On 06/04/2023 20.25, Daniel Vetter wrote:
On Thu, Apr 06, 2023 at 02:02:55PM +0900, Asahi Lina wrote:
On 05/04/2023 23.44, Daniel Vetter wrote:
On Tue, Mar 07, 2023 at 11:25:43PM +0900, Asahi Lina wrote:
+/// Look up a GEM object handle for a `File` and return an `ObjectRef` for it.
+pub(crate) fn lookup_handle(file: &DrmFile, handle: u32) -> Result<ObjectRef> {
+ Ok(ObjectRef::new(shmem::Object::lookup_handle(file, handle)?))
+}
So maybe my expectations for rust typing is a bit too much, but I kinda
expected this to be fully generic:
- trait Driver (drm_driver) knows the driver's object type
- a generic create_handle function could ensure that for drm_file (which
is always for a specific drm_device and hence Driver) can ensure at the
type level that you only put the right objects into the drm_file
- a generic lookup_handle function on the drm_file knows the Driver trait
and so can give you back the right type right away.
Why the wrapping, what do I miss?
Sigh, so this is one of the many ways I'm trying to work around the "Rust
doesn't do subclasses" problem (so we can figure out what the best one is
^^).
The generic shmem::Object::lookup_handle() call *is* fully generic and will
get you back a driver-specific object. But since Rust doesn't do
subclassing, what you get back isn't a driver-specific type T, but rather a
(reference to a) shmem::Object<T>. T represents the inner driver-specific
data/functionality (only), and the outer shmem::Object<T> includes the
actual drm_gem_shmem_object plus a T. This is backwards from C, where you
expect the opposite situation where T contains a shmem object, but that just
doesn't work with Rust because there's no way to build a safe API around
that model as far as I know.
Ah I think I just got confused. I did untangle (I think at least) the
Object<T> trick, I guess the only thing that confused me here is why this
is in the shmem module? Or is that the rust problem again?
I'd kinda have expected that we'd have a gem::Object<T> here that the
lookup_handle function returns. So for the shmem case I guess that would
then be gem::Object<shmem::Object<T>> for the driver type T with driver
specific stuff? I guess not very pretty ...
Ahh, uh... Yeah, so shmem objects are allocated their own way (the shmem
core expects to kfree them in drm_gem_shmem_free) and
bindings::drm_gem_shmem_object already contains a
bindings::drm_gem_object. Since the composition is already done in the C
side, we can't just do it again in Rust cleanly. That's why I have this
weird setup with both a common trait for common GEM functionality and
separate actual types that both implement it.
Honestly the whole GEM codepath is untested other than the bits
inherited by shmem. I'm not sure I'll be able to verify that this all
makes sense until another Rust driver comes along that needs something
other than shmem. I just felt I had to do *something* for GEM since the
hierarchy is there and I needed shmem...
This whole gem stuff is IMO the messiest part of the abstractions
though, so I'm happy to turn it on its head if it makes it better and
someone has an idea of how to do that ^^
Now the problem is from the higher layers I want object operations that
interact with the shmem::Object<T> (that is, they call generic GEM functions
on the object). Options so far:
1. Add an outer wrapper and put that functionality there.
2. Just have the functions on T as helpers, so you need to call T::foo(obj)
instead of obj.foo().
3. Use the undocumented method receiver trait thing to make shmem::Object<T>
a valid `self` type, plus add auto-Deref to shmem::Object. Then obj.foo()
works.
#1 is what I use here. #2 is how the driver-specific File ioctl callbacks
are implemented, and also sched::Job<T>. #3 is used for fence callbacks
(FenceObject<T>). None of them are great, and I'd love to hear what people
think of the various options...
There are other unexplored options, like in this GEM case it could be
covered with a driver-internal auxiliary trait impl'd on shmem::Object<T>
buuut that doesn't work when you actually need callbacks on T itself to
circle back to shmem::Object<T>, as is the case with File/Job/FenceObject.
Ok I think I'm completely lost here. But I also havent' looked at how this
is all really used in the driver, it's really just the shmem:: module in
the lookup_handle function which looked strange to me.
Ah, sorry, I misunderstood what you were talking about in my previous
email then. That's just a default trait function. It comes from common
functionality in the gem module, but shmem::Object implements the trait
so it ends up offering it too (lookup_handle() is not duplicated, it
only lives in gem, shmem only has to implement going to/from the
drm_gem_object pointer so the rest of the methods can use it). That's
part of why the type/trait hierarchy is kind of messy here, it's so I
can share functionality between both types even though they are
pre-composed on the C side.
In the end the object types are specialized for any given driver, so
you're always getting your own unique kind of object anyway. It's just
that drivers based on shmem will go through it to reach the common code
and work with a shmem::Object<T>, and drivers using raw gem will use
gem::Object<T> instead.
~~ Lina
