drm/msm: a6xx: 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: a2c3c0a5 drm/msm/a6xx: Add devfreq support for a6xx
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/a6xx_gpu.c

@@ -761,18 +761,21 @@ static unsigned long a6xx_gpu_busy(struct msm_gpu *gpu)
 {
 	struct adreno_gpu *adreno_gpu = to_adreno_gpu(gpu);
 	struct a6xx_gpu *a6xx_gpu = to_a6xx_gpu(adreno_gpu);
-	u64 busy_cycles;
-	unsigned long busy_time;
+	u64 busy_cycles, busy_time;
 
 	busy_cycles = gmu_read64(&a6xx_gpu->gmu,
 			REG_A6XX_GMU_CX_GMU_POWER_COUNTER_XOCLK_0_L,
 			REG_A6XX_GMU_CX_GMU_POWER_COUNTER_XOCLK_0_H);
 
-	busy_time = ((busy_cycles - gpu->devfreq.busy_cycles) * 10) / 192;
+	busy_time = (busy_cycles - gpu->devfreq.busy_cycles) * 10;
+	do_div(busy_time, 192);
 
 	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 = {