linux-kernel - Re: [PATCH v2] drm: bridge: fsl-ldb: fixup mode on freq mismatch

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Message-ID: <5a6ab24d-6c74-497f-828e-b3e7645d664a@denx.de>
Date: Sat, 7 Dec 2024 12:46:15 +0100
From: Marek Vasut <marex@...x.de>
To: Nikolaus Voss <nv@...n.de>
Cc: 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@...tlin.com, nikolaus.voss@...g-streit.com,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] drm: bridge: fsl-ldb: fixup mode on freq mismatch

On 12/4/24 11:40 AM, Nikolaus Voss wrote:

Hi,

>>> LDB clock has to be a fixed multiple of the pixel clock.
>>> As LDB and pixel clock are derived from different clock sources
>>
>> Can you please share the content of /sys/kernel/debug/clk/clk_summary ?
> 
> Sure. Without my patch:
> 
>      video_pll1_ref_sel               1       1        0        24000000 
>    0          0     50000      Y      deviceless no_connection_id
>         video_pll1                    1       1        0 1039500000  
> 0          0     50000      Y         deviceless          no_connection_id
>            video_pll1_bypass          1       1        0 1039500000  
> 0          0     50000      Y            deviceless             
> no_connection_id
>               video_pll1_out          2       2        0 1039500000  
> 0          0     50000      Y               deviceless                
> no_connection_id
>                  media_ldb            1       1        0        
> 346500000   0          0     50000      Y 32ec0000.blk- 
> ctrl:bridge@5c     ldb
>                                                   deviceless         
> no_connection_id
>                     media_ldb_root_clk 0       0        0 346500000   
> 0          0     50000      Y                     deviceless 
>                       no_connection_id
>                  media_disp2_pix      1       1        0        51975000 
>    0          0     50000      Y                  deviceless         
> no_connection_id
>                     media_disp2_pix_root_clk 1       1        0 
> 51975000    0          0     50000      Y 32e90000.display- 
> controller     pix
> 
> Here 346500000 (media_ldb) != 7 * 51975000 (media_disp2_pix)
>    -> distorted panel image (if any).
> The requested panel pixel clock from EDID is 51200000.

Right, this is what Miquel is trying to solve with their series.

> This is the same with my patch:
> 
>      video_pll1_ref_sel               1       1        0        24000000 
>    0          0     50000      Y      deviceless no_connection_id
>         video_pll1                    1       1        0 1039500000  
> 0          0     50000      Y         deviceless          no_connection_id
>            video_pll1_bypass          1       1        0 1039500000  
> 0          0     50000      Y            deviceless             
> no_connection_id
>               video_pll1_out          2       2        0 1039500000  
> 0          0     50000      Y               deviceless                
> no_connection_id
>                  media_ldb            1       1        0        
> 346500000   0          0     50000      Y 32ec0000.blk- 
> ctrl:bridge@5c     ldb
>                                                   deviceless         
> no_connection_id
>                     media_ldb_root_clk 0       0        0 346500000   
> 0          0     50000      Y                     deviceless 
>                       no_connection_id
>                  media_disp2_pix      1       1        0        49500000 
>    0          0     50000      Y                  deviceless         
> no_connection_id
>                     media_disp2_pix_root_clk 1       1        0 
> 49500000    0          0     50000      Y 32e90000.display- 
> controller     pix
> 
> So, here 346500000 (media_ldb) = 7 * 49500000 (media_disp2_pix).
>    -> stable panel image, but pixel clock reduced to 49.5 MHz from 
> requested 51.2 MHz.

Inaccurate pixel clock and non-60Hz frame rate is not a win either.

> My conclusion: The clock source is the same

I agree .

You wrote "derived from different clock sources" above, 
keyword:different, which is not correct.

> , nevertheless the
> ldb/pixel clock constraint cannot be satisfied without either
> modifying the pll clock or the pixel clock.
In this particular case, you surely do want to modify the PLL settings 
to achieve accurate pixel clock.