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

[Ville Syrjälä]

On Wed, Apr 04, 2018 at 02:37:41PM -0400, Lyude Paul wrote:
> On Wed, 2018-04-04 at 18:34 +0300, Ville Syrjälä 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 Syrjälä <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

And how exactly do we lose irqs? The hub/whatever throws the up req msgs
away if we don't read them within some really short time?

>  * 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

-- 
Ville Syrjälä
Intel OTC