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

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

Hello Francesco, all,

On Tue Nov 25, 2025 at 11:38 AM CET, Francesco Dolcini wrote:
> From: Francesco Dolcini <francesco.dolcini@toradex.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@gmail.com>
> Closes: https://lore.kernel.org/all/bhkn6hley4xrol5o3ytn343h4unkwsr26p6s6ltcwexnrsjsdx@mgkdf6ztow42/
> Fixes: ad5c6ecef27e ("drm: bridge: ti-sn65dsi83: Add error recovery mechanism")
> Cc: stable@vger.kernel.org
> Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.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

[0] https://lore.kernel.org/lkml/20251112-drm-bridge-atomic-vs-remove-v3-2-85db717ce094@bootlin.com/

Luca

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