linux-kernel - Re: [PATCH] drm: bridge: fsl-ldb: fixup mode on freq mismatch

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Message-ID: <72f924e5f881b66c3e187890d0a200ca@vosn.de>
Date: Wed, 11 Dec 2024 17:54:13 +0100
From: Nikolaus Voss <nv@...n.de>
To: Marek Vasut <marex@...x.de>
Cc: Liu Ying <victor.liu@....nxp.com>, Alexander Stein
 <alexander.stein@...tq-group.com>, Liu Ying <victor.liu@....com>, Luca
 Ceresoli <luca.ceresoli@...tlin.com>, Fabio Estevam <festevam@...x.de>,
 Andrzej Hajda <andrzej.hajda@...el.com>, Neil Armstrong
 <neil.armstrong@...aro.org>, Robert Foss <rfoss@...nel.org>, Laurent
 Pinchart <Laurent.pinchart@...asonboard.com>, Jonas Karlman
 <jonas@...boo.se>, Jernej Skrabec <jernej.skrabec@...il.com>, David Airlie
 <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 nikolaus.voss@...g-streit.com, miquel.raynal@...tlin.com
Subject: Re: [PATCH] drm: bridge: fsl-ldb: fixup mode on freq mismatch

Hi Marek,

On 09.12.2024 22:46, Marek Vasut wrote:
> On 12/9/24 9:46 AM, Nikolaus Voss wrote:
>>>>>> and store the panel's timing in EDID EEPROM.
>>>>> Oh, that is a new one. Does the EDID EEPROM store the entirety of
>>>>> 'struct display_timing {}' somehow , or is that a custom format ?
>>>> 
>>>> Well, sort of ;-). VESA has taken care of this 30 years ago
>>>> (https://en.wikipedia.org/wiki/Extended_Display_Identification_Data).
>>>> 
>>>> DRM handles this with drm_get_edid() and siblings, e.g. :
>>> 
>>> EDID can not encode all the information in struct display_timing {} ,
>>> or can it ?
>>> 
>>> I think what you would be missing are bus_flags , bus_format and
>>> possibly the single/dual link and channel (odd/even) mapping, won't
>>> you ?
>> 
>> Yes, that's right. I use the vendor block for bus_flags and bus_format
>> now, but that's not standard and not portable of course.
>> 
>> My first idea was to store the DT overlay in the display EEPROM but
>> a standard 1k EEPROM is too small for that.
> Understood. I had the same problem, in the end I went for custom
> encoding in the EEPROM but the amount of panels was limited in my
> case.
> 
> Indeed, DTO does not fit the EEPROM and EDID is not really fitting too
> well to DPI/LVDS panels, it has too many fields that are specific to
> regular pluggable panels and not useful on DPI/LVDS ones.

The curse of backward-compatibility ;-). Nevertheless, we are doing well
with EDID for years now. 95% is implemented in standard kernel, you just
need a few quirks.

-- 
Nikolaus Voss