drm/sun4i: validate modes for HDMI

When I connected my cubieboard running 4.15-rc1 to my 4k display I got no
picture. Some digging found that there is no check against the upper
pixelclock limit of the HDMI output, so X selects a 4kp60 format at 594
MHz, which obviously won't work.

The patch below adds a check for the upper bound of what this hardware can
do, and it checks if the requested tmds clock can be obtained.

It also allows for the +/- 0.5% pixel clock variation that the HDMI spec permits.

That code is based on commit 22d0be2a557e ("drm: arcpgu: Allow some clock
deviation in crtc->mode_valid() callback") from Jose Abreu for drm/arc.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Thanks-to: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/162854cb-c7bd-d9ce-9fa0-9a6cd89c621b@xs4all.nl

drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c

@@ -208,8 +208,27 @@ static int sun4i_hdmi_get_modes(struct drm_connector *connector)
 	return ret;
 }
 
+static int sun4i_hdmi_mode_valid(struct drm_connector *connector,
+				 struct drm_display_mode *mode)
+{
+	struct sun4i_hdmi *hdmi = drm_connector_to_sun4i_hdmi(connector);
+	long rate = mode->clock * 1000;
+	long diff = rate / 200; /* +-0.5% allowed by HDMI spec */
+	long rounded_rate;
+
+	/* 165 MHz is the typical max pixelclock frequency for HDMI <= 1.2 */
+	if (rate > 165000000)
+		return MODE_CLOCK_HIGH;
+	rounded_rate = clk_round_rate(hdmi->tmds_clk, rate);
+	if (max(rounded_rate, rate) - min(rounded_rate, rate) < diff &&
+	    rounded_rate > 0)
+		return MODE_OK;
+	return MODE_NOCLOCK;
+}
+
 static const struct drm_connector_helper_funcs sun4i_hdmi_connector_helper_funcs = {
 	.get_modes	= sun4i_hdmi_get_modes,
+	.mode_valid	= sun4i_hdmi_mode_valid,
 };
 
 static enum drm_connector_status