On Thu, 16 Feb 2017 07:30:07 PST (-0800), ville.syrjala@linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>
> Apparently some DP sinks are a little nuts and cause HPD to drop
> intermittently during modesets. This happens eg. on an ASUS PB287Q.
> In oder to recover from this we can't really use the previous
> connector status to determine if the link needs retraining, so let's
> just ignore that piece of information and do the retrain
> unconditionally. We do of course still check whether the link is
> supposed to be running or not.

With this patch on top of linux-4.9 I have DPMS on/off working again, but I
don't have the correct resolutions on my monitor.  I just see

DP1 connected 1024x768+2880+0 (normal left inverted right x axis y axis) 0mm x 0mm
   1024x768      60.00*
   800x600       60.32    56.25
   848x480       60.00
   640x480       59.94

>
> Cc: stable@vger.kernel.org
> Cc: Palmer Dabbelt <palmer@dabbelt.com>
> Reported-by: Palmer Dabbelt <palmer@dabbelt.com>
> References: https://lists.freedesktop.org/archives/intel-gfx/2017-February/119779.html
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 024798a9c016..37a746f7fbc3 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -4648,11 +4648,18 @@ intel_dp_long_pulse(struct intel_connector *intel_connector)
>  		 */
>  		status = connector_status_disconnected;
>  		goto out;
> -	} else if (connector->status == connector_status_connected) {
> +	} else {
>  		/*
> -		 * If display was connected already and is still connected
> -		 * check links status, there has been known issues of
> -		 * link loss triggerring long pulse!!!!
> +		 * If display is now connected check links status,
> +		 * there has been known issues of link loss triggerring
> +		 * long pulse.
> +		 *
> +		 * Some sinks (eg. ASUS PB287Q) seem to perform some
> +		 * weird HPD ping pong during modesets. So we can apparely
> +		 * end up with HPD going low during a modeset, and then
> +		 * going back up soon after. And once that happens we must
> +		 * retrain the link to get a picture. That's in case no
> +		 * userspace component reacted to intermittent HPD dip.
>  		 */
>  		drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
>  		intel_dp_check_link_status(intel_dp);