drm/msm: a5xx: Fix improper u64 division

This patch uses the proper do_div() macro to perform u64 division and
guards against overflow if the result is too large for the unsigned long
return type

Fixes: de0a3d09 drm/msm: re-factor devfreq code
Cc: Sharat Masetty <smasetty@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

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

@@ -1438,18 +1438,20 @@ static struct msm_ringbuffer *a5xx_active_ring(struct msm_gpu *gpu)
 
 static unsigned long a5xx_gpu_busy(struct msm_gpu *gpu)
 {
-	u64 busy_cycles;
-	unsigned long busy_time;
+	u64 busy_cycles, busy_time;
 
 	busy_cycles = gpu_read64(gpu, REG_A5XX_RBBM_PERFCTR_RBBM_0_LO,
 			REG_A5XX_RBBM_PERFCTR_RBBM_0_HI);
 
-	busy_time = (busy_cycles - gpu->devfreq.busy_cycles) /
-		(clk_get_rate(gpu->core_clk) / 1000000);
+	busy_time = (busy_cycles - gpu->devfreq.busy_cycles);
+	do_div(busy_time, (clk_get_rate(gpu->core_clk) / 1000000));
 
 	gpu->devfreq.busy_cycles = busy_cycles;
 
-	return busy_time;
+	if (WARN_ON(busy_time > ~0LU))
+		return ~0LU;
+
+	return (unsigned long)busy_time;
 }
 
 static const struct adreno_gpu_funcs funcs = {