Hi, On Wed, Oct 19, 2022 at 11:18 AM Stephen Boyd <swboyd@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> > > --- > > Reviewed-by: Stephen Boyd <swboyd@xxxxxxxxxxxx> 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