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

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Message-ID: <abcc89936f44fd884b9c5da65ea64c42@vosn.de>
Date: Wed, 04 Dec 2024 11:40:37 +0100
From: Nikolaus Voss <nv@...n.de>
To: Marek Vasut <marex@...x.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

Hi Marek,

On 03.12.2024 21:15, Marek Vasut wrote:
> On 12/3/24 8:09 PM, 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
> 
> 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.

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.

My conclusion: The clock source is the same, nevertheless the
ldb/pixel clock constraint cannot be satisfied without either
modifying the pll clock or the pixel clock.

-- 
Nikolaus Voss