[PATCH v6 14/26] drm/bridge: add support for refcounted DRM bridges

[Maxime Ripard]

On Fri, Feb 07, 2025 at 09:54:06PM +0200, Dmitry Baryshkov wrote:
> On Fri, Feb 07, 2025 at 12:47:51PM +0100, Maxime Ripard wrote:
> > Hi,
> > 
> > On Thu, Feb 06, 2025 at 07:14:29PM +0100, Luca Ceresoli wrote:
> > > DRM bridges are currently considered as a fixed element of a DRM card, and
> > > thus their lifetime is assumed to extend for as long as the card
> > > exists. New use cases, such as hot-pluggable hardware with video bridges,
> > > require DRM bridges to be added and removed to a DRM card without tearing
> > > the card down. This is possible for connectors already (used by DP MST), so
> > > add this possibility to DRM bridges as well.
> > > 
> > > Implementation is based on drm_connector_init() as far as it makes sense,
> > > and differs when it doesn't. A difference is that bridges are not exposed
> > > to userspace, hence struct drm_bridge does not embed a struct
> > > drm_mode_object which would provide the refcount. Instead we add to struct
> > > drm_bridge a refcount field (we don't need other struct drm_mode_object
> > > fields here) and instead of using the drm_mode_object_*() functions we
> > > reimplement from those functions the few lines that drm_bridge needs for
> > > refcounting.
> > > 
> > > Also add a new devm_drm_bridge_alloc() macro to allocate a new refcounted
> > > bridge.
> > > 
> > > Signed-off-by: Luca Ceresoli <luca.ceresoli at bootlin.com>
> > 
> > So, a couple of general comments:
> > 
> > - I've said it a couple of times already, but I really think you're
> >   making it harder than necessary for you here. This (and only this!)
> >   should be the very first series you should be pushing. The rest can
> >   only ever work if that work goes through, and it's already hard enough
> >   as it is. So, split that patch into a series of its own, get that
> >   merged, and then we will be able to deal with panels conversion and
> >   whatever. That's even more true with panels since there's ongoing work
> >   that will make it easier for you too. So the best thing here is
> >   probably to wait.
> 
> Luca and I had a quick chat on this during FOSDEM. I really think that
> panel (part of the) series can go in first as it fixes a very well known
> bug _and_ allows a pretty good cleanup to a whole set of drivers.

I don't necessarily disagree on principle, but if you state that it can
get first, and fixes a known problem (which one?), then it should be a
separate, standalone, series.

Ever-expanding features are bad for both the reviewers and the
contributors, even more so when the discussion happens off-list.

> With all those panel / bridge wrappers gone we should be able to see a
> clearer picture of what individual drivers are doing. In other words,
> which memory and which code actually hosts and uses internal
> 'next_bridge' reference.
> 
> > - This patch really needs to be split into several patches, something
> >   along the lines of:
> > 
> >   + Creating devm_drm_bridge_alloc()
> >   + Adding refcounting
> >   + Taking the references in all the needed places
> >   + Converting a bunch of drivers
> 
> The last two parts seem troublematic to me, but, I must admit, I didn't
> spend so much time reviewing all drm_bridge usage patterns.

Why? the third one is already done by that patch, the fourth can
relatively easily be done using coccinelle.

Maxime
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