drm/i915: Improve vlv_gpu_freq() and vlv_freq_opcode()

We're currently miscalculating the VLV graphics clock a little bit.
This is caused by rounding the step to integer MHz, which does not
match reality. Change the formula to match the GUnit HAS to give
more accurate answers.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -5947,57 +5947,46 @@ int sandybridge_pcode_write(struct drm_i915_private *dev_priv, u8 mbox, u32 val)
 
 int vlv_gpu_freq(int ddr_freq, int val)
 {
-	int mult, base;
+	int div;
 
+	/* 4 x czclk */
 	switch (ddr_freq) {
 	case 800:
-		mult = 20;
-		base = 120;
+		div = 10;
 		break;
 	case 1066:
-		mult = 22;
-		base = 133;
+		div = 12;
 		break;
 	case 1333:
-		mult = 21;
-		base = 125;
+		div = 16;
 		break;
 	default:
 		return -1;
 	}
 
-	return ((val - 0xbd) * mult) + base;
+	return DIV_ROUND_CLOSEST(ddr_freq * (val + 6 - 0xbd), 4 * div);
 }
 
 int vlv_freq_opcode(int ddr_freq, int val)
 {
-	int mult, base;
+	int mul;
 
+	/* 4 x czclk */
 	switch (ddr_freq) {
 	case 800:
-		mult = 20;
-		base = 120;
+		mul = 10;
 		break;
 	case 1066:
-		mult = 22;
-		base = 133;
+		mul = 12;
 		break;
 	case 1333:
-		mult = 21;
-		base = 125;
+		mul = 16;
 		break;
 	default:
 		return -1;
 	}
 
-	val /= mult;
-	val -= base / mult;
-	val += 0xbd;
-
-	if (val > 0xea)
-		val = 0xea;
-
-	return val;
+	return DIV_ROUND_CLOSEST(4 * mul * val, ddr_freq) + 0xbd - 6;
 }
 
 void intel_pm_init(struct drm_device *dev)