Re: [PATCH v9 2/2] drm/bridge: add support for lontium lt9211c bridge

[Philipp Zabel]

On Fr, 2026-09-25 at 15:23 +0200, Philipp Zabel wrote:
> On Do, 2026-09-24 at 21:29 +0530, vishnu.saini@xxxxxxxxxxxxxxxx wrote:
> > From: Yi Zhang <zhanyi@xxxxxxxxxxxxxxxx>
> > 
> > LT9211C is a Single/Dual-Link DSI/LVDS or Single DPI input to
> > Single-Link/Dual-Link DSI/LVDS or Single DPI output bridge chip.
> > Extend the existing lontium-lt9211 driver to support DSI-to-LVDS
> > bridge configuration by detecting and handling both LT9211 and LT9211C
> > variants from a single driver.
> > 
> > Chip detection in lt9211_read_chipid() is extended to identify the
> > LT9211C by its distinct chip ID registers, and cross-checked against
> > the chip type requested by the DT compatible string to catch a
> > mismatched board/compatible combination.
> > 
> > Add LT9211C-specific regmap support and use lt9211_chip_data with
> > i2c_get_match_data() to provide per-chip configuration.
> > 
> > Five new functions implement the LT9211C DSI-to-LVDS initialisation
> > sequence: lt9211c_configure_rx(), lt9211c_autodetect_rx(),
> > lt9211c_configure_timing(), lt9211c_configure_plls() and
> > lt9211c_configure_tx().
> > 
> > Defer the remaining LT9211C initialization to a work item scheduled
> > from atomic_enable(), since RX auto-detection requires an active DSI
> > stream.
> 
> This is still wrong, and I don't understand why you need it.
> 
> All scheduling initialization as a work item should allow is for
> downstream bridges and/or panels to be atomic_enabled while
> lt9211_work_func() is waiting for a vblank interrupt.
> They expect the LVDS signal to be active at this point. If LVDS is
> enabled at some unknown later point in time by the work item, any
> startup timing requirements the panel might have can not be applied
> correctly.
> 
> Also, deferring initialization as a work item shouldn't have any
> influence on the upstream DSI signal. That should already be active
> when lt9211c atomic_enable is called. Could it be that there is a bug
> in your display controller or DSI bridge driver that causes the DSI
> signal to still not be completely active at this point?
> 
> > Signed-off-by: Yi Zhang <zhanyi@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Nilesh Laad <nilesh.laad@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Gopi Botlagunta <venkata.botlagunta@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Vishnu Saini <vishnu.saini@xxxxxxxxxxxxxxxx>
> > Tested-by: Philipp Zabel <p.zabel@xxxxxxxxxxxxxx>
> 
> I have not tested this version (yet).

I have now retested this on RK3576 - both as-is and with the
"drm/bridge: lt9211: drop delayed work" patch I just sent.

I still think we have to figure out why the DSI signal doesn't appear
to be streaming as it should when lt9211_atomic_enable is called in
your setup.

When enabling the lt9211c synchronously, is there any error?
Is just the Sot/RX: debug output incorrect?
Which display controller / DSI bridge drivers are used to send the DSI
signal?

regards
Philipp