[Freedreno] [PATCH v5 6/9] drm/msm/dp: wait for hpd high before any sink interaction

[Doug Anderson]

Hi,

On Fri, Mar 25, 2022 at 8:54 AM Sankeerth Billakanti (QUIC)
<quic_sbillaka at quicinc.com> wrote:
>
> > -----Original Message-----
> > From: Doug Anderson <dianders at chromium.org>
> > Sent: Saturday, March 19, 2022 5:26 AM
> > To: Stephen Boyd <swboyd at chromium.org>
> > Cc: Sankeerth Billakanti (QUIC) <quic_sbillaka at quicinc.com>; open list:OPEN
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> > msm at vger.kernel.org>; LKML <linux-kernel at vger.kernel.org>; Rob Clark
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> > Subject: Re: [PATCH v5 6/9] drm/msm/dp: wait for hpd high before any sink
> > interaction
> >
> > Hi,
> >
> > On Fri, Mar 18, 2022 at 4:27 PM Stephen Boyd <swboyd at chromium.org>
> > wrote:
> > >
> > > > > Pushing hpd state checking into aux transactions looks like the
> > > > > wrong direction. Also, as I said up above I am concerned that even
> > > > > checking the GPIO won't work and we need some way to ask the
> > > > > bridge if HPD is asserted or not and then fallback to the GPIO
> > > > > method if the display phy/controller doesn't have support to check
> > > > > HPD internally. Something on top of DRM_BRIDGE_OP_HPD?
> > > >
> > > > If we could somehow get the HPD status from the bridge in the panel
> > > > driver it definitely would be convenient. It does feel like that's
> > > > an improvement that could be done later, though. We've already
> > > > landed a few instances of doing what's done here, like for
> > > > parade-ps8640 and analogix_dp. I suspect designing a new mechanism
> > > > might not be the most trivial.
> > >
> > > What is done in the bridge drivers is to wait for a fixed timeout and
> > > assume aux is ready? Or is it something else? If there's just a fixed
> > > timeout for the eDP case it sounds OK to do that for now and we can
> > > fine tune it later to actually check HPD status register before the
> > > panel tries to read EDID.
> >
> > Right. For the parade chip (which is only used for eDP as far as I know--never
> > DP) waits for up to 200 ms. See ps8640_ensure_hpd().
> >
> > So I guess tl;dr to Sankeerth that it's OK for his patch to have the wait in the
> > aux transfer function, but only for eDP. Other discussions here are about
> > how we could make it better in future patches.
> >
> >
>
> The aux transactions for external DP are initiated by the dp_display driver only after the
> display is hot plugged to the connector. The phy_init is necessary for the aux transactions
> to take place. So, for the DP case, like Doug mentioned below, this patch is introducing
> an overhead of three register reads to detect hpd_high before performing aux transactions.
> So, we felt this was okay to do for DP.

Personally I'm not that upset about the 3 register reads. The problem
Stephen pointed out is bigger. It's possible that a DP cable is
unplugged _just_ as we started an AUX transaction. In that case we'll
have a big delay here when we don't actually need one.


> On the other hand, for eDP, it is necessary to wait for panel ready through this hpd connect status.
> Currently there is no way to know which type of connector it is in the dp_aux sub-module.
>
> However, as the discussion suggested, to have the wait only for eDP, I am thinking to pass the
> connector_type information to aux sub-module and register different aux_transfer functions
> for eDP and DP. The eDP transfer function will wait for hpd_high and the DP transfer function
> will be same as the one before this patch.

Personally I wouldn't register two separate functions. You could just
store a boolean in your structure and only wait for HPD if this is
eDP. One extra "if" test doesn't seem like it justifies splitting off
into two functions...

-Doug