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@xxxxxxxxxxxx>
> ---
Reviewed-by: Stephen Boyd <swboyd@xxxxxxxxxxxx>
