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

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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 ? 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 ?

> Again: AFAICU, I'm trying to understand and improve things.

-- 
Regards,

Laurent Pinchart