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Date:   Wed, 5 Apr 2023 21:29:02 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Asahi Lina <lina@asahilina.net>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
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        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?iso-8859-1?Q?Bj=F6rn?= Roy Baron <bjorn3_gh@protonmail.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
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        Luben Tuikov <luben.tuikov@amd.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Alyssa Rosenzweig <alyssa@rosenzweig.io>,
        Karol Herbst <kherbst@redhat.com>,
        Ella Stanforth <ella@iglunix.org>,
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Subject: Re: [PATCH RFC 12/18] rust: drm: sched: Add GPU scheduler abstraction
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References: <20230307-rust-drm-v1-0-917ff5bc80a8@asahilina.net>
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 <ZC2XBfJGAdNMQjpZ@phenom.ffwll.local>
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On Wed, Apr 05, 2023 at 05:43:01PM +0200, Daniel Vetter wrote:
> On Tue, Mar 07, 2023 at 11:25:37PM +0900, Asahi Lina wrote:
> > +/// An armed DRM scheduler job (not yet submitted)
> > +pub struct ArmedJob<'a, T: JobImpl>(Box<Job<T>>, PhantomData<&'a T>);
> > +
> > +impl<'a, T: JobImpl> ArmedJob<'a, T> {
> > +    /// Returns the job fences
> > +    pub fn fences(&self) -> JobFences<'_> {
> > +        JobFences(unsafe { &mut *self.0.job.s_fence })
> > +    }
> > +
> > +    /// Push the job for execution into the scheduler
> > +    pub fn push(self) {
> > +        // After this point, the job is submitted and owned by the scheduler
> > +        let ptr = match self {
> > +            ArmedJob(job, _) => Box::<Job<T>>::into_raw(job),
> > +        };
> 
> If I get this all right then this all makes sure that drivers can't use
> the job after push and they don't forgot to call arm.
> 
> What I'm not seeing is how we force drivers to call push once they've
> called arm? I haven't check what the code does, but from the docs it
> sounds like if you don't call push then drop will get called. Which wreaks
> the book-keeping on an armed job. Or is there someting that prevents
> ArmedJob<T> from having the Drop trait and so the only way to not go boom
> is by pushing it?
> 
> Googling for "rust undroppable" seems to indicate that this isn't a thing
> rust can do?

Another thing that I just realized: The driver must ensure that the
arm->push sequence on a given drm_sched_entity isn't interrupte by another
thread doing the same, i.e. you need to wrap it all in a lock, and it
always needs to be the same lock for a given entity.

I have no idea how to guarantee that, but I guess somehow we should?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch