[PATCH 3.16.y-ckt 054/133] DRM - radeon: Don't link train DisplayPort on HPD until we get the dpcd

From: Luis Henriques
Date: Wed Sep 30 2015 - 06:24:15 EST

3.16.7-ckt18 -stable review patch. If anyone has any objections, please let me know.


From: Stephen Chandler Paul <cpaul@xxxxxxxxxx>

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@xxxxxxxxxx>
Reviewed-by: Jerome Glisse <jglisse@xxxxxxxxxx>
Signed-off-by: Alex Deucher <alexander.deucher@xxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
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 eb76aa72466c..508eed9f8071 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.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/