Re: [RFC v3 03/33] rust: drm/kms: Introduce the main ModeConfigObject traits

[Lyude Paul <lyude@redhat.com>]

On Fri, 2025-03-14 at 11:44 +0100, Maxime Ripard wrote:
> On Wed, Mar 05, 2025 at 05:59:19PM -0500, Lyude Paul wrote:
> > The KMS API has a very consistent idea of a "mode config object", which
> > includes any object with a drm_mode_object struct embedded in it. These
> > objects have their own object IDs which DRM exposes to userspace, and we
> > introduce the ModeConfigObject trait to represent any object matching these
> > characteristics.
> > 
> > One slightly less consistent trait of these objects however: some mode
> > objects have a reference count, while others don't. Since rust requires
> > that we are able to define the lifetime of an object up-front, we introduce
> > two other super-traits of ModeConfigObject for this:
> 
> I'm not entirely sure what you mean by that, sorry. Would you have a
> small example of the challenge that forced you to split it into two
> separate traits?

So - ModeObject itself of course defines the methods that should be available
on any mode object, the ability to get a raw pointer to the drm_mode_object
struct - and a reference to the DRM Device. I assume you might understand
struct mode_config already on the C side of things, e.g. we have mode objects
like drm_connector, drm_framebuffer, etc. - they have various object IDs.
Additionally, some mode objects are refcounted (drm_connector and
drm_framebuffer are two examples), while others aren't (drm_crtc, drm_plane).

Now, object lifetimes in Rust and C are pretty different. You need to have
well defined lifetimes for everything in both languages, but C leaves this as
an exercise for the programmer with little to no formal verification. Rust on
the other hand is very explicit about this, and requires that you ensure in
some way that the compiler is able to verify these lifetimes. Even for
resources that might live for as long as the driver itself does, we still need
to be able to ensure this is /always/ the case and prove it to the compiler.
For short function scopes where we're passed a reference (&'a Device) to the
device, we can just pass a reference to the mode object using a lifetime that
lives as long as the device reference 'a, or define a new lifetime that is a
separate borrow but lives just as long ('b: 'a). For long living structures
that leave the scope of a &'a Device though, it's impossible for us to define
a lifetime to express this.

In rvkms we do actually have an example of this, where we use an hrtimer to
emulate a vblank interrupt. Since we need access to the respective CRTC for
the vblank interrupt, the structure we embed our hrtimer in either be the Crtc
itself (I'll explain a little below why we can, but don't really want to do
this in rust), or to be able to somehow ensure that Crtc can't be destroyed
for as long as the hrtimer containing vblank timer struct lives.

So, in rust when you face situations like this: generally the solution is to
do something that ensures the object in question lives for as long as
necessary. The easiest form of this of course is refcounting, which we can
easily fallback to for mode objects that have a refcount (hence RcModeObject).
For RcModeObject, this super-trait is mainly just there to make implementing
the kernel's types for objects with embedded refcounts (AlwaysRefCounted)
easy; since every reference counted mode object uses the exact same functions
for ref/unref. So instead of making every ref-counted mode object implement
AlwaysRefCounted, we just introduce RcModeObject - and then have
AlwaysRefCounted automatically implemented using drm_mode_object_get()/put()
and ModeObject for any given type T that implements RcModeObject.

Note that we can't do this solely through ModeObject, simply because there's
no way to say "Implement AlwaysRefCounted for T, but only if T implements some
specific refcounting method". Trait methods can be optional, but we can't
really check if they're specified or not through just a trait bound. So
instead we make it an unsafe super-trait of ModeObject. This is also called
the "new type pattern", and is very heavily used in a lot of rust code.

But how do we actively ensure a mode object without a refcount stays alive?
What about drm_crtc, we need it for the vblank timer? The answer of course is
that we can't, BUT! We already established a static mode object is valid for
as long as it's respective Device is active. Which subsequently implies that
if can take a refcount for the Device, the static mode object will remain
alive for as long as that refcount is held. This is the main purpose of
StaticModeObject and KmsRef. StaticModeObject just indicates a ModeObject with
no refcount, and KmsRef can contain any StaticModeObject. And KmsRef will keep
the mode object alive by using ModeObject's trait methods to acquire a owned
refcount to the parent device for as long as the KmsRef lives.

Rememeber too: this needs to use the new type pattern as well, there's no way
for us to create a trait bound that relies on not implementing a trait
(!Send/!Sync are exceptions to this, but that's out of scope for this
explanation).

> 
> > * StaticModeObject - this trait represents any mode object which does not
> >   have a reference count of its own. Such objects can be considered to
> >   share the lifetime of their parent KMS device
> 
> I think that part is true for both cases. I'm not aware of any
> reference-counted object that might outlive the DRM device. Do you have
> an example?

Nope - none of them would, but we should be ensuring that the DRM device is
alive (and at least allocated) for as long as any owned reference (static or
rc) to a mode object. Though…

You did just make me have the sad realization that a reference counted mode
object does not in fact, ensure that it's parent device stays alive with
drm_device_get(). I guess that's probably something worth me sending a patch
for D:!.

> 
> > * RcModeObject - this trait represents any mode object which does have its
> >   own reference count. Objects implementing this trait get a free blanket
> >   implementation of AlwaysRefCounted, and as such can be used with the ARef
> >   container without us having to implement AlwaysRefCounted for each
> >   individual mode object.
> > 
> > This will be able to handle most lifetimes we'll need with one exception:
> > it's entirely possible a driver may want to hold a "owned" reference to a
> > static mode object.
> 
> I guess it kind of derives from the conversation above, but would you
> have an example of a driver wanting to have a reference to a mode object
> that isn't on the same lifetime than the DRM device?
> 
> Maxime

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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