Re: [PATCH v1] drm/bridge: ssd2825: Fix panel enable sequence

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

On Sun Oct 25, 2026 at 2:36 PM CET, Svyatoslav Ryhel wrote:
> Currently, in the case of a single bridge and a single panel, the DRM
> framework calls panel prepare() > bridge pre_enable() > bridge enable() >
> panel enable(). While this is fine for most cases, the SSD2825 requires
> the last two steps to be swapped. There is no meaningful way to achieve
> this using standard bridge chain configuration, so the bridge previously
> called panel_enable() directly at the end of its pre_enable() hook to
> gain fine-grained control over the sequence.
>
> Commit 554d36acf5e0 ("drm/bridge: ssd2825: don't create a panel_bridge")
> removed this call, resulting in a regression where display output is
> missing because the panel enable function is now called after the bridge
> completes its configuration.
>
> Since panels are now automatically wrapped into the bridge framework,
> invoke the next bridge's atomic enable directly, similar to how panel
> enabling was handled previously.
>
> This restores panel support on the LG Optimus 4X HD (P880) and
> Optimus Vu (P895).
>
> Fixes: 554d36acf5e0 ("drm/bridge: ssd2825: don't create a panel_bridge")
> Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
> ---
>  drivers/gpu/drm/bridge/ssd2825.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/drivers/gpu/drm/bridge/ssd2825.c b/drivers/gpu/drm/bridge/ssd2825.c
> index 71877699dfab6..9216cee09115f 100644
> --- a/drivers/gpu/drm/bridge/ssd2825.c
> +++ b/drivers/gpu/drm/bridge/ssd2825.c
> @@ -475,6 +475,7 @@ static void ssd2825_bridge_atomic_pre_enable(struct drm_bridge *bridge,
>  					     struct drm_atomic_commit *state)
>  {
>  	struct ssd2825_priv *priv = bridge_to_ssd2825(bridge);
> +	struct drm_bridge *next_bridge = priv->output.bridge;
>  	struct mipi_dsi_device *dsi_dev = priv->output.dev;
>  	const struct drm_crtc_state *crtc_state;
>  	const struct drm_display_mode *mode;
> @@ -569,6 +570,14 @@ static void ssd2825_bridge_atomic_pre_enable(struct drm_bridge *bridge,
>  	/* Initial DSI configuration register set */
>  	ssd2825_write_reg(priv, SSD2825_CONFIGURATION_REG, config);
>  	ssd2825_write_reg(priv, SSD2825_VC_CTRL_REG, 0);
> +
> +	/*
> +	 * The SSD2825 requires that the next bridge be enabled first;
> +	 * otherwise, the SSD2825 cannot complete its configuration and
> +	 * there will be no output.
> +	 */
> +	if (next_bridge->funcs && next_bridge->funcs->atomic_enable)
> +		next_bridge->funcs->atomic_enable(next_bridge, state);
>  }

I wonder whether this logic could be moved to the drm_bridge core, based on
a flag somewhat like pre_enable_prev_first. I think it would be tricky to
implement though.

Anyway, this fix seems to correctly re-instate the behaviour that was in
place before 554d36acf5e0, so:

Reviewed-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

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