[Intel-gfx] [PATCH] drm/i915: Don't recheck link status for eDP display.

[Manasi Navare]

On Tue, Oct 17, 2017 at 03:46:00PM -0700, Puthikorn Voravootivat wrote:
> I tried  https://patchwork.freedesktop.org/series/30670/ but it doesn't work.
> 
> > When you see the display blinking, do you know for sure that drm_dp_channel_eq_ok() returns false?
> Yes. Repro is running "modetest" while PSR is on (easier to repro with
> PSR2 but PSR1 is fine too)
> 
> > lane_align = dp_link_status(link_status,
> >    DP_LANE_ALIGN_STATUS_UPDATED);
> > if ((lane_align & DP_INTERLANE_ALIGN_DONE) == 0)
> >    return false;
> 
> I always got lane_align = 128 (DP_LINK_STATUS_UPDATED)
> So drm_dp_channel_eq_ok() always return false.

Hmm, it looks like there is a bug somewhere else that is causing the
link status to get updated and bit LINK_STATUS_UPDATED from LANE_ALIGN_STATUS_UPDATED register
is not being read so not being cleared causing the link to be retrained again.

So I still feel that this patch is a workaround for another bug in the PSR code.
But I will let others comment on this.

Regards
Manasi

> 
> If I disable PSR, drm_dp_channel_eq_ok() will return true and no
> blinking occurs.
> 
> On Tue, Oct 17, 2017 at 2:21 PM, Manasi Navare
> <manasi.d.navare at intel.com> wrote:
> > On Tue, Oct 17, 2017 at 02:01:56PM -0700, Puthikorn Voravootivat wrote:
> >> intel_dp_long_pulse() is always checking link status because
> >> there has been known issues of link loss triggerring long pulse.
> >>
> >> However this is not needed for eDP display since we won't have
> >> link loss for internal display. Also there are reports that
> >> screens are flickering during link status check. (repro by
> >> running modetest command repeatedly)
> >>
> >> Signed-off-by: Puthikorn Voravootivat <puthik at chromium.org>
> >> ---
> >>  drivers/gpu/drm/i915/intel_dp.c | 7 ++++++-
> >>  1 file changed, 6 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> >> index 4b65cf137f79..75a77ef257e2 100644
> >> --- a/drivers/gpu/drm/i915/intel_dp.c
> >> +++ b/drivers/gpu/drm/i915/intel_dp.c
> >> @@ -4763,7 +4763,8 @@ intel_dp_long_pulse(struct intel_connector *intel_connector)
> >>                */
> >>               status = connector_status_disconnected;
> >>               goto out;
> >> -     } else {
> >> +     } else if (status != connector_status_connected ||
> >> +                intel_encoder->type != INTEL_OUTPUT_EDP) {
> >>               /*
> >>                * If display is now connected check links status,
> >>                * there has been known issues of link loss triggerring
> >> @@ -4775,6 +4776,10 @@ intel_dp_long_pulse(struct intel_connector *intel_connector)
> >>                * 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.
> >> +              *
> >> +              * Skip checking links status for connected eDP display.
> >> +              * There are known issues of display blinking during checking
> >> +              * link status and we don't have link loss for internal display.
> >>                */
> >
> > Inside intel_dp_check_link_status(), it actually checks if link status is good by
> > checking both clock recovery and Channel EQ bits in DPCD, only then it will retrain.
> > So in case of eDP panel, if there is no link loss then it should always return link
> > status as good and not retrain.
> > So IMHO I dont think we need to explicitly avoid this for eDP.
> >
> > When you see the display blinking, do you know for sure that drm_dp_channel_eq_ok() returns false?
> > Also look at this patch: https://patchwork.freedesktop.org/series/30670/
> > This makes sure none of the eDP training parameters change unless link is bad. So with this patch, if the
> > link is good then it should never retrain even in intel_dp_check_link_status() for eDP.
> >
> > Manasi
> >>               intel_dp_check_link_status(intel_dp);
> >>       }
> >> --
> >> 2.15.0.rc0.271.g36b669edcc-goog
> >>
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