linux-kernel - Re: [PATCH v3 1/2] drm/panel: jd9365da: Modify Kingdisplay and Melfas panel timing

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Message-ID: <CAD=FV=Vm-MzhNrotMz1n-iYvm_VAfyVRDtTp4yrQ=6sCTzX1Vw@mail.gmail.com>
Date: Wed, 2 Oct 2024 13:01:03 -0700
From: Doug Anderson <dianders@...omium.org>
To: Zhaoxiong Lv <lvzhaoxiong@...qin.corp-partner.google.com>
Cc: neil.armstrong@...aro.org, quic_jesszhan@...cinc.com, sam@...nborg.org, 
	maarten.lankhorst@...ux.intel.com, mripard@...nel.org, tzimmermann@...e.de, 
	airlied@...il.com, simona@...ll.ch, hsinyi@...gle.com, 
	awarnecke002@...mail.com, dmitry.baryshkov@...aro.org, 
	dri-devel@...ts.freedesktop.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] drm/panel: jd9365da: Modify Kingdisplay and Melfas
 panel timing

Hi,

On Fri, Sep 27, 2024 at 2:44 AM Zhaoxiong Lv
<lvzhaoxiong@...qin.corp-partner.google.com> wrote:
>
> In MTK chips, if the DRM runtime resume has not yet completed and the
> system enters sleep mode at the same time, there is a possibility of
> a black screen after waking the machine. Reduce the disable delay
> resolves this issue,

This sounds _really_ suspicious to me and it feels like you're just
pushing around timing to get lucky and avoid the problem. Generally if
decreasing delays like this fixes a functional problem then the
problem is still there (just hidden) and has the potential to come
back if a little extra delay shows up.

I don't understand _why_ it's important for DRM runtime resume to
complete before the system enters sleep. Is this causing the Mediatek
DRM driver to get confused? I would have expected that if the system
went fully into suspend that it would have totally powered off the
panel and then when we resume the panel shouldn't maintain any state
from before the suspend.

...so this needs to be debugged more and a real fix needs to be made.


> The "backlight_off_to_display_off_delay_ms" was added between
> "backlight off" and "display off"  to prevent "display off" from being
> executed when the backlight is not fully powered off, which may cause
> a white screen. However, we removed this
> "backlight_off_to_display_off_delay_ms" and found that this situation
> did not occur. Therefore, in order to solve the problem mentioned
> above, we reduced it from 100ms to 3ms (tCMD_OFF >= 1ms).
>
> This is the timing specification for the two panels:
> 1. Kingdisplay panel timing spec:
> https://github.com/KD54183/-JD9365DA_Power-On-Off-Sequence_V0120240923
> 2. LMFBX101117480 timing spec: https://github.com/chiohsin-lo/TDY-JD_LIB
>
> Fixes: 2b976ad760dc ("drm/panel: jd9365da: Support for kd101ne3-40ti MIPI-DSI panel")
> Fixes: c4ce398cf18a ("drm/panel: jd9365da: Support for Melfas lmfbx101117480 MIPI-DSI panel")
>
> Signed-off-by: Zhaoxiong Lv <lvzhaoxiong@...qin.corp-partner.google.com>

For future reference: please don't put a blank line between the
"Fixes" and the "Signed-off-by".


-Doug