[PATCHv3] drm/sun4i: validate modes for HDMI

[Hans Verkuil]

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 22d0be2a557e53a22feb484e8fce255fe09e6ad5 from
Jose Abreu for drm/arc.

Signed-off-by: Hans Verkuil <hans.verkuil at cisco.com>
Thanks-to: Jose Abreu <Jose.Abreu at synopsys.com>
---
Changes since v2:
- Allow for the 0.5% variation.
---
 drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
index dda904ec0534..a7e9eb93c378 100644
--- a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c
+++ b/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);
+	unsigned 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
-- 
2.15.0