Re: [PATCH v2 0/2] drm: bridge: ti-sn65dsi83: Improve dual-link LVDS support

[tessolveupstream]

On 12-03-2026 21:19, Luca Ceresoli wrote:
> Hello Sudarshan,
> 
> and thanks Marek for copying me, I hadn't noticed this series.
> 
> On Thu Mar 12, 2026 at 1:35 PM CET, tessolveupstream wrote:
> 
> [...]
> 
>>> +CC Luca
>>>
>>> You might want to look at recently posted:
>>>
>>> [PATCH 2/3] drm/bridge: ti-sn65dsi83: halve horizontal syncs for dual LVDS output
>>
>> Thanks for pointing this out.
>> I tried applying the patch “[PATCH 2/3] drm/bridge: ti-sn65dsi83: halve horizontal syncs for dual LVDS output” on top of the current tree and
>> removed the changes that I had previously added in the driver.
>> However, with this patch applied, I am currently seeing only the backlight turning on and no image on the LVDS panel.
>> For reference, the LVDS panel used on our platform is G133HAN01.1 and the
>> DSI-to-dual-link LVDS bridge is SN65DSI84ZXHR.
> 
> Thanks for having tried.
> 
> Can you please test with both the fixes in the series applied + the test
> pattern feature and report the results you get with and without test
> pattern enabled?
> 
> The patches to apply are:
> 
>  - https://lore.kernel.org/all/20260226-ti-sn65dsi83-dual-lvds-fixes-and-test-pattern-v1-1-2e15f5a9a6a0@xxxxxxxxxxx/
>  - https://lore.kernel.org/all/20260226-ti-sn65dsi83-dual-lvds-fixes-and-test-pattern-v1-2-2e15f5a9a6a0@xxxxxxxxxxx/
>  - https://lore.kernel.org/lkml/20260309-ti-sn65dsi83-dual-lvds-fixes-and-test-pattern-v2-1-e6aaa7e1d181@xxxxxxxxxxx/
> 

Thanks for the suggestions.

I tested the three patches together as mentioned, but the LVDS panel 
still only shows the backlight and no image. I also tried removing the 
test-pattern patch and retesting with only the remaining two fixes, but 
the result remained the same — only the backlight turns on and no image 
is displayed.
>> During our earlier debugging, we went through several trial-and-error
>> iterations and also received support from TI. According to TI, when
>> operating in dual-link mode the horizontal timing parameters must be
>> divided by two before being written to the device. Without this
>> adjustment, the panel either does not light up or shows corrupted output.
>>
>> TI also shared a set of recommended register settings for dual-link mode,
>> which were derived using the TI DSI-Tuner tool. These settings helped us
>> get the panel working on our hardware during testing.
>> For reference, the register configuration suggested by TI is as follows:
>>
>> 	regmap_write(ctx->regmap, REG_RC_LVDS_PLL, 0x05);
>> 	regmap_write(ctx->regmap, REG_RC_PLL_EN, 0x00);
>> 	regmap_write(ctx->regmap, REG_DSI_CLK, 0x53);
>> 	regmap_write(ctx->regmap, REG_LVDS_FMT, 0x6f);
>> 	regmap_write(ctx->regmap, REG_LVDS_VCOM, 0x00);
>> 	regmap_write(ctx->regmap,
>> 		     REG_VID_CHA_VERTICAL_DISPLAY_SIZE_LOW, 0x00);
>> 	regmap_write(ctx->regmap,
>> 		     REG_VID_CHA_VERTICAL_DISPLAY_SIZE_HIGH, 0x00);
>> 	regmap_write(ctx->regmap,
>> 		     REG_VID_CHA_HSYNC_PULSE_WIDTH_LOW, 0x10);
>> 	regmap_write(ctx->regmap,
>> 		     REG_VID_CHA_HORIZONTAL_BACK_PORCH, 0x28);
>> 	regmap_write(ctx->regmap,
>> 		     REG_VID_CHA_VERTICAL_BACK_PORCH, 0x00);
>> 	regmap_write(ctx->regmap,
>> 		     REG_VID_CHA_HORIZONTAL_FRONT_PORCH, 0x00);
>> 	regmap_write(ctx->regmap,
>> 		     REG_VID_CHA_VERTICAL_FRONT_PORCH, 0x00);
>>
>> If it would help, we can test any proposed changes on our hardware.
> 
> The first thing I suggest doing on your side is testing with the 3 patches
> mentioned above.
> 
> If you display works, good! Let us know (you can also add your Tested-by /
> Reviewed-by tags to the test_pattern patch too if applicable).
> 
> If it doesn't work, compare the individual register values to find the
> differences, try to figure out why the working setting works and how to
> apply that change to the driver in away that keeps other boards
> working. You're welcome to come back here to discuss it in case you can't
> find out on your own.
> 

I tested the three patches as suggested, but the panel still shows only the 
backlight with no visible image. I’m unsure how to translate the working 
register values into a generic fix based on display timings. Any guidance 
on the right direction would be helpful.
> Hope this helps,
> Luca
> 
> --
> Luca Ceresoli, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com