drm/msm/a6xx: Add a helper for software-resetting the GPU

Introduce a6xx_gpu_sw_reset() in preparation for adding GMU wrapper
GPUs and reuse it in a6xx_gmu_force_off().

This helper, contrary to the original usage in GMU code paths, adds
a readback+delay sequence to ensure that the reset is never deasserted
too quickly due to e.g. OoO execution going crazy.
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/542758/Signed-off-by: Rob Clark <robdclark@chromium.org>

drivers/gpu/drm/msm/adreno/a6xx_gmu.c

@@ -899,8 +899,7 @@ static void a6xx_gmu_force_off(struct a6xx_gmu *gmu)
 	a6xx_bus_clear_pending_transactions(adreno_gpu, true);
 
 	/* Reset GPU core blocks */
-	gpu_write(gpu, REG_A6XX_RBBM_SW_RESET_CMD, 1);
-	udelay(100);
+	a6xx_gpu_sw_reset(gpu, true);
 }
 
 static void a6xx_gmu_set_initial_freq(struct msm_gpu *gpu, struct a6xx_gmu *gmu)

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

@@ -1742,6 +1742,18 @@ void a6xx_bus_clear_pending_transactions(struct adreno_gpu *adreno_gpu, bool gx_
 	gpu_write(gpu, REG_A6XX_GBIF_HALT, 0x0);
 }
 
+void a6xx_gpu_sw_reset(struct msm_gpu *gpu, bool assert)
+{
+	gpu_write(gpu, REG_A6XX_RBBM_SW_RESET_CMD, assert);
+	/* Perform a bogus read and add a brief delay to ensure ordering. */
+	gpu_read(gpu, REG_A6XX_RBBM_SW_RESET_CMD);
+	udelay(1);
+
+	/* The reset line needs to be asserted for at least 100 us */
+	if (assert)
+		udelay(100);
+}
+
 static int a6xx_pm_resume(struct msm_gpu *gpu)
 {
 	struct adreno_gpu *adreno_gpu = to_adreno_gpu(gpu);

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

@@ -89,5 +89,6 @@ struct msm_gpu_state *a6xx_gpu_state_get(struct msm_gpu *gpu);
 int a6xx_gpu_state_put(struct msm_gpu_state *state);
 
 void a6xx_bus_clear_pending_transactions(struct adreno_gpu *adreno_gpu, bool gx_off);
+void a6xx_gpu_sw_reset(struct msm_gpu *gpu, bool assert);
 
 #endif /* __A6XX_GPU_H__ */