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

[Maxime Ripard <mripard@kernel.org>]

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On Mon, Feb 10, 2025 at 06:12:03PM +0100, Luca Ceresoli wrote:
> Hello Maxime, Dmitry,
> 
> On Mon, 10 Feb 2025 16:23:44 +0200
> Dmitry Baryshkov <dmitry.baryshkov@linaro.org> 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.  
> 
> The idea you proposed was to handle the issues current panel bridge
> code adds to the hotplug work by adding a .destroy callback and some
> more devm magic. I explored the idea but even after some clarifications
> from you it still didn't appear clearly doable and correct to me. And
> even in the case it were perfectly correct and doable, it is based on
> adding more complexity and "magic" on top of a topic that is already
> hard to understand: panel_bridge lifetime.

Not really, no. I told you several time that you shouldn't deal with
panels yet.

> So I opted for the other way: rework panel_bridge code so its lifetime
> is clear and as one would expect (panel_bridge lifetime == panel
> lifetime).
> 
> Possibly more work for me, but now it's done and it's in these patches
> so why waiting?

No, it's not done. You have the same issue with panels than you are
trying to fix with bridges: it's allocated through devm so they'll get
destroyed too soon. The panel_bridge might work fine now, but the panel
won't.

So it's more work, more scope-creep, and more discussions. For example,
I'm really not convinced on moving the drm_panel code under bridge.

Splitting it up will allow you to at least merge the parts that are
somewhat agreed upon. But do however you want :)

Maxime

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