Re: [RFC PATCH 3/4] rust/drm: Add DRM Jobqueue

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Feb 11, 2026 at 12:19:56PM +0100, Philipp Stanner wrote:
> On Wed, 2026-02-11 at 12:07 +0100, Boris Brezillon wrote:
> > On Wed, 11 Feb 2026 11:47:27 +0100
> > Philipp Stanner <phasta@mailbox.org> wrote:
> > 
> > > On Tue, 2026-02-10 at 15:57 +0100, Boris Brezillon wrote:
> > > > On Tue,  3 Feb 2026 09:14:02 +0100
> > > > Philipp Stanner <phasta@kernel.org> wrote:
> > > >   
> > > > > +/// A jobqueue Job.
> > > > > +///
> > > > > +/// You can stuff your data in it. The job will be borrowed back to your driver
> > > > > +/// once the time has come to run it.
> > > > > +///
> > > > > +/// Jobs are consumed by [`Jobqueue::submit_job`] by value (ownership transfer).
> > > > > +/// You can set multiple [`DmaFence`] as dependencies for a job. It will only
> > > > > +/// get run once all dependency fences have been signaled.
> > > > > +///
> > > > > +/// Jobs cost credits. Jobs will only be run if there are is enough capacity in
> > > > > +/// the jobqueue for the job's credits. It is legal to specify jobs costing 0
> > > > > +/// credits, effectively disabling that mechanism.
> > > > > +#[pin_data]
> > > > > +pub struct Job<T: 'static + Send> {
> > > > > +    cost: u32,
> > > > > +    #[pin]
> > > > > +    pub data: T,
> > > > > +    done_fence: Option<ARef<DmaFence<i32>>>,
> > > > > +    hardware_fence: Option<ARef<DmaFence<i32>>>,
> > > > > +    nr_of_deps: AtomicU32,
> > > > > +    dependencies: List<Dependency>,  
> > > > 
> > > > Given how tricky Lists are in rust, I'd recommend going for an XArray,
> > > > like we have on the C side. There's a bit of overhead when the job only
> > > > has a few deps, but I think simplicity beats memory-usage-optimizations
> > > > in that case (especially since the overhead exists and is accepted in
> > > > C).  
> > > 
> > > I mean, the list is now already implemented and works. Considering the
> > > XArray would have made sense during the development difficulties.
> > 
> > I'm sure it does, but that's still more code/tricks to maintain than
> > what you'd have with the XArray abstraction.
> 
> The solution than will rather be to make the linked list implementation
> better.
> 
> A list is the correct data structure in a huge number of use cases in
> the kernel. We should not begin here to defer to other structures
> because of convenience.

Rust vs C aside, linked lists are often used in the kernel despite not
being the best choice. They are extremely cache unfriendly and
inefficient; most of the time a vector or xarray is far faster if you
can accept an ENOMEM failure path when adding elements. I have heard
several times from C maintainers that overuse of list is making the
kernel slow in a death from a thousand cuts situation.

This applies to the red/black tree too, by the way.

Alice