drm: rcar-du: Restrict DPLL duty cycle workaround to H3 ES1.x

The H3 ES1.x exhibits dot clock duty cycle stability issues. We can work
around them by configuring the DPLL to twice the desired frequency,
coupled with a /2 post-divider. This isn't needed on other SoCs and
breaks HDMI output on M3-W for a currently unknown reason, so restrict
the workaround to H3 ES1.x.

From an implementation point of view, move work around handling outside
of the rcar_du_dpll_divider() function by requesting a x2 DPLL output
frequency explicitly. The existing post-divider calculation mechanism
will then take care of dividing the clock by two automatically.

While at it, print a more useful debugging message to ease debugging
clock rate issues.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>

drivers/gpu/drm/rcar-du/rcar_du_crtc.c

@@ -13,6 +13,7 @@
 
 #include <linux/clk.h>
 #include <linux/mutex.h>
+#include <linux/sys_soc.h>
 
 #include <drm/drmP.h>
 #include <drm/drm_atomic.h>
@@ -129,10 +130,8 @@ static void rcar_du_dpll_divider(struct rcar_du_crtc *rcrtc,
 			for (fdpll = 1; fdpll < 32; fdpll++) {
 				unsigned long output;
 
-				/* 1/2 (FRQSEL=1) for duty rate 50% */
 				output = input * (n + 1) / (m + 1)
-				       / (fdpll + 1) / 2;
-
+				       / (fdpll + 1);
 				if (output >= 400000000)
 					continue;
 
@@ -158,6 +157,11 @@ done:
 		 best_diff);
 }
 
+static const struct soc_device_attribute rcar_du_r8a7795_es1[] = {
+	{ .soc_id = "r8a7795", .revision = "ES1.*" },
+	{ /* sentinel */ }
+};
+
 static void rcar_du_crtc_set_display_timing(struct rcar_du_crtc *rcrtc)
 {
 	const struct drm_display_mode *mode = &rcrtc->crtc.state->adjusted_mode;
@@ -186,7 +190,20 @@ static void rcar_du_crtc_set_display_timing(struct rcar_du_crtc *rcrtc)
 
 		extclk = clk_get_rate(rcrtc->extclock);
 		if (rcdu->info->dpll_ch & (1 << rcrtc->index)) {
-			rcar_du_dpll_divider(rcrtc, &dpll, extclk, mode_clock);
+			unsigned long target = mode_clock;
+
+			/*
+			 * The H3 ES1.x exhibits dot clock duty cycle stability
+			 * issues. We can work around them by configuring the
+			 * DPLL to twice the desired frequency, coupled with a
+			 * /2 post-divider. This isn't needed on other SoCs and
+			 * breaks HDMI output on M3-W for a currently unknown
+			 * reason, so restrict the workaround to H3 ES1.x.
+			 */
+			if (soc_device_match(rcar_du_r8a7795_es1))
+				target *= 2;
+
+			rcar_du_dpll_divider(rcrtc, &dpll, extclk, target);
 			extclk = dpll.output;
 		}
 
@@ -198,8 +215,6 @@ static void rcar_du_crtc_set_display_timing(struct rcar_du_crtc *rcrtc)
 
 		if (abs((long)extrate - (long)mode_clock) <
 		    abs((long)rate - (long)mode_clock)) {
-			dev_dbg(rcrtc->group->dev->dev,
-				"crtc%u: using external clock\n", rcrtc->index);
 
 			if (rcdu->info->dpll_ch & (1 << rcrtc->index)) {
 				u32 dpllcr = DPLLCR_CODE | DPLLCR_CLKE
@@ -216,12 +231,14 @@ static void rcar_du_crtc_set_display_timing(struct rcar_du_crtc *rcrtc)
 
 				rcar_du_group_write(rcrtc->group, DPLLCR,
 						    dpllcr);
-
-				escr = ESCR_DCLKSEL_DCLKIN | 1;
-			} else {
-				escr = ESCR_DCLKSEL_DCLKIN | extdiv;
 			}
+
+			escr = ESCR_DCLKSEL_DCLKIN | extdiv;
 		}
+
+		dev_dbg(rcrtc->group->dev->dev,
+			"mode clock %lu extrate %lu rate %lu ESCR 0x%08x\n",
+			mode_clock, extrate, rate, escr);
 	}
 
 	rcar_du_group_write(rcrtc->group, rcrtc->index % 2 ? ESCR2 : ESCR,