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

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Date:	Mon, 24 Aug 2015 09:47:41 -0400
From:	Jerome Glisse <j.glisse@...il.com>
To:	cpaul@...hat.com
Cc:	Alex Deucher <alexander.deucher@....com>,
	Christian König <christian.koenig@....com>,
	David Airlie <airlied@...ux.ie>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	Jerome Glisse <jglisse@...hat.com>,
	Benjamin Tissoires <benjamin.tissoires@...hat.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH] DRM - radeon: Don't link train DisplayPort on HPD until
 we get the dpcd

On Fri, Aug 21, 2015 at 02:16:12PM -0400, cpaul@...hat.com wrote:
> From: Stephen Chandler Paul <cpaul@...hat.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@...hat.com>
> Reviewed-by: Jerome Glisse <jglisse@...hat.com>

This should be added to stable

cc: <stable@...r.kernel.org>



> ---
>  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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