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SubjectRe: [PATCH] drm/msm/dp: make eDP pa nel as the first connected connector

On 30 June 2022 04:57:35 GMT+03:00, Rob Clark <> wrote:
>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)

Thanks for the description. I think the userspace code should be fixed too, but this patch can go in on its own.

Reviewed-by: Dmitry Baryshkov <>

>> 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 <>

With best wishes

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