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SubjectRe: [PATCH] drm/msm/dp: make eDP panel as the first connected connector
On Wed, Jun 29, 2022 at 5:36 PM Doug Anderson <> wrote:
> Hi,
> On Tue, Jun 28, 2022 at 1:14 PM Dmitry Baryshkov
> <> wrote:
> >
> > On 28 June 2022 18:20:06 GMT+03:00, Kuogee Hsieh <> 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)


> 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 <>
> Tested-by: Douglas Anderson <>

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