drm/i915: factor out vlv_PLL_is_optimal

Factor out the logic to decide whether the newly calculated dividers are
better than the best found so far. Do this for clarity and to prepare
for the upcoming BXT helper needing the same.

No functional change.
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -776,6 +776,33 @@ g4x_find_best_dpll(const intel_limit_t *limit, struct intel_crtc *crtc,
 	return found;
 }
 
+/*
+ * Check if the calculated PLL configuration is more optimal compared to the
+ * best configuration and error found so far. Return the calculated error.
+ */
+static bool vlv_PLL_is_optimal(struct drm_device *dev, int target_freq,
+			       const intel_clock_t *calculated_clock,
+			       const intel_clock_t *best_clock,
+			       unsigned int best_error_ppm,
+			       unsigned int *error_ppm)
+{
+	*error_ppm = div_u64(1000000ULL *
+				abs(target_freq - calculated_clock->dot),
+			     target_freq);
+	/*
+	 * Prefer a better P value over a better (smaller) error if the error
+	 * is small. Ensure this preference for future configurations too by
+	 * setting the error to 0.
+	 */
+	if (*error_ppm < 100 && calculated_clock->p > best_clock->p) {
+		*error_ppm = 0;
+
+		return true;
+	}
+
+	return *error_ppm + 10 < best_error_ppm;
+}
+
 static bool
 vlv_find_best_dpll(const intel_limit_t *limit, struct intel_crtc *crtc,
 		   int target, int refclk, intel_clock_t *match_clock,
@@ -800,7 +827,7 @@ vlv_find_best_dpll(const intel_limit_t *limit, struct intel_crtc *crtc,
 				clock.p = clock.p1 * clock.p2;
 				/* based on hardware requirement, prefer bigger m1,m2 values */
 				for (clock.m1 = limit->m1.min; clock.m1 <= limit->m1.max; clock.m1++) {
-					unsigned int ppm, diff;
+					unsigned int ppm;
 
 					clock.m2 = DIV_ROUND_CLOSEST(target * clock.p * clock.n,
 								     refclk * clock.m1);
@@ -811,20 +838,15 @@ vlv_find_best_dpll(const intel_limit_t *limit, struct intel_crtc *crtc,
 								&clock))
 						continue;
 
-					diff = abs(clock.dot - target);
-					ppm = div_u64(1000000ULL * diff, target);
-
-					if (ppm < 100 && clock.p > best_clock->p) {
-						bestppm = 0;
-						*best_clock = clock;
-						found = true;
-					}
+					if (!vlv_PLL_is_optimal(dev, target,
+								&clock,
+								best_clock,
+								bestppm, &ppm))
+						continue;
 
-					if (bestppm >= 10 && ppm < bestppm - 10) {
-						bestppm = ppm;
-						*best_clock = clock;
-						found = true;
-					}
+					*best_clock = clock;
+					bestppm = ppm;
+					found = true;
 				}
 			}
 		}