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

Maxime Ripard
2025-02-10T18:17:42Z

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On Mon, Feb 10, 2025 at 04:23:44PM +0200, Dmitry Baryshkov wrote:
> On Mon, 10 Feb 2025 at 14:31, Maxime Ripard <mripard@kernel.org> wrote:
> >
> > 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@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.
>
> A problem of panel bridges having the wrong lifetime because of devm_
> attachment to a wrong device and so either being kept for too long or
> being destroyed too early.

Yeah, and panels too. Which isn't solved there.

> > 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.
>
> I have doubts about cocci. It doesn't have a way to know, what is the
> lifetime of the references to the reference-holding memory. Maybe I'm
> missing a point there.

It doesn't matter? Cocci doesn't (have to) understand the code, it
generates a patch for you. It's up to the author to make a correct
patch.

Maxime

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