[PATCH] drm/bridge: ps8640: Add back the 50 ms mystery delay after HPD

[Doug Anderson]

Hi,

On Wed, Oct 19, 2022 at 11:18 AM Stephen Boyd <swboyd at chromium.org> wrote:
>
> Quoting Douglas Anderson (2022-10-17 12:18:51)
> > Back in commit 826cff3f7ebb ("drm/bridge: parade-ps8640: Enable
> > runtime power management") we removed a mysterious 50 ms delay because
> > "Parade's support [couldn't] explain what the delay [was] for".
> >
> > While I'm always a fan of removing mysterious delays, I suspect that
> > we need this mysterious delay to avoid some problems.
> >
> > Specifically, what I found recently is that on sc7180-trogdor-homestar
> > sometimes the AUX backlight wasn't initializing properly. Some
> > debugging showed that the drm_dp_dpcd_read() function that the AUX
> > backlight driver was calling was returning bogus data about 1% of the
> > time when I booted up. This confused
> > drm_panel_dp_aux_backlight(). From continued debugging:
> > - If I retried the read then the read worked just fine.
> > - If I added a loop to perform the same read that
> >   drm_panel_dp_aux_backlight() was doing 30 times at bootup I could
> >   see that some percentage of the time the first read would give bogus
> >   data but all 29 additional reads would always be fine.
> > - If I added a large delay _after_ powering on the panel but before
> >   powering on PS8640 I could still reproduce the problem.
> > - If I added a delay after PS8640 powered on then I couldn't reproduce
> >   the problem.
> > - I couldn't reproduce the problem on a board with the same panel but
> >   the ti-sn65dsi86 bridge chip.
> >
> > To me, the above indicated that there was a problem with PS8640 and
> > not the panel.
> >
> > I don't really have any insight into what's going on in the MCU, but
> > my best guess is that when the MCU itself sees the HPD go high that it
> > does some AUX transfers itself and this is confusing things.
> >
> > Let's go back and add back in the mysterious 50 ms delay. We only want
> > to do this the first time we see HPD go high after booting the MCU,
> > not every time we double-check HPD.
> >
> > With this, the backlight initializes reliably on homestar.
> >
> > Fixes: 826cff3f7ebb ("drm/bridge: parade-ps8640: Enable runtime power management")
> > Signed-off-by: Douglas Anderson <dianders at chromium.org>
> > ---
>
> Reviewed-by: Stephen Boyd <swboyd at chromium.org>

I'm not expecting any other reviews of this patch, though I'm happy to
be proven wrong. As a heads up, I'll plan to land this on Friday
(roughly 2 days from now) in "drm-misc-fixes" barring anything else.
If anyone else plans to offer any opinions about this patch or just
wants more time to review, please shout.

-Doug