drm/radeon: rework finding display PLL numbers v2

This completely reworks how the PLL parameters are generated and
should result in better matching dot clock frequencies.

Probably needs quite a bit of testing.

bugs: https://bugs.freedesktop.org/show_bug.cgi?id=76564

v2: more cleanup and comments.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_display.c

@@ -34,6 +34,8 @@
 #include <drm/drm_crtc_helper.h>
 #include <drm/drm_edid.h>
 
+#include <linux/gcd.h>
+
 static void avivo_crtc_load_lut(struct drm_crtc *crtc)
 {
 	struct radeon_crtc *radeon_crtc = to_radeon_crtc(crtc);
@@ -799,66 +801,57 @@ int radeon_ddc_get_modes(struct radeon_connector *radeon_connector)
 }
 
 /* avivo */
-static void avivo_get_fb_div(struct radeon_pll *pll,
-			     u32 target_clock,
-			     u32 post_div,
-			     u32 ref_div,
-			     u32 *fb_div,
-			     u32 *frac_fb_div)
-{
-	u32 tmp = post_div * ref_div;
 
-	tmp *= target_clock;
-	*fb_div = tmp / pll->reference_freq;
-	*frac_fb_div = tmp % pll->reference_freq;
-
-        if (*fb_div > pll->max_feedback_div)
-		*fb_div = pll->max_feedback_div;
-        else if (*fb_div < pll->min_feedback_div)
-                *fb_div = pll->min_feedback_div;
-}
-
-static u32 avivo_get_post_div(struct radeon_pll *pll,
-			      u32 target_clock)
+/**
+ * avivo_reduce_ratio - fractional number reduction
+ *
+ * @nom: nominator
+ * @den: denominator
+ * @nom_min: minimum value for nominator
+ * @den_min: minimum value for denominator
+ *
+ * Find the greatest common divisor and apply it on both nominator and
+ * denominator, but make nominator and denominator are at least as large
+ * as their minimum values.
+ */
+static void avivo_reduce_ratio(unsigned *nom, unsigned *den,
+			       unsigned nom_min, unsigned den_min)
 {
-	u32 vco, post_div, tmp;
+	unsigned tmp;
 
-	if (pll->flags & RADEON_PLL_USE_POST_DIV)
-		return pll->post_div;
+	/* reduce the numbers to a simpler ratio */
+	tmp = gcd(*nom, *den);
+	*nom /= tmp;
+	*den /= tmp;
 
-	if (pll->flags & RADEON_PLL_PREFER_MINM_OVER_MAXP) {
-		if (pll->flags & RADEON_PLL_IS_LCD)
-			vco = pll->lcd_pll_out_min;
-		else
-			vco = pll->pll_out_min;
-	} else {
-		if (pll->flags & RADEON_PLL_IS_LCD)
-			vco = pll->lcd_pll_out_max;
-		else
-			vco = pll->pll_out_max;
+	/* make sure nominator is large enough */
+        if (*nom < nom_min) {
+		tmp = (nom_min + *nom - 1) / *nom;
+		*nom *= tmp;
+		*den *= tmp;
 	}
 
-	post_div = vco / target_clock;
-	tmp = vco % target_clock;
-
-	if (pll->flags & RADEON_PLL_PREFER_MINM_OVER_MAXP) {
-		if (tmp)
-			post_div++;
-	} else {
-		if (!tmp)
-			post_div--;
+	/* make sure the denominator is large enough */
+	if (*den < den_min) {
+		tmp = (den_min + *den - 1) / *den;
+		*nom *= tmp;
+		*den *= tmp;
 	}
-
-	if (post_div > pll->max_post_div)
-		post_div = pll->max_post_div;
-	else if (post_div < pll->min_post_div)
-		post_div = pll->min_post_div;
-
-	return post_div;
 }
 
-#define MAX_TOLERANCE 10
-
+/**
+ * radeon_compute_pll_avivo - compute PLL paramaters
+ *
+ * @pll: information about the PLL
+ * @dot_clock_p: resulting pixel clock
+ * fb_div_p: resulting feedback divider
+ * frac_fb_div_p: fractional part of the feedback divider
+ * ref_div_p: resulting reference divider
+ * post_div_p: resulting reference divider
+ *
+ * Try to calculate the PLL parameters to generate the given frequency:
+ * dot_clock = (ref_freq * feedback_div) / (ref_div * post_div)
+ */
 void radeon_compute_pll_avivo(struct radeon_pll *pll,
 			      u32 freq,
 			      u32 *dot_clock_p,
@@ -867,53 +860,123 @@ void radeon_compute_pll_avivo(struct radeon_pll *pll,
 			      u32 *ref_div_p,
 			      u32 *post_div_p)
 {
-	u32 target_clock = freq / 10;
-	u32 post_div = avivo_get_post_div(pll, target_clock);
-	u32 ref_div = pll->min_ref_div;
-	u32 fb_div = 0, frac_fb_div = 0, tmp;
+	unsigned fb_div_min, fb_div_max, fb_div;
+	unsigned post_div_min, post_div_max, post_div;
+	unsigned ref_div_min, ref_div_max, ref_div;
+	unsigned post_div_best, diff_best;
+	unsigned nom, den, tmp;
 
-	if (pll->flags & RADEON_PLL_USE_REF_DIV)
-		ref_div = pll->reference_div;
+	/* determine allowed feedback divider range */
+	fb_div_min = pll->min_feedback_div;
+	fb_div_max = pll->max_feedback_div;
 
 	if (pll->flags & RADEON_PLL_USE_FRAC_FB_DIV) {
-		avivo_get_fb_div(pll, target_clock, post_div, ref_div, &fb_div, &frac_fb_div);
-		frac_fb_div = (100 * frac_fb_div) / pll->reference_freq;
-		if (frac_fb_div >= 5) {
-			frac_fb_div -= 5;
-			frac_fb_div = frac_fb_div / 10;
-			frac_fb_div++;
-		}
-		if (frac_fb_div >= 10) {
-			fb_div++;
-			frac_fb_div = 0;
+		fb_div_min *= 10;
+		fb_div_max *= 10;
 	}
+
+	/* determine allowed ref divider range */
+	if (pll->flags & RADEON_PLL_USE_REF_DIV)
+		ref_div_min = pll->reference_div;
+	else
+		ref_div_min = pll->min_ref_div;
+	ref_div_max = pll->max_ref_div;
+
+	/* determine allowed post divider range */
+	if (pll->flags & RADEON_PLL_USE_POST_DIV) {
+		post_div_min = pll->post_div;
+		post_div_max = pll->post_div;
 	} else {
-		while (ref_div <= pll->max_ref_div) {
-			avivo_get_fb_div(pll, target_clock, post_div, ref_div,
-					 &fb_div, &frac_fb_div);
-			if (frac_fb_div >= (pll->reference_freq / 2))
-				fb_div++;
-			frac_fb_div = 0;
-			tmp = (pll->reference_freq * fb_div) / (post_div * ref_div);
-			tmp = (tmp * 10000) / target_clock;
-
-			if (tmp > (10000 + MAX_TOLERANCE))
-				ref_div++;
-			else if (tmp >= (10000 - MAX_TOLERANCE))
-				break;
+		unsigned target_clock = freq / 10;
+		unsigned vco_min, vco_max;
+
+		if (pll->flags & RADEON_PLL_IS_LCD) {
+			vco_min = pll->lcd_pll_out_min;
+			vco_max = pll->lcd_pll_out_max;
+		} else {
+			vco_min = pll->pll_out_min;
+			vco_max = pll->pll_out_max;
+		}
+
+		post_div_min = vco_min / target_clock;
+		if ((target_clock * post_div_min) < vco_min)
+			++post_div_min;
+		if (post_div_min < pll->min_post_div)
+			post_div_min = pll->min_post_div;
+
+		post_div_max = vco_max / target_clock;
+		if ((target_clock * post_div_max) > vco_max)
+			--post_div_max;
+		if (post_div_max > pll->max_post_div)
+			post_div_max = pll->max_post_div;
+	}
+
+	/* represent the searched ratio as fractional number */
+	nom = pll->flags & RADEON_PLL_USE_FRAC_FB_DIV ? freq : freq / 10;
+	den = pll->reference_freq;
+
+	/* reduce the numbers to a simpler ratio */
+	avivo_reduce_ratio(&nom, &den, fb_div_min, post_div_min);
+
+	/* now search for a post divider */
+	if (pll->flags & RADEON_PLL_PREFER_MINM_OVER_MAXP)
+		post_div_best = post_div_min;
 	else
-				ref_div++;
+		post_div_best = post_div_max;
+	diff_best = ~0;
+
+	for (post_div = post_div_min; post_div <= post_div_max; ++post_div) {
+		unsigned diff = abs(den - den / post_div * post_div);
+		if (diff < diff_best || (diff == diff_best &&
+		    !(pll->flags & RADEON_PLL_PREFER_MINM_OVER_MAXP))) {
+
+			post_div_best = post_div;
+			diff_best = diff;
 		}
 	}
+	post_div = post_div_best;
 
-	*dot_clock_p = ((pll->reference_freq * fb_div * 10) + (pll->reference_freq * frac_fb_div)) /
-		(ref_div * post_div * 10);
+	/* get matching reference and feedback divider */
+	ref_div = max(den / post_div, 1u);
+	fb_div = nom;
+
+	/* we're almost done, but reference and feedback
+	   divider might be to large now */
+
+	tmp = ref_div;
+
+        if (fb_div > fb_div_max) {
+		ref_div = ref_div * fb_div_max / fb_div;
+		fb_div = fb_div_max;
+	}
+
+	if (ref_div > ref_div_max) {
+		ref_div = ref_div_max;
+		fb_div = nom * ref_div_max / tmp;
+	}
+
+	/* reduce the numbers to a simpler ratio once more */
+	/* this also makes sure that the reference divider is large enough */
+	avivo_reduce_ratio(&fb_div, &ref_div, fb_div_min, ref_div_min);
+
+	/* and finally save the result */
+	if (pll->flags & RADEON_PLL_USE_FRAC_FB_DIV) {
+		*fb_div_p = fb_div / 10;
+		*frac_fb_div_p = fb_div % 10;
+	} else {
 		*fb_div_p = fb_div;
-	*frac_fb_div_p = frac_fb_div;
+		*frac_fb_div_p = 0;
+	}
+
+	*dot_clock_p = ((pll->reference_freq * *fb_div_p * 10) +
+			(pll->reference_freq * *frac_fb_div_p)) /
+		       (ref_div * post_div * 10);
 	*ref_div_p = ref_div;
 	*post_div_p = post_div;
-	DRM_DEBUG_KMS("%d, pll dividers - fb: %d.%d ref: %d, post %d\n",
-		      *dot_clock_p, fb_div, frac_fb_div, ref_div, post_div);
+
+	DRM_DEBUG_KMS("%d - %d, pll dividers - fb: %d.%d ref: %d, post %d\n",
+		      freq, *dot_clock_p, *fb_div_p, *frac_fb_div_p,
+		      ref_div, post_div);
 }
 
 /* pre-avivo */