FAILED: patch "[PATCH] drm/i915: Try EDID bitbanging on HDMI after failed read" failed to apply to 4.16-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.16-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

>From cfb926e148e99acc02351d72e8b85e32b5f786ef Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Stefan=20Br=C3=BCns?= <stefan.bruens@rwth-aachen.de>
Date: Sun, 31 Dec 2017 23:34:54 +0100
Subject: [PATCH] drm/i915: Try EDID bitbanging on HDMI after failed read
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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 Brüns <stefan.bruens@rwth-aachen.de>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/a39f080b-81a5-4c93-b3f7-7cb0a58daca3@rwthex-w2-a.rwth-ad.de

diff --git a/drivers/gpu/drm/i915/intel_hdmi.c b/drivers/gpu/drm/i915/intel_hdmi.c
index bced7b954d93..179d0ad3889d 100644
--- a/drivers/gpu/drm/i915/intel_hdmi.c
+++ b/drivers/gpu/drm/i915/intel_hdmi.c
@@ -1595,12 +1595,20 @@ intel_hdmi_set_edid(struct drm_connector *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);