drm/msm/a6xx: Clear shadow on suspend

Clear the shadow rptr on suspend.  Otherwise, when we resume, we can
have a stale value until CP_WHERE_AM_I executes.  If we suspend near
the ringbuffer wraparound point, this can lead to a chicken/egg
situation where we are waiting for ringbuffer space to write the
CP_WHERE_AM_I (or CP_INIT) packet, because we mistakenly believe that
the ringbuffer is full (due to stale rptr value in the shadow).

Fixes errors like:

  [drm:adreno_wait_ring [msm]] *ERROR* timeout waiting for space in ringbuffer 0

in the resume path.

Fixes: d3a569fc ("drm/msm: a6xx: Use WHERE_AM_I for eligible targets")
Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/adreno/a6xx_gpu.c

@@ -1043,12 +1043,21 @@ static int a6xx_pm_suspend(struct msm_gpu *gpu)
 {
 	struct adreno_gpu *adreno_gpu = to_adreno_gpu(gpu);
 	struct a6xx_gpu *a6xx_gpu = to_a6xx_gpu(adreno_gpu);
+	int i, ret;
 
 	trace_msm_gpu_suspend(0);
 
 	devfreq_suspend_device(gpu->devfreq.devfreq);
 
-	return a6xx_gmu_stop(a6xx_gpu);
+	ret = a6xx_gmu_stop(a6xx_gpu);
+	if (ret)
+		return ret;
+
+	if (adreno_gpu->base.hw_apriv || a6xx_gpu->has_whereami)
+		for (i = 0; i < gpu->nr_rings; i++)
+			a6xx_gpu->shadow[i] = 0;
+
+	return 0;
 }
 
 static int a6xx_get_timestamp(struct msm_gpu *gpu, uint64_t *value)