[Intel-gfx] [PATCH] drm/i915/ddi: Avoid long delays during system suspend / eDP disabling

[Imre Deak]

On Thu, Jun 01, 2017 at 04:58:50PM +0300, Ville Syrjälä wrote:
> On Thu, Jun 01, 2017 at 03:55:13PM +0300, Jani Nikula wrote:
> > On Wed, 31 May 2017, Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:
> > > On Wed, May 31, 2017 at 08:05:35PM +0300, Imre Deak wrote:
> > >> Atm disabling either DP or eDP outputs can generate a spurious short
> > >> pulse interrupt. The reason is that after disabling the port the source
> > >> will stop sending a valid stream data, while the sink expects either a
> > >> valid stream or the idle pattern. Since neither of this is sent the sink
> > >> assumes (after an arbitrary delay) that the link is lost and requests
> > >> for link retraining with a short pulse.
> > >> 
> > >> The spurious pulse is a real problem at least for eDP panels with long
> > >> power-off / power-cycle delays: as part of disabling the output we
> > >> disable the panel power. The subsequent spurious short pulse handling
> > >> will have to turn the power back on, which means the driver has to do a
> > >> redundant wait for the power-off and power-cycle delays. During system
> > >> suspend this leads to an unnecessary delay up to ~1s on systems with
> > >> such panels as reported by Rui.
> > >> 
> > >> To fix this put the sink to DPMS D3 state before turning off the port.
> > >> According to the DP spec in this state the sink should not request
> > >> retraining. This is also what we do already on pre-ddi platforms.
> > >> 
> > >> As an alternative I also tried configuring the port to send idle pattern
> > >> - which is against BSPec - and leave the port in normal mode before
> > >> turning off the port. Neither of these resolved the problem.
> > >> 
> > >> Cc: Zhang Rui <rui.zhang at intel.com>
> > >> Cc: David Weinehall <david.weinehall at linux.intel.com>
> > >> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > >> Reported-and-tested-by: Zhang Rui <rui.zhang at intel.com>
> > >> Signed-off-by: Imre Deak <imre.deak at intel.com>
> > >
> > > Makes sense to me.
> > 
> > I wonder if we should write D0 on hotplug.
> 
> D0 is the default power state IIRC, so when you plug something in it
> should automagically go into D0. That's actually a slight problem
> power-wise if there's no subsequent modeset to drop it into D3.

There was this DP->VGA adaptor that seems to require that we set D0
explicitly:
https://bugs.freedesktop.org/show_bug.cgi?id=99114#c5

But it's an odd behaviour, the DPCD read itself succeeds, only reading a
certain register fails (DPCD_SINK_COUNT). Based on the DP spec a sink
should respond to AUX transfers even after being put to D3, possibly
with a longer wake-up delay (up to 20ms AFAIR).

--Imre