Re: [PATCH v2 34/34] drm/bridge: panel: convert to devm_drm_bridge_alloc() API

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

On Mon, 5 May 2025 08:23:26 +0200
Maxime Ripard <mripard@kernel.org> wrote:

> On Mon, Apr 28, 2025 at 05:25:16PM +0200, Luca Ceresoli wrote:
> > Hi Maxime,
> > 
> > On Mon, 28 Apr 2025 13:39:23 +0200
> > Maxime Ripard <mripard@kernel.org> wrote:
> >   
> > > On Thu, Apr 24, 2025 at 10:05:49PM +0200, Luca Ceresoli wrote:  
> > > > This is the new API for allocating DRM bridges.
> > > > 
> > > > The devm lifetime management of this driver is peculiar. The underlying
> > > > device for the panel_bridge is the panel, and the devm lifetime is tied the
> > > > panel device (panel->dev). However the panel_bridge allocation is not
> > > > performed by the panel driver, but rather by a separate entity (typically
> > > > the previous bridge in the encoder chain).
> > > > 
> > > > Thus when that separate entoty is destroyed, the panel_bridge is not
> > > > removed automatically by devm, so it is rather done explicitly by calling
> > > > drm_panel_bridge_remove(). This is the function that does devm_kfree() the
> > > > panel_bridge in current code, so update it as well to put the bridge
> > > > reference instead.
> > > > 
> > > > Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>    
> > > 
> > > This looks fine, but we need a TODO entry to clean this up later on, and
> > > a comment on devm_drm_put_bridge that this is inherently unsafe and
> > > must not be used.  
> > 
> > Ah, I see, OK.
> > 
> > Quick draft:
> > 
> >  /**
> >   * devm_drm_put_bridge - Release a bridge reference obtained via devm
> >   * @dev: device that got the bridge via devm
> >   * @bridge: pointer to a struct drm_bridge obtained via devm
> >   *
> >   * Same as drm_bridge_put() for bridge pointers obtained via devm functions
> >   * such as devm_drm_bridge_alloc().
> > + *
> > + * This function is a temporary workaround and MUST NOT be used. Manual
> > + * handling of bridge lifetime is inherently unsafe.
> >   */  
> 
> That part looks good to me
> 
> > and:
> > 
> > -	devm_kfree(panel_bridge->panel->dev, bridge);
> > +       /* TODO remove this after reworking panel_bridge lifetime */
> > +	devm_drm_put_bridge(panel_bridge->panel->dev, bridge);
> >  }
> > 
> > Does it look good enough?  
> 
> That too, but I was talking about an entry in
> https://www.kernel.org/doc/html/latest/gpu/todo.html

Ah, sure!

I queued this for v3, if you have better suggestions don't hesitate to
let me know:

-----8<-----

Remove devm_drm_put_bridge()
----------------------------

Due to how the panel bridge handles the drm_bridge object lifetime, special
care must be taken to dispose of the drm_bridge object when the
panel_bridge is removed. This is currently managed using
devm_drm_put_bridge(), but that is an unsafe, temporary workaround. To fix
that, the DRM panel lifetime needs to be reworked. After the rework is
done, remove devm_drm_put_bridge() and the TODO in drm_panel_bridge_remove().

Contact: Maxime Ripard <mripard@kernel.org>,
         Luca Ceresoli <luca.ceresoli@bootlin.com>

Level: Intermediate

-----8<-----

Luca

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