drm/sun4i: rgb: Change the pixel clock validation check

The current code, since commit bb43d40d ("drm/sun4i: rgb: Validate the
clock rate"), perform some validation on the pixel clock to filter out the
EDID modes provided by monitors (through bridges) that we wouldn't be able
to reach. For the usual modes, we're able to generate a perfect clock rate,
so a strict check was enough.

However, this had the side effect of preventing displays that would work
otherwise to operate properly, since we would pretty much never be able to
generate an exact rate for those displays, even though we would fall within
that panel tolerance.

This was also shown to happen for unusual modes exposed through EDIDs, for
example on eDP panels.

We can work around this by simplifying a bit the problem: no panels we've
encountered so far actually needed that check. All of them are tied to a
particular board when it is produced, and made to work with the Allwinner
BSP. That pretty much guarantees that we never have a pixel clock out of
reach.

On the other hand, the EDIDs modes that needed to be validated have always
been exposed through bridges.

Let's just use that metric to instead of validating all modes, only
validate modes when we have a bridge attached. It should be good enough for
now, while we still have room for improvements or refinements using the
display_timings structure for example for panels.

We also add a tolerance for EDID-based modes instead of doing a strict
check. This tolerance is of 0.5% which is the one advertised in the VESA
DVT and CVT specs. If that needed to be extended in the future, we can add
a custom module parameter to relax it a bit.

Fixes: bb43d40d ("drm/sun4i: rgb: Validate the clock rate")
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Vasily Khoruzhick <anarsoul@gmail.com> # tested on pinebook
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ec2dc2a7b3d4bd44f7a2a6e1c1813f92449a7310.1551191081.git-series.maxime.ripard@bootlin.com

drivers/gpu/drm/sun4i/sun4i_rgb.c

@@ -53,6 +53,14 @@ static int sun4i_rgb_get_modes(struct drm_connector *connector)
 	return drm_panel_get_modes(rgb->panel);
 }
 
+/*
+ * VESA DMT defines a tolerance of 0.5% on the pixel clock, while the
+ * CVT spec reuses that tolerance in its examples, so it looks to be a
+ * good default tolerance for the EDID-based modes. Define it to 5 per
+ * mille to avoid floating point operations.
+ */
+#define SUN4I_RGB_DOTCLOCK_TOLERANCE_PER_MILLE	5
+
 static enum drm_mode_status sun4i_rgb_mode_valid(struct drm_encoder *crtc,
 						 const struct drm_display_mode *mode)
 {
@@ -61,6 +69,7 @@ static enum drm_mode_status sun4i_rgb_mode_valid(struct drm_encoder *crtc,
 	u32 hsync = mode->hsync_end - mode->hsync_start;
 	u32 vsync = mode->vsync_end - mode->vsync_start;
 	unsigned long long rate = mode->clock * 1000;
+	unsigned long long lowest, highest;
 	unsigned long long rounded_rate;
 
 	DRM_DEBUG_DRIVER("Validating modes...\n");
@@ -93,15 +102,39 @@ static enum drm_mode_status sun4i_rgb_mode_valid(struct drm_encoder *crtc,
 
 	DRM_DEBUG_DRIVER("Vertical parameters OK\n");
 
+	/*
+	 * TODO: We should use the struct display_timing if available
+	 * and / or trying to stretch the timings within that
+	 * tolerancy to take care of panels that we wouldn't be able
+	 * to have a exact match for.
+	 */
+	if (rgb->panel) {
+		DRM_DEBUG_DRIVER("RGB panel used, skipping clock rate checks");
+		goto out;
+	}
+
+	/*
+	 * That shouldn't ever happen unless something is really wrong, but it
+	 * doesn't harm to check.
+	 */
+	if (!rgb->bridge)
+		goto out;
+
 	tcon->dclk_min_div = 6;
 	tcon->dclk_max_div = 127;
 	rounded_rate = clk_round_rate(tcon->dclk, rate);
-	if (rounded_rate < rate)
+
+	lowest = rate * (1000 - SUN4I_RGB_DOTCLOCK_TOLERANCE_PER_MILLE);
+	do_div(lowest, 1000);
+	if (rounded_rate < lowest)
 		return MODE_CLOCK_LOW;
 
-	if (rounded_rate > rate)
+	highest = rate * (1000 + SUN4I_RGB_DOTCLOCK_TOLERANCE_PER_MILLE);
+	do_div(highest, 1000);
+	if (rounded_rate > highest)
 		return MODE_CLOCK_HIGH;
 
+out:
 	DRM_DEBUG_DRIVER("Clock rate OK\n");
 
 	return MODE_OK;