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

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

On Fri, 7 Feb 2025 04:49:21 +0200
Dmitry Baryshkov <dmitry.baryshkov@linaro.org> wrote:

> 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(-)
> >   
> 
> LGTM, minor issue below.
> 
> > @@ -1018,6 +1067,11 @@ struct drm_bridge *devm_drm_panel_bridge_add_typed(struct device *dev,
> >  {
> >  	struct drm_bridge **ptr, *bridge;
> >  
> > +	if (panel->bridge) {
> > +		DRM_DEBUG("panel %s: returning existing bridge=%p", dev_name(dev), panel->bridge);
> > +		return panel->bridge;
> > +	}  
> 
> Shouldn't the rest of the function also be removed as you do in other
> cases?

Indeed it should.

And even more, I now realize drm_panel_bridge_add_typed() should also
become a simple 'return panel->bridge', like its devm and drmm
variants, and its code, implementing the actual creation of a panel
bridge, move to an internal function. Otherwise this patch is a bug:
existing drivers which do call drm_panel_bridge_add_typed() would end
up in having two panel_bridges for the same panel.

I must say the process of developing this patch together with the
hotplug work was "convoluted" to say the least, which probably explains
why this got unnoticed so far.

Luca

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