RE: [PATCH] drm/bridge: panel: Fix device link for DRM_BRIDGE_ATTACH_NO_CONNECTOR

[Ying Liu]

Hi Angelo,

Thank you for the patch.

On Monday, September 18, 2023 11:01 PM, AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx> wrote:
> When external bridges are attached with
> DRM_BRIDGE_ATTACH_NO_CONNECTOR,
> the panel bridge may also get the same flag, but in the .attach()
> callback for the panel bridge a device link is added only when this
> flag is not present; To make things worse, the .detach() callback
> tries to delete the device link unconditionally and without checking
> if it was created in the first place, crashing the kernel with a NULL
> pointer kernel panic upon calling panel_bridge_detach().
> 
> Fix that by moving the device_link_add() call before checking if the
> DRM_BRIDGE_ATTACH_NO_CONNECTOR flag is present.
> 
> Fixes: 199cf07ebd2b ("drm/bridge: panel: Add a device link between drm
> device and panel device")
> Signed-off-by: AngeloGioacchino Del Regno
> <angelogioacchino.delregno@xxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/bridge/panel.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/panel.c
> b/drivers/gpu/drm/bridge/panel.c
> index e00d2e94c751..8c507dfd589e 100644
> --- a/drivers/gpu/drm/bridge/panel.c
> +++ b/drivers/gpu/drm/bridge/panel.c
> @@ -67,14 +67,6 @@ static int panel_bridge_attach(struct drm_bridge
> *bridge,
>  	struct drm_device *drm_dev = bridge->dev;
>  	int ret;
> 
> -	if (flags & DRM_BRIDGE_ATTACH_NO_CONNECTOR)
> -		return 0;
> -
> -	if (!bridge->encoder) {
> -		DRM_ERROR("Missing encoder\n");
> -		return -ENODEV;
> -	}
> -
>  	panel_bridge->link = device_link_add(drm_dev->dev, panel->dev,
>  					     DL_FLAG_STATELESS);
>  	if (!panel_bridge->link) {
> @@ -83,6 +75,14 @@ static int panel_bridge_attach(struct drm_bridge
> *bridge,
>  		return -EINVAL;
>  	}
> 
> +	if (flags & DRM_BRIDGE_ATTACH_NO_CONNECTOR)
> +		return 0;
> +
> +	if (!bridge->encoder) {
> +		DRM_ERROR("Missing encoder\n");

Shouldn't the device link be deleted in case of error?

Regards,
Liu Ying

> +		return -ENODEV;
> +	}
> +
>  	drm_connector_helper_add(connector,
>  				 &panel_bridge_connector_helper_funcs);
> 
> --
> 2.42.0