Patch "drm/i915: Try EDID bitbanging on HDMI after failed read" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

drm/i915: Try EDID bitbanging on HDMI after failed read

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drm-i915-try-edid-bitbanging-on-hdmi-after-failed-read.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: =?UTF-8?q?Stefan=20Br=C3=BCns?= <[email protected]>
Date: Sun, 31 Dec 2017 23:34:54 +0100
Subject: drm/i915: Try EDID bitbanging on HDMI after failed read
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From: Stefan Brüns <[email protected]>

commit 90024a5951029685acc5396258f1b0de9b23cf4a 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

---
 drivers/gpu/drm/i915/intel_hdmi.c |   14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

@@ -1573,12 +1573,20 @@ intel_hdmi_set_edid(struct drm_connector
 	struct intel_hdmi *intel_hdmi = intel_attached_hdmi(connector);
 	struct edid *edid;
 	bool connected = false;
+	struct i2c_adapter *i2c;
 
 	intel_display_power_get(dev_priv, POWER_DOMAIN_GMBUS);
 
-	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);
+	}
 
 	intel_hdmi_dp_dual_mode_detect(connector, edid != NULL);
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/drm-i915-try-edid-bitbanging-on-hdmi-after-failed-read.patch