[3.19.y-ckt stable] Patch "DRM - radeon: Don't link train DisplayPort on HPD until we get the dpcd" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    DRM - radeon: Don't link train DisplayPort on HPD until we get the dpcd

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt8.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From ebf9d650c0bdf36456dd4a32e43bae4cbe5ba465 Mon Sep 17 00:00:00 2001
From: Stephen Chandler Paul <cpaul at redhat.com>
Date: Fri, 21 Aug 2015 14:16:12 -0400
Subject: DRM - radeon: Don't link train DisplayPort on HPD until we get the
 dpcd

commit 924f92bf12bfbef3662619e3ed24a1cea7c1cbcd upstream.

Most of the time this isn't an issue since hotplugging an adaptor will
trigger a crtc mode change which in turn, causes the driver to probe
every DisplayPort for a dpcd. However, in cases where hotplugging
doesn't cause a mode change (specifically when one unplugs a monitor
from a DisplayPort connector, then plugs that same monitor back in
seconds later on the same port without any other monitors connected), we
never probe for the dpcd before starting the initial link training. What
happens from there looks like this:

	- GPU has only one monitor connected. It's connected via
	  DisplayPort, and does not go through an adaptor of any sort.

	- User unplugs DisplayPort connector from GPU.

	- Change in HPD is detected by the driver, we probe every
	  DisplayPort for a possible connection.

	- Probe the port the user originally had the monitor connected
	  on for it's dpcd. This fails, and we clear the first (and only
	  the first) byte of the dpcd to indicate we no longer have a
	  dpcd for this port.

	- User plugs the previously disconnected monitor back into the
	  same DisplayPort.

	- radeon_connector_hotplug() is called before everyone else,
	  and tries to handle the link training. Since only the first
	  byte of the dpcd is zeroed, the driver is able to complete
	  link training but does so against the wrong dpcd, causing it
	  to initialize the link with the wrong settings.

	- Display stays blank (usually), dpcd is probed after the
	  initial link training, and the driver prints no obvious
	  messages to the log.

In theory, since only one byte of the dpcd is chopped off (specifically,
the byte that contains the revision information for DisplayPort), it's
not entirely impossible that this bug may not show on certain monitors.
For instance, the only reason this bug was visible on my ASUS PB238
monitor was due to the fact that this monitor using the enhanced framing
symbol sequence, the flag for which is ignored if the radeon driver
thinks that the DisplayPort version is below 1.1.

Signed-off-by: Stephen Chandler Paul <cpaul at redhat.com>
Reviewed-by: Jerome Glisse <jglisse at redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher at amd.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/gpu/drm/radeon/radeon_connectors.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/radeon/radeon_connectors.c b/drivers/gpu/drm/radeon/radeon_connectors.c
index 26baa9c..15f0906 100644
--- a/drivers/gpu/drm/radeon/radeon_connectors.c
+++ b/drivers/gpu/drm/radeon/radeon_connectors.c
@@ -72,6 +72,11 @@ void radeon_connector_hotplug(struct drm_connector *connector)
 			if (!radeon_hpd_sense(rdev, radeon_connector->hpd.hpd)) {
 				drm_helper_connector_dpms(connector, DRM_MODE_DPMS_OFF);
 			} else if (radeon_dp_needs_link_train(radeon_connector)) {
+				/* Don't try to start link training before we
+				 * have the dpcd */
+				if (!radeon_dp_getdpcd(radeon_connector))
+					return;
+
 				/* set it to OFF so that drm_helper_connector_dpms()
 				 * won't return immediately since the current state
 				 * is ON at this point.
--
1.9.1