Hi, On Thu, Jul 16, 2020 at 1:21 PM Douglas Anderson <dianders@xxxxxxxxxxxx> wrote: > > On boe_nv133fhm_n62 (and presumably on boe_nv133fhm_n61) a scope shows > a small spike on the HPD line right when you power the panel on. The > picture looks something like this: > > +-------------------------------------- > | > | > | > Power ---+ > +--- > | > ++ | > +----+| | > HPD -----+ +---------------------------+ > > So right when power is applied there's a little bump in HPD and then > there's small spike right before it goes low. The total time of the > little bump plus the spike was measured on one panel as being 8 ms > long. The total time for the HPD to go high on the same panel was > 51.2 ms, though the datasheet only promises it is < 200 ms. > > When asked about this glitch, BOE indicated that it was expected and > persisted until the TCON has been initialized. > > If this was a real hotpluggable DP panel then this wouldn't matter a > whole lot. We'd debounce the HPD signal for a really long time and so > the little blip wouldn't hurt. However, this is not a hotpluggable DP > panel and the the debouncing logic isn't needed and just shows down > the time needed to get the display working. This is why the code in > panel_simple_prepare() doesn't do debouncing and just waits for HPD to > go high once. Unfortunately if we get unlucky and happen to poll the > HPD line right at the spike we can try talking to the panel before > it's ready. > > Let's handle this situation by putting in a 15 ms prepare delay and > decreasing the "hpd absent delay" by 15 ms. That means: > * If you don't have HPD hooked up at all you've still got the > hardcoded 200 ms delay. > * If you've got HPD hooked up you will always wait at least 15 ms > before checking HPD. The only case where this could be bad is if > the panel is sharing a voltage rail with something else in the > system and was already turned on long before the panel came up. In > such a case we'll be delaying 15 ms for no reason, but it's not a > huge delay and I don't see any other good solution to handle that > case. > > Even though the delay was measured as 8 ms, 15 ms was chosen to give a > bit of margin. > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> > --- > I don't actually have a device in front of me that is exhibiting these > problems. I believe that it is only some devices and some of the > time. Still, this patch seems safe and seems likely to fix the issue > given the scope shots. Just to follow-up, I just heard that someone who had a panel exhibiting this problem tried my patch and it fixed it for them. :-) So this is not such a shot in the dark anymore. -Doug
