Re: [Freedreno] [PATCH] drm/msm/dp: make eDP panel as the first connected connector

[Abhinav Kumar <quic_abhinavk@quicinc.com>]

On 7/4/2022 11:14 AM, Dmitry Baryshkov wrote:
> On 30/06/2022 09:14, Dmitry Baryshkov wrote:
>>
>>
>> On 30 June 2022 04:57:35 GMT+03:00, Rob Clark <robdclark@gmail.com> 
>> wrote:
>>> On Wed, Jun 29, 2022 at 5:36 PM Doug Anderson <dianders@chromium.org> 
>>> wrote:
>>>>
>>>> Hi,
>>>>
>>>> On Tue, Jun 28, 2022 at 1:14 PM Dmitry Baryshkov
>>>> <dmitry.baryshkov@linaro.org> wrote:
>>>>>
>>>>> On 28 June 2022 18:20:06 GMT+03:00, Kuogee Hsieh 
>>>>> <quic_khsieh@quicinc.com> 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 <dmitry.baryshkov@linaro.org>
> 
> 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 
> panel.
> 
Ah. True, but just to add. It should be after kms_init() but before 
drm_dev_register().

>>
>>
>>>
>>> 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@chromium.org>
>>>> Tested-by: Douglas Anderson <dianders@chromium.org>
>>
> 
>