Re: [PATCH v2] drm/i915: Keep AUX block running when disabling DPMS for MST

[Lyude Paul]

On Wed, 2018-04-04 at 18:34 +0300, Ville SyrjÃlà wrote:
> On Mon, Apr 02, 2018 at 05:26:16PM -0400, Lyude Paul wrote:
> > While enabling/disabling DPMS before link training with MST hubs is
> > perfectly valid; unfortunately disabling DPMS results in some devices
> > disabling their AUX CH block as well. For SST this isn't as much of a
> > problem, but for MST we need to be able to continue handling aux
> > transactions even when none of the sinks are turned on since it's
> > possible for us to have a single atomic commit which results in
> > disabling each downstream sink, followed by subsequently re-enabling
> > each sink.
> > 
> > If we don't do this, we'll end up stalling any pending ESI interrupts
> > from the sink for up to 1ms. Unfortunately, dropping ESIs during this
> > timespan makes it so that link fallback retraining for MST (which I will
> > be submitting to the ML shortly) fails due to the channel EQ failure
> > interrupts potentially getting dropped. Additionally, when performing a
> > modeset that brings the hub status's link status from bad -> good having
> > ESIs disabled for that long causes us to miss the hub's response to us
> > trying to start link training as well.
> > 
> > Since any sink with MST is going to support DisplayPort 1.2 anyway, save
> > us the hassle of trying to wait until the sink comes back up and just
> > never shut the aux block down.
> > 
> > Changes since v2:
> >  - Fix patch name, no functional changes
> > 
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > Cc: Laura Abbott <labbott@xxxxxxxxxx>
> > Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@xxxxxxxxx>
> > Cc: Ville SyrjÃlà <ville.syrjala@xxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: ad260ab32a4d9 ("drm/i915/dp: Write to SET_POWER dpcd to enable MST
> > hub.")
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c
> > b/drivers/gpu/drm/i915/intel_dp.c
> > index 62f82c4298ac..0479c377981b 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -2589,11 +2589,13 @@ void intel_dp_sink_dpms(struct intel_dp *intel_dp,
> > int mode)
> >  		return;
> >  
> >  	if (mode != DRM_MODE_DPMS_ON) {
> > +		unsigned char data = intel_dp->is_mst ?
> > +			DP_SET_POWER_D3_AUX_ON : DP_SET_POWER_D3;
> 
> This smells like a workaround for an actual bug somewhere. Why exactly
> is the slower wakeup or the AUX block a problem for MST but not for SST
> when the link training is exactly the same for SST and MST?
I actually thought about this but I still think this is the appropriate fix.
So; the real reason for the wakeup not being a problem with SST is that for
DPMS on with SST, we actually do a wait to make sure that the hub is ready
before continuing. And yes: I'm fairly sure SST does actually have around the
same wakeup time that MST does, but with the wait we do it doesn't reallhy
make a difference. With MST, we could do this but there's a few reasons I
don't think we should:
 * We don't need to. D3_AUX_ON is a part of the 1.2 spec, so any hub that has
   MST is going to be guaranteed to have this.
 * Turning off the aux block means that there's a high chance we're going to
   miss ESIs from sinks
 * It's faster to keep the aux block on anyway


> 
> > +
> >  		if (downstream_hpd_needs_d0(intel_dp))
> >  			return;
> >  
> > -		ret = drm_dp_dpcd_writeb(&intel_dp->aux, DP_SET_POWER,
> > -					 DP_SET_POWER_D3);
> > +		ret = drm_dp_dpcd_writeb(&intel_dp->aux, DP_SET_POWER,
> > data);
> >  	} else {
> >  		struct intel_lspcon *lspcon = dp_to_lspcon(intel_dp);
> >  
> > -- 
> > 2.14.3
> 
> 
-- 
Cheers,
	Lyude Paul