Re: [PATCH 19/37] drm/bridge: samsung-dsim: move drm_bridge_add() call to probe

[Maxime Ripard]

On Tue, May 19, 2026 at 12:37:36PM +0200, Luca Ceresoli wrote:
> This bridge driver calls drm_bridge_add() in the DSI host .attach callback
> instead of in the probe function.
> 
> This works for current use cases but is problematic for supporting hotplug
> of DRM bridges. The problematic case is when this DSI host is always
> present while its DSI device is hot-pluggable. In such case with the
> current code the DRM card will not be populated until after the DSI device
> attaches to the host, which could happen a very long time after booting, or
> even not happen at all.
> 
> The reason is that the previous pipeline component (the encoder in this
> case) when probing cannot find the samsung-dsim bridge. What happens is:
> 
>  [1 and 2 can happen in any order, same result]
>  1) samsung-dsim probes (does not drm_bridge_add() itself)
>  2) The lcdif starts probing multiple times, but
>     lcdif_probe
>     -> lcdif_load
>        -> lcdif_attach_bridge
>           -> devm_drm_of_get_bridge() returns -EPROBE_DEFER because
>              the samsung-dsim is not in the global bridge_list
>              (deferred probe pending: imx-lcdif: Cannot connect bridge)
> 
> The samsung-dsim will not drm_bridge_add() itself until a DSI device will
> try to mipi_dsi_attach() to the DSI Host, which can happen arbitratily late
> on hot-pluggable hardware.
> 
> As a preliminary step to supporting hotplug move drm_bridge_add() at probe
> time, so that the samsung-dsim DSI host bridge is available during boot,
> even without a connected DSI device. This results in:
> 
>  1) samsung-dsim probes (and adds to drm_bridge_add() itself)
>  2) The lcdif starts probing multiple times, but
>     lcdif_probe
>     -> lcdif_load
>        -> lcdif_attach_bridge
>           -> devm_drm_of_get_bridge() --> OK, returns samsung-dsim ptr
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>

We should probably amend
https://www.kernel.org/doc/html/latest/gpu/drm-kms-helpers.html#special-care-with-mipi-dsi-bridges

To mention this use case here

Maxime

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