Re: [PATCH v6 08/26] drm/bridge: panel: add a panel_bridge to every panel

[Maxime Ripard <mripard@kernel.org>]

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On Thu, Feb 06, 2025 at 07:14:23PM +0100, Luca Ceresoli wrote:
> Adding a panel does currently not add a panel_bridge wrapping it. Usually
> the panel_bridge creation happens when some other driver (e.g. the previous
> bridge or the encoder) calls *_of_get_bridge() and the following element in
> the pipeline is a panel.
> 
> This has some drawbacks:
> 
>  * the panel_bridge is not created in the context of the driver of the
>    underlying physical device (the panel driver), but of some other driver
>  * that "other driver" is not aware of whether the returned drm_bridge
>    pointer is a panel_bridge created on the fly, a pre-existing
>    panel_bridge or a non-panel bridge
>  * removal of a panel_bridge requires calling drm_panel_bridge_remove(),
>    but the "other driver" doesn't know whether this is needed because it
>    doesn't know whether it has created a panel_bridge or not
> 
> So far this approach has been working because devm and drmm ensure the
> panel bridge would be dealloacted at some later point. However with the
> upcoming implementation of dynamic bridge lifetime this will get more
> complicated.
> 
> Correct removal of a panel_bridge might possibly be obtained by adding more
> devm/drmm technology to have it freed correctly at all times. However this
> would add more complexity and not help making lifetime more understandable.
> 
> Use a different approach instead: always create a panel_bridge with a
> drm_panel, thus matching the lifetime of the drm_panel and the panel_bridge
> wrapping it. This makes lifetime much more straightforward to understand
> and to further develop on.
> 
> With the panel_bridge always created, the functions to get a bridge
> [devm_drm_of_get_bridge() and drmm_of_get_bridge()] become simpler because
> the bridge they are looking for exists already (if it can exist at all). In
> turn, this is implemented based on a variant of
> drm_of_find_panel_or_bridge() that only looks for panels:
> of_drm_find_bridge_by_endpoint(). In the future
> drm_of_find_panel_or_bridge() can be progressively removed because there
> will never be a panel not exposing a bridge.
> 
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> 
> ---
> 
> This patch was added in v6.
> ---
>  drivers/gpu/drm/bridge/panel.c | 74 +++++++++++++++++++++++++++++++++---------
>  include/drm/drm_panel.h        |  8 +++++
>  2 files changed, 66 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/panel.c b/drivers/gpu/drm/bridge/panel.c
> index 58570ff6952ca313b3def084262c9bb3772272ef..6995de605e7317dd1eb153afd475746ced764712 100644
> --- a/drivers/gpu/drm/bridge/panel.c
> +++ b/drivers/gpu/drm/bridge/panel.c
> @@ -69,6 +69,9 @@ EXPORT_SYMBOL(drm_panel_init);
>   */
>  void drm_panel_add(struct drm_panel *panel)
>  {
> +	panel->bridge = drm_panel_bridge_add(panel);
> +	WARN_ON(!panel->bridge);
> +
>  	mutex_lock(&panel_lock);
>  	list_add_tail(&panel->list, &panel_list);
>  	mutex_unlock(&panel_lock);
> @@ -86,6 +89,9 @@ void drm_panel_remove(struct drm_panel *panel)
>  	mutex_lock(&panel_lock);
>  	list_del_init(&panel->list);
>  	mutex_unlock(&panel_lock);
> +
> +	drm_panel_bridge_remove(panel->bridge);
> +	panel->bridge = NULL;
>  }
>  EXPORT_SYMBOL(drm_panel_remove);

Given that drm_panel_add and drm_panel_remove are typically called at
probe/remove, it's pretty much equivalent to using devm. Both of these
solutions aren't safe, and the drm_panel lifetime is still broken.

I'd rather work on a solution that actually fixes those lifetime issues.

Maxime

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