On Wed, Jun 29, 2022 at 5:36 PM Doug Anderson <dianders@xxxxxxxxxxxx> wrote: > > Hi, > > On Tue, Jun 28, 2022 at 1:14 PM Dmitry Baryshkov > <dmitry.baryshkov@xxxxxxxxxx> wrote: > > > > On 28 June 2022 18:20:06 GMT+03:00, Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx> wrote: > > >Some userspace presumes that the first connected connector is the main > > >display, where it's supposed to display e.g. the login screen. For > > >laptops, this should be the main panel. > > > > > >This patch call drm_helper_move_panel_connectors_to_head() after > > >drm_bridge_connector_init() to make sure eDP stay at head of > > >connected connector list. This fixes unexpected corruption happen > > >at eDP panel if eDP is not placed at head of connected connector > > >list. > > > > The change itself is a good fix anyway. (And I'd ack it.) However I would like to understand why does it fix the corruption issue. What is we have eDP and DSI, with DSI ending up before the eDP? Would we see the issue? > > Also could you please describe the mind of corruption you are observing? > > I've spent a whole bunch of time poking at this and in the end my > conclusion is this: > > 1. The glitchyness seems to be a result of the Chrome OS userspace > somehow telling the kernel to do something wrong. > > 2. I believe (though I have no proof other than Kuogee's patch fixing > things) that the Chrome OS userspace is simply confused by the eDP > connector being second. This would imply that Kuogee's patch is > actually the right one. > > 3. It would be ideal if the Chrome OS userspace were fixed to handle > this, but it's an area of code that I've never looked at. It also > seems terribly low priority to fix since apparently other OSes have > similar problems (seems like this code was originally added by > RedHat?) > > > Specifically, I tested with a similar but "persistent" glitch that I > reproduced. The glitch Kuogee was digging into was a transitory glitch > on the eDP (internal) display when you plugged in a DP (external) > display. It would show up for a frame or two and then be fixed. I can > get a similar-looking glitch (vertical black and white bars) that > persists by doing these steps on a Chrome OS device (and Chrome OS > kernel): > > a) Observe screen looks good. > b) Observe DP not connected. > c) Plug in DP > d) See transitory glitch on screen, then it all looks fine. > e) set_power_policy --ac_screen_dim_delay=5 --ac_screen_off_delay=10 > f) Wait for screen to turn off > g) Unplug DP > h) Hit key on keyboard to wake device. > i) See glitchy. > j) Within 5 seconds: set_power_policy --ac_screen_dim_delay=5000 > --ac_screen_off_delay=10000 > > Once I'm in the persistent glitch: > > * The "screenshot" command in Chrome OS shows corruption. Not exactly > black and white bars, but the image produced has distinct bands of > garbage. > > * I can actually toggle between VT2 and the main screen (VT1). Note > that VT1/VT2 are not quite the normal Linux managed solution--I > believe they're handled by frecon. In any case, when I switch to VT2 > it looks normal (I can see the login prompt). Then back to VT1 and the > vertical bars glitch. Back to VT2 and it's normal. Back to VT1 and the > glitch again. This implies (especially with the extra evidence of > screenshot) that the display controller hardware is all fine and that > it's the underlying data that's somehow messed up. fwiw, from looking at this a bit w/ Doug, I think the "glitch" is simply just an un-renderered buffer being interpreted by the display controller as UBWC (because userspace tells it to) BR, -R > When I pick Kuogee's patch then this "persistent" glitch goes away > just like the transitory one does. > > I'm going to go ahead and do: > > Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx> > Tested-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
