into it.
+		 * Use wait_for_atomic() with 20ms delay to let engine resumes from
+		 * correct RING_HEAD. Experimented different values and determined
+		 * that 20ms works best based on testing.
+		 */
+		if (wait_for_atomic((!stop_ring(engine) == 0), 20)) {
 			drm_err(&engine->i915->drm,
 				"failed to set %s head to zero "
 				"ctl %08x head %08x tail %08x start %08x\n",