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

From: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
To: Rob Clark <robdclark@gmail.com>, Doug Anderson <dianders@chromium.org>
Cc: Sean Paul <sean@poorly.run>, Stephen Boyd <swboyd@chromium.org>,
	Vinod Koul <vkoul@kernel.org>, Daniel Vetter <daniel@ffwll.ch>,
	David Airlie <airlied@linux.ie>, Andy Gross <agross@kernel.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	"Abhinav Kumar (QUIC)" <quic_abhinavk@quicinc.com>,
	"Aravind Venkateswaran (QUIC)" <quic_aravindh@quicinc.com>,
	"Kuogee Hsieh (QUIC)" <quic_khsieh@quicinc.com>,
	Sankeerth Billakanti <quic_sbillaka@quicinc.com>,
	freedreno <freedreno@lists.freedesktop.org>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] drm/msm/dp: make eDP panel as the first connected connector
Date: Thu, 30 Jun 2022 09:14:42 +0300	[thread overview]
Message-ID: <E87F561E-22B4-46B6-AD9B-4995E92A8E29@linaro.org> (raw)
In-Reply-To: <CAF6AEGvFYK3x_CmErkKsmYNEv9y8AksE9UB_sfp-Z7RieX=tVg@mail.gmail.com>

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>

>
>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>

-- 
With best wishes
Dmitry