[PATCH 3.16 001/366] drm/i915: Try EDID bitbanging on HDMI after failed read

[Ben Hutchings]

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Stefan BrÃns <stefan.bruens@xxxxxxxxxxxxxx>

commit cfb926e148e99acc02351d72e8b85e32b5f786ef upstream.

The ACK/NACK implementation as found in e.g. the G965 has the falling
clock edge and the release of the data line after the ACK for the received
byte happen at the same time.

This is conformant with the I2C specification, which allows a zero hold
time, see footnote [3]: "A device must internally provide a hold time of
at least 300 ns for the SDA signal (with respect to the V IH(min) of the
SCL signal) to bridge the undefined region of the falling edge of SCL."

Some HDMI-to-VGA converters apparently fail to adhere to this requirement
and latch SDA at the falling clock edge, so instead of an ACK
sometimes a NACK is read and the slave (i.e. the EDID ROM) ends the
transfer.

The bitbanging releases the data line for the ACK only 1/4 bit time after
the falling clock edge, so a slave will see the correct value no matter
if it samples at the rising or the falling clock edge or in the center.

Fallback to bitbanging is already done for the CRT connector.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92685
Signed-off-by: Stefan BrÃns <stefan.bruens@xxxxxxxxxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/a39f080b-81a5-4c93-b3f7-7cb0a58daca3@xxxxxxxxxxxxxxxxxxxxxx
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 drivers/gpu/drm/i915/intel_hdmi.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

--- a/drivers/gpu/drm/i915/intel_hdmi.c
+++ b/drivers/gpu/drm/i915/intel_hdmi.c
@@ -971,6 +971,7 @@ intel_hdmi_detect(struct drm_connector *
 	struct edid *edid;
 	enum intel_display_power_domain power_domain;
 	enum drm_connector_status status = connector_status_disconnected;
+	struct i2c_adapter *i2c;
 
 	DRM_DEBUG_KMS("[CONNECTOR:%d:%s]\n",
 		      connector->base.id, connector->name);
@@ -981,9 +982,16 @@ intel_hdmi_detect(struct drm_connector *
 	intel_hdmi->has_hdmi_sink = false;
 	intel_hdmi->has_audio = false;
 	intel_hdmi->rgb_quant_range_selectable = false;
-	edid = drm_get_edid(connector,
-			    intel_gmbus_get_adapter(dev_priv,
-						    intel_hdmi->ddc_bus));
+	i2c = intel_gmbus_get_adapter(dev_priv, intel_hdmi->ddc_bus);
+
+	edid = drm_get_edid(connector, i2c);
+
+	if (!edid && !intel_gmbus_is_forced_bit(i2c)) {
+		DRM_DEBUG_KMS("HDMI GMBUS EDID read failed, retry using GPIO bit-banging\n");
+		intel_gmbus_force_bit(i2c, true);
+		edid = drm_get_edid(connector, i2c);
+		intel_gmbus_force_bit(i2c, false);
+	}
 
 	if (edid) {
 		if (edid->input & DRM_EDID_INPUT_DIGITAL) {