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

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Message-ID: <808d4092a9e97b95480d47c1bd84d930@vosn.de>
Date: Tue, 03 Dec 2024 08:20:05 +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

On 03.12.2024 04:12, Marek Vasut wrote:
> On 12/3/24 3:22 AM, Liu Ying wrote:
> 
> [...]
> 
>>>> I doubt that pixel clock tree cannot find appropriate division 
>>>> ratios
>>>> for some pixel clock rates, especially for dual-link LVDS on i.MX8MP
>>>> and i.MX93 platforms, because PLL clock rate should be 7x faster 
>>>> than
>>>> pixel clock rate and 2x faster than "ldb" clock rate so that the 3.5
>>>> folder between "ldb" clock and pixel clock can be met. That means 
>>>> the
>>>> PLL clock rate needs to be explicitly set first for this case.
>>>> 
>>>> Can you assign the PLL clock rate in DT to satisfy the "ldb" and 
>>>> pixel
>>>> clock rates like the below commit does, if you use a LVDS panel?
>>>> 
>>>> 4fbb73416b10 ("arm64: dts: imx8mp-phyboard-pollux: Set Video PLL1
>>>> frequency to 506.8 MHz")
>>> 
>>> I probably could. The point of my patch is you don't have to know in
>>> advance which LVDS panel is connected, and you don't have to 
>>> calculate
>>> the base PLL clock by hand and store it in the device tree.
>>> 
>>> In my test system, I have three different LVDS panels with EDID 
>>> EEPROM,
>>> none of which worked with the stock driver, but all work with this
>>> patch.
>>> With slightly adapted pixel clocks though.
>> 
>> If each of the three LVDS panels has only one display mode, you may
>> assign the PLL clock rates in DT overlays for the panels.
> I temporarily agree.
> 
> I also currently use DTOs for various panels including their PLL
> setting, but in the end, I think/hope the work of Miquel and co. is
> going to make that PLL setting part unnecessary.

That is exactly what my patch is about. I want to use one DT for all
panels and store the panel's timing in EDID EEPROM.

-- 
Nikolaus Voss