Re: [PATCH v5 04/10] drm/bridge: add documentation of refcounted bridges

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

Hi Maxime,

On Tue, 28 Jan 2025 15:49:23 +0100
Maxime Ripard <mripard@kernel.org> wrote:

> Hi,
> 
> On Wed, Jan 22, 2025 at 05:12:30PM +0100, Luca Ceresoli wrote:
> > On Wed, 8 Jan 2025 17:02:04 +0100
> > Maxime Ripard <mripard@kernel.org> wrote:
> > 
> > [...]
> >   
> > > > > > And we'll also need some flag in drm_bridge to indicate that the device
> > > > > > is gone, similar to what drm_dev_enter()/drm_dev_exit() provides,
> > > > > > because now your bridge driver sticks around for much longer than your
> > > > > > device so the expectation that your device managed resources (clocks,
> > > > > > registers, etc.) are always going to be around.      
> > > > 
> > > > Yes, makes sense too. That should be a drm_bridge_enter/exit(), and
> > > > drm_bridge.c will need to be sprinkled with them I guess.    
> > > 
> > > The users would be the drivers, most likely. There's not much we can do
> > > at the framework level, unfortunately.  
> > 
> > Back to the idea of a "gone" flag, or perhaps an "unplugged" flag to
> > be consistent with the struct drm_device naming, and
> > drm_bridge_enter()/drm_bridge_exit(), I did a few experiments and have
> > a question.
> > 
> > In case:
> > 
> >   a) there is a notification callback to inform about bridges
> >      being removed, and
> >   b) all entities owning a struct drm_bridge pointer stop using
> >      that pointer when notified
> > 
> > 
> > With the above, there should be no need for
> > drm_bridge_enter()/drm_bridge_exit(). Nobody will be using a pointer to
> > a bridge that is being removed.
> > 
> > Now, about a), patch 1 in this series implements such a mechanism to
> > inform all bridges when a bridge is being removed. Note that the
> > "unplugged" flag would be set immediately after the notifier callback
> > is currently called: "unplugged == true" will never happen before the
> > callback, and after the callback there will be no pointer at all.
> > 
> > Patch 1 however is only notifying bridges, so other entities (e.g.
> > encoders) cannot be notified with this implementation. However a
> > different notification mechanism can be implemented. E.g. until v3 this
> > series was using a generic struct notifier_block for this goal [0], so
> > any part of the kernel can be notified.
> > 
> > About b), the notification appears simpler to implement in the various
> > drivers as it needs to be added in one place per driver. Also adding
> > drm_bridge_enter()/exit() can be trickier to get right for non-trivial
> > functions.
> > 
> > Do you see any drawback in using a notification mechanism instead of
> > drm_bridge_enter()/exit() + unplugged flag?  
> 
> Yeah, because we're not considering the same thing :)
> 
> The issue you're talking about is that you want to be notified that the
> next bridge has been removed and you shouldn't use the drm_bridge
> pointer anymore.
> 
> A notification mechanism sounds like a good solution there.
> 
> The other issue we have is that now, we will have the drm_bridge pointer
> still allocated and valid after its device has been removed.
> 
> In which case, you need to be able to tell the bridge driver whose
> device got removed that the devm resources aren't there anymore, and it
> shouldn't try to access them.
> 
> That's what drm_bridge_enter()/exit is here for.

Let me rephrase to check I got what you mean.

A) On bridge removal, use a notifier to notify all consumers of that
bridge that they have to stop using the pointer to the bridge about to
be removed.

B) Internally in the bridge driver (provider) use
drm_bridge_enter()/exit() to forbid access to resources when the
hardware is unplugged.

And also: bridge consumers won't need to use drm_bridge_enter()/exit()
as they will clear their pointer before setting the unplugged flag.

Is my understanding of your idea correct?

If it is, I tend to agree, and I like it.

I like it, except for one point  I'm afraid. Why do we need enter/exit
inside the driver (provider) code? At driver release, the driver
instance won't exist anymore. Sure the private struct embedding a
struct drm_bridge will be still allocated for some time, but the struct
device will not exist, and the device driver instance as well.

Luca

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