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Date: Tue, 01 Sep 2026 09:14:42 +0200
Message-Id: <DL3SM4UA1QC5.3KA2M1DE6F0F7@kernel.org>
Subject: Re: [PATCH v5 01/11] gpu: nova-core: Add public driver API to
 nova-core
Cc: "Gary Guo" <gary@garyguo.net>, "nova-gpu" <nova-gpu@lists.linux.dev>, "M
 Henning" <mhenning@darkrefraction.com>, "Alice Ryhl"
 <aliceryhl@google.com>, "David Airlie" <airlied@gmail.com>, "Alexandre
 Courbot" <acourbot@nvidia.com>, "Benno Lossin" <lossin@kernel.org>, "Eliot
 Courtney" <ecourtney@nvidia.com>, "John Hubbard" <jhubbard@nvidia.com>,
 <linux-kernel@vger.kernel.org>, <dri-devel@lists.freedesktop.org>,
 <rust-for-linux@vger.kernel.org>
To: "Alistair Popple" <apopple@nvidia.com>
From: "Danilo Krummrich" <dakr@kernel.org>
References: <20260828033531.1117754-1-apopple@nvidia.com>
 <20260828033531.1117754-2-apopple@nvidia.com>
 <DL3EGE1IY4FL.3X49WKCS67UT@kernel.org>
 <DL3F6EKL2F6Z.2DRXPQLYLE7FM@garyguo.net>
 <apZ3_o8hxlMr6SoP@nvdebian.thelocal>
In-Reply-To: <apZ3_o8hxlMr6SoP@nvdebian.thelocal>

On Tue Sep 1, 2026 at 9:07 AM CEST, Alistair Popple wrote:
> On 2026-09-01 at 06:42 +1000, Gary Guo <gary@garyguo.net> wrote...
>> On Mon Aug 31, 2026 at 9:08 PM BST, Danilo Krummrich wrote:
>> > On Fri Aug 28, 2026 at 5:35 AM CEST, Alistair Popple wrote:
>> >> +/// API handle for the auxiliary bus child drivers to interact with =
nova-core.
>> >> +pub struct NovaCoreApi<'bound> {
>> >> +    #[expect(unused)]
>> >> +    pub(crate) gpu: Pin<&'bound Gpu<'bound>>,
>> >> +}
>> >> +
>> >> +impl NovaCoreApi<'_> {
>> >> +    /// Obtain a [`NovaCoreApi`] handle from an auxiliary device reg=
istered
>> >> +    /// by nova-core.
>> >> +    pub fn of(adev: &auxiliary::Device<Bound>) -> Result<Pin<&NovaCo=
reApi<'_>>> {
>> >> +        adev.registration_data::<CovariantForLt!(NovaCoreApi<'_>)>()
>> >> +    }
>> >> +}
>> >
>> > CovariantForLt does not hold anymore on latest drm-rust-next, as Cmdq =
has a
>> > Mutex. So, this needs ForLt now and therefore the approach that I shar=
ed in [1]
>> > a while ago. I applied the changes in [2] to fix it up.
>
> I can see why the mutex means covariance no longer holds, but I would kin=
d
> of also expect it to not compile given it surely can't be safe here to tr=
eat
> an invariant type as covariant. Is this just a limitation of the current
> CovariantForLt implementation not being able to prove covariance or am I =
missing
> something else? Thanks.

Well, it did not compile (as expected) on my end, which is how I caught it.
What's your base revision?