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SubjectRe: [PATCH] drm/msm/dp: make eDP panel as the first connected connector
On 30/06/2022 09:14, Dmitry Baryshkov wrote:
> 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 <>

After some time (please excuse me), musing with the code and even
picking up the commit for the merge branch, I understood the fact that I
did not like about this change. It moves all panel connectors (generic
code) from the DP-specific driver.

I'd like to retract my R-b. Please move this call to the msm_drm_init().
Calling this function somewhere after the ->kms_init() would make sure
that all panel connectors are close to the top of the list, whichever
MDP/DPU driver is used and whichever actual interface is bound to this

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

With best wishes

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