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.
Signed-off-by: Hans Verkuil hans.verkuil@cisco.com --- diff --git a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c index 6d19d2ac68d1..62d52102d0f7 100644 --- a/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c +++ b/drivers/gpu/drm/sun4i/sun4i_hdmi_enc.c @@ -211,7 +211,17 @@ static int sun4i_hdmi_get_modes(struct drm_connector *connector) static int sun4i_hdmi_mode_valid(struct drm_connector *connector, struct drm_display_mode *mode) { - if (mode->clock > 170000) + struct sun4i_hdmi *hdmi = drm_connector_to_sun4i_hdmi(connector); + unsigned long rate = mode->clock * 1000; + 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 (rounded_rate < rate) + return MODE_CLOCK_LOW; + if (rounded_rate > rate) return MODE_CLOCK_HIGH; return MODE_OK; }