linux-kernel - Re: [PATCH v1] Revert "drm: bridge: ti-sn65dsi83: Add error recovery mechanism"

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Message-ID: <20251201184039.79dfe0bb@bootlin.com>
Date: Mon, 1 Dec 2025 18:40:39 +0100
From: Herve Codina <herve.codina@...tlin.com>
To: Maxime Ripard <mripard@...nel.org>, Francesco Dolcini
 <francesco@...cini.it>, João Paulo Gonçalves <jpaulo.silvagoncalves@...il.com>, Emanuele Ghidoli
 <emanuele.ghidoli@...adex.com>
Cc: Luca Ceresoli <luca.ceresoli@...tlin.com>, 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>, Maarten Lankhorst
 <maarten.lankhorst@...ux.intel.com>, Thomas Zimmermann
 <tzimmermann@...e.de>, David Airlie <airlied@...il.com>, Simona Vetter
 <simona@...ll.ch>, Francesco Dolcini <francesco.dolcini@...adex.com>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org, Tomi Valkeinen <tomi.valkeinen@...asonboard.com>
Subject: Re: [PATCH v1] Revert "drm: bridge: ti-sn65dsi83: Add error
 recovery mechanism"

Hi Maxime,

On Mon, 1 Dec 2025 17:36:13 +0100
Maxime Ripard <mripard@...nel.org> wrote:

> Hi,
> 
> On Thu, Nov 27, 2025 at 09:46:07AM +0100, Luca Ceresoli wrote:
> > Hello Francesco, all,
> > 
> > On Tue Nov 25, 2025 at 11:38 AM CET, Francesco Dolcini wrote:  
> > > From: Francesco Dolcini <francesco.dolcini@...adex.com>
> > >
> > > This reverts commit ad5c6ecef27e ("drm: bridge: ti-sn65dsi83: Add error
> > > recovery mechanism").
> > >
> > > The reverted commit introduces a regression on Verdin AM62, and
> > > potentially on more devices, not being able to generate a clock
> > > that the TI SN65DSI83 PLL can lock to, with the display periodically
> > > blinking.
> > >
> > > Verdin AM62 SoM has a Toshiba TC358778 DPI to DSI bridge, that can be
> > > connected to an LVDS display over a TI SN65DSI83 bridge. Before this
> > > change despite the TI SN65DSI83 reporting with a debug print a PLL
> > > locking error the display was working fine with no visible glitches.
> > >
> > > The reasons for this issue was investigated without getting to a final
> > > conclusion:
> > >
> > >  - the DPI clock was measure and it is stable/accurate
> > >  - the DSI clock was not possible to measure, but this setup is used
> > >    with other display/bridges with no known issues
> > >  - the DSI clock is configured in continuous mode
> > >  - the actual DSI clock generated from the TC358778 is generate with a
> > >    PLL from a 25MHz reference clock
> > >  - it's not clear why some frequencies are working and some are not, for
> > >    example 50000000, 68750000, 72750000, 75000000 frequencies are fine,
> > >    while 69750000, 71100000, 72500000 are not
> > >
> > > Given that the safest approach is to just revert the commit, till a
> > > proper solution for error recovery that is not introducing regression
> > > is figured out.
> > >
> > > Reported-by: João Paulo Gonçalves <jpaulo.silvagoncalves@...il.com>
> > > Closes: https://lore.kernel.org/all/bhkn6hley4xrol5o3ytn343h4unkwsr26p6s6ltcwexnrsjsdx@mgkdf6ztow42/
> > > Fixes: ad5c6ecef27e ("drm: bridge: ti-sn65dsi83: Add error recovery mechanism")
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Francesco Dolcini <francesco.dolcini@...adex.com>  
> > 
> > Thanks for having sent this revert patch.
> > 
> > However after evaluating the overall situation I decided to send a
> > different patch to address this issue in the short term. The idea is to
> > just ignore the PLL_UNLOCK error, keeping the existing
> > structure. Rationale:
> > 
> >  * this sloves the issue for Toradex, based on João's initial report
> >  * there is no evidence of any bugs in the recovery mechanism, it's
> >    just exposing a pre-existing problem that was only producing a
> >    non-fatal dev_err() before
> >  * a full revert would remove error checking for all errors, including
> >    those not creating any issue, thus removing a useful feature
> >  * a full revert would require rewriting patches such as [0] (not a big
> >    deal per se, but see next bullet)
> >  * after patches such as [0] are applied, re-adding the error recovery
> >    mechanism would require another rework, so more work for authors,
> >    reviewers, testers and maintainers  
> 
> Were are we on this? Both patches work for me, but we need to take a decision.
> 

IMHO, Luca's patch [0] is more interesting than this current patch doing a
full revert. Indeed, Luca's patch keeps the monitoring active except for
cases we known broken.

Francesco, Emanuele, João have you had time to test Luca's patch ?

[0] https://lore.kernel.org/all/20251127-drm-ti-sn65dsi83-ignore-pll-unlock-v1-1-8a03fdf562e9@bootlin.com/

Best regards,
Hervé