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

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Message-ID: <230b3977-fa4f-4297-a05e-796c0df90d48@denx.de>
Date: Mon, 2 Dec 2024 20:11:40 +0100
From: Marek Vasut <marex@...x.de>
To: Nikolaus Voss <nv@...n.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>,
 Miquel Raynal <miquel.raynal@...tlin.com>, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, nikolaus.voss@...g-streit.com
Subject: Re: [PATCH] drm: bridge: fsl-ldb: fixup mode on freq mismatch

On 12/2/24 6:03 PM, Nikolaus Voss wrote:
> On 02.12.2024 13:56, Marek Vasut wrote:
>> On 12/2/24 7:32 AM, Liu Ying wrote:
>>> On 11/27/2024, Nikolaus Voss wrote:
>>>> LDB clock has to be a fixed multiple of the pixel clock.
>>>> As LDB and pixel clock are derived from different clock sources
>>>> (at least on imx8mp), this constraint cannot be satisfied for
>>>> any pixel clock, which leads to flickering and incomplete
>>>> lines on the attached display.
>>>>
>>>> To overcome this, check this condition in mode_fixup() and
>>>> adapt the pixel clock accordingly.
>>>>
>>>> Cc: <stable@...r.kernel.org>
>>>
>>> It looks like stable is not effectively Cc'ed.
>>> Need a Fixes tag?
>> Isn't this fix effectively superseded by series
>>
>> [PATCH 0/5] clk: Fix simple video pipelines on i.MX8
>>
>> ?
> 
> Maybe. I wasn't aware of the series. Looking at it, the change is
> rather complex and not suitable for the stable series.
> 
> My intention was to get a simple fix which doesn't potentially
> break anything. It wouldn't even break the patch series you mentioned.
I know, the proper fix is indeed complex and not yet fully figured out.

The fix for stable for existing hardware is similar to this commit I think:

4fbb73416b10 ("arm64: dts: imx8mp-phyboard-pollux: Set Video PLL1 
frequency to 506.8 MHz")