[PATCH v7 3/5] drm/i915: Check pixel rate for DP to VGA dongle

[Mika Kahola]

Filter out a mode that exceeds the max pixel rate setting
for DP to VGA dongle. This is defined in DPCD register 0x81
if detailed cap info i.e. info field is 4 bytes long and
it is available for DP downstream port.

The register defines the pixel rate divided by 8 in MP/s.

v2: DPCD read outs and computation moved to drm (Ville, Daniel)
v3: Sink pixel rate computation moved to drm_dp_max_sink_dotclock()
    function (Daniel)
v4: Use of drm_dp_helper.c routines to compute max pixel clock (Ville)
v5: Use of intel_dp->downstream_ports to read out port capabilities.
    Code restructuring (Ville)
v6: Move DP branch device check to drm_dp_helper.c (Daniel)

Signed-off-by: Mika Kahola <mika.kahola at intel.com>
---
 drivers/gpu/drm/i915/intel_dp.c | 25 ++++++++++++++++++++++++-
 1 file changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
index 21b04c3..e990c8b 100644
--- a/drivers/gpu/drm/i915/intel_dp.c
+++ b/drivers/gpu/drm/i915/intel_dp.c
@@ -190,6 +190,26 @@ intel_dp_max_data_rate(int max_link_clock, int max_lanes)
 	return (max_link_clock * max_lanes * 8) / 10;
 }
 
+static int
+intel_dp_downstream_max_dotclock(struct intel_dp *intel_dp, int dotclk)
+{
+	int ds_dotclk;
+	int type;
+
+	ds_dotclk = drm_dp_downstream_max_clock(intel_dp->dpcd,
+						intel_dp->downstream_ports);
+
+	if (ds_dotclk == 0)
+		return dotclk;
+
+	type = intel_dp->downstream_ports[0] & DP_DS_PORT_TYPE_MASK;
+
+	if (type != DP_DS_PORT_TYPE_VGA)
+		return dotclk;
+
+	return min(dotclk, ds_dotclk);
+}
+
 static enum drm_mode_status
 intel_dp_mode_valid(struct drm_connector *connector,
 		    struct drm_display_mode *mode)
@@ -199,7 +219,10 @@ intel_dp_mode_valid(struct drm_connector *connector,
 	struct drm_display_mode *fixed_mode = intel_connector->panel.fixed_mode;
 	int target_clock = mode->clock;
 	int max_rate, mode_rate, max_lanes, max_link_clock;
-	int max_dotclk = to_i915(connector->dev)->max_dotclk_freq;
+	int max_dotclk;
+
+	max_dotclk = intel_dp_downstream_max_dotclock(intel_dp,
+						      to_i915(connector->dev)->max_dotclk_freq);
 
 	if (is_edp(intel_dp) && fixed_mode) {
 		if (mode->hdisplay > fixed_mode->hdisplay)
-- 
1.9.1