Re: [PATCH v4 2/2] drm: bridge: ti-sn65dsi83: Disable video burst mode for LVDS stability

["Luca Ceresoli" <luca.ceresoli@bootlin.com>]

Hello Laurent, Marek,

On Fri Jun 5, 2026 at 7:31 PM CEST, Laurent Pinchart wrote:
> On Fri, Jun 05, 2026 at 06:56:54PM +0200, Luca Ceresoli wrote:
>> On Fri Jun 5, 2026 at 6:09 PM CEST, Laurent Pinchart wrote:
>> > On Fri, Jun 05, 2026 at 05:18:43PM +0200, Luca Ceresoli wrote:
>> >> Hello Sean,
>> >>
>> >> +Cc Marek, Maxime.
>> >>
>> >> On Sat May 30, 2026 at 8:51 PM CEST, Sean Nyekjaer wrote:
>> >> > On Wed, May 27, 2026 at 02:27:36PM +0100, Sudarshan Shetty wrote:
>> >> >> The current DSI configuration enables MIPI_DSI_MODE_VIDEO_BURST.
>> >> >> while burst mode is supported by the hardware, its use
>> >> >> depends on continuous clock behavior from the DSI host. In practice,
>> >> >> burst mode may introduce instability depending on the host controller
>> >> >> implementation, as the DSI link may transition to low-power state
>> >> >> between bursts.
>> >> >>
>> >> >> Testing showed improved display stability when using non-burst mode on
>> >> >> affected panels.
>> >> >>
>> >> >> Remove MIPI_DSI_MODE_VIDEO_BURST and use non-burst video mode.
>> >> >
>> >> > We briefly talked about this at Embedded Recipes
>> >> > I promised to sent a link:
>> >> > https://lore.kernel.org/all/E35054BA-FBE5-4CEE-905C-1F5D20140590@geanix.com/
>> >>
>> >> Thanks for the discussion at ER and for this follow-up e-mail.
>> >>
>> >> > When burst mode is enabled, the LVDS clock gets way to high for my
>> >> > panel. I don't know if it's the DSI controller in the STM32MP1 or
>> >> > something not supported on the TI side.
>> >> >
>> >> > We have been running with this fix for 2 years :)
>> >>
>> >> If I can summarize the situation in the last 4 years:
>> >>
>> >>  * Several users reported the same trouble
>> >>  * Those users patch their kernel out of tree to disable burst mode as a
>> >>    workaround
>> >>  * According to Marek the correct way to make burst mode work is
>> >>    implementing link negotiation
>> >>  * Nobody is willing to implement link negotiation as of now
>> >>
>> >> And this leads me to some questions.
>> >>
>> >>  * Do we want to keep the current situation (everybody beats their head on
>> >>    the wall until they discover disabling burst mode "fixes" their panel,
>> >>    and keep an out of tree patch)?
>> >>
>> >>  * Assuming the priority is getting a screen working (and not saving power
>> >>    on a black screen), would it make sense to apply this patch, and let
>> >>    people improve in the future by implementing link negotiation?
>> >>
>> >>    Let's pretend for a moment this is a new driver being developed: would
>> >>    it be OK to have a basic working driver, without some power optimization
>> >>    features which can be added later on? The only valid answer to this
>> >>    question is obviously "yes". Doesn't the same principle apply here? If
>> >>    it doesn't, why?
>> >>
>> >>  * What is the expected power saving with burst mode?
>> >>
>> >>    I'm afraid I don't have precise numbers but I measured the total board
>> >>    consumption with or without burst mode (the former with a black screen
>> >>    but backlight enabled) and found no difference: exactly 12.74 W in both
>> >>    cases.
>> >>
>> >> Thanks for you rpatience in reading this. I hope it helps in finding a
>> >> better solution.
>> >
>> > Rephrasing this a bit, is the discussion about dropping support for a
>> > supported feature (burst mode) because users who suffer from the lack of
>> > another feature (link negotiation) are not willing to spend time
>> > implementing it,
>>
>> That's the question I have, more or less. I have no answer yet, I'm mostly
>> trying to clarify the situation in the first place, for myself and anyone
>> interested.
>>
>> Maybe it's worth pointing out that AFAICU any driver enabling burst mode is
>> buggy because in lack of link negotiation it may work or not, based on pure
>> luck.
>>
>> > and would prefer if users of burst mode were forced to
>> > do the work instead ? That doesn't seem very fair to me.
>>
>> Link negotiation is not just "another feature" w.r.t. burts mode. It's a
>> prerequisite for burst mode to work reliably. So hard-enabling burst mode
>> was building a roof without solid walls (link negotiation).
>>
>> So I'm rephrasing your the question :) as: shouldn't users of burst mode be
>> forced to implement link negotiation, since _they_ need it?
>
> It's quite annoying when both positions have compelling arguments :-)
>
> Has anyone analyzed what work would be needed to implement the link
> negotiation ?

Marek did multiple attempts in the past, I think it's the best analysis
available in public as of now. Following the link provided by Sean Nyekjaer
in this thread on May 30 leads to them:

 * https://lore.kernel.org/dri-devel/20220801131113.182487-1-marex@denx.de/
 * https://lore.kernel.org/all/20220219002844.362157-1-marex@denx.de/

> Dropping burst mode without any plan to support it will be
> demotivating for some people. If asking for link negotiation support to
> support non-burst mode is too much yak shaving, would researching a
> technical plan be an acceptable middleground ?

On my side I haven't gone into the details. But I'm not sure having a plan
would help: Marek had one, but it didn't fly.

Maybe Marek can comment on this?

Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com