linux-kernel - Re: [PATCH RFC 18/18] drm/asahi: Add the Asahi driver for Apple AGX GPUs

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Message-ID: <df68305d-fbf0-8f03-d9fb-4502632beec6@asahilina.net>
Date:   Thu, 6 Apr 2023 14:09:48 +0900
From:   Asahi Lina <lina@...hilina.net>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        Luben Tuikov <luben.tuikov@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Karol Herbst <kherbst@...hat.com>,
        Ella Stanforth <ella@...unix.org>,
        Faith Ekstrand <faith.ekstrand@...labora.com>,
        Mary <mary@...y.zone>, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, rust-for-linux@...r.kernel.org,
        linux-media@...r.kernel.org, linaro-mm-sig@...ts.linaro.org,
        linux-sgx@...r.kernel.org, asahi@...ts.linux.dev,
        David Airlie <airlied@...il.com>
Subject: Re: [PATCH RFC 18/18] drm/asahi: Add the Asahi driver for Apple AGX
 GPUs

Same as the prior email, this was supposed to go to Daniel...

On 06/04/2023 14.02, 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.
> 
> 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.
> 
> ~~ Lina
> 

~~ Lina