drm/msm: a5xx: Make preemption reset case reentrant

nr_rings is reset to 1, but when this function is called for a second
(and third!) time nr_rings > 1 is false, thus the else case is entered
to set up a buffer for the RPTR shadow and consequently written to
RB_RPTR_ADDR, hanging platforms without WHERE_AM_I firmware support.

Restructure the condition in such a way that shadow buffer setup only
ever happens when has_whereami is true; otherwise preemption is only
finalized when the number of ring buffers has not been reset to 1 yet.

Fixes: 8907afb4 ("drm/msm: Allow a5xx to mark the RPTR shadow as privileged")
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@somainline.org>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/adreno/a5xx_gpu.c

@@ -755,12 +755,8 @@ static int a5xx_hw_init(struct msm_gpu *gpu)
 	gpu_write(gpu, REG_A5XX_CP_RB_CNTL,
 		MSM_GPU_RB_CNTL_DEFAULT | AXXX_CP_RB_CNTL_NO_UPDATE);
 
-	/* Disable preemption if WHERE_AM_I isn't available */
-	if (!a5xx_gpu->has_whereami && gpu->nr_rings > 1) {
-		a5xx_preempt_fini(gpu);
-		gpu->nr_rings = 1;
-	} else {
 	/* Create a privileged buffer for the RPTR shadow */
+	if (a5xx_gpu->has_whereami) {
 		if (!a5xx_gpu->shadow_bo) {
 			a5xx_gpu->shadow = msm_gem_kernel_new(gpu->dev,
 				sizeof(u32) * gpu->nr_rings,
@@ -774,6 +770,10 @@ static int a5xx_hw_init(struct msm_gpu *gpu)
 
 		gpu_write64(gpu, REG_A5XX_CP_RB_RPTR_ADDR,
 			REG_A5XX_CP_RB_RPTR_ADDR_HI, shadowptr(a5xx_gpu, gpu->rb[0]));
+	} else if (gpu->nr_rings > 1) {
+		/* Disable preemption if WHERE_AM_I isn't available */
+		a5xx_preempt_fini(gpu);
+		gpu->nr_rings = 1;
 	}
 
 	a5xx_preempt_hw_init(gpu);