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

[Alex Deucher]

On Fri, Aug 21, 2015 at 2:16 PM,  <cpaul at redhat.com> wrote:
> From: Stephen Chandler Paul <cpaul at redhat.com>
>
> 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>

Applied.  thanks!

Alex

> ---
>  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 94b21ae..5a2cafb 100644
> --- a/drivers/gpu/drm/radeon/radeon_connectors.c
> +++ b/drivers/gpu/drm/radeon/radeon_connectors.c
> @@ -95,6 +95,11 @@ 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.
> --
> 2.4.3
>
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