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Date: Tue, 03 Mar 2026 21:50:55 +0100
Message-Id: <DGTFZX0OF96N.18P63FULRD8GN@kernel.org>
Cc: "Zhi Wang" <zhiw@nvidia.com>, <rust-for-linux@vger.kernel.org>,
 <linux-pci@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
 <gary@garyguo.net>, <joelagnelf@nvidia.com>, <aliceryhl@google.com>,
 <bhelgaas@google.com>, <kwilczynski@kernel.org>, <ojeda@kernel.org>,
 <alex.gaynor@gmail.com>, <boqun.feng@gmail.com>,
 <bjorn3_gh@protonmail.com>, <lossin@kernel.org>, <a.hindborg@kernel.org>,
 <tmgross@umich.edu>, <markus.probst@posteo.de>, <helgaas@kernel.org>,
 <cjia@nvidia.com>, <smitra@nvidia.com>, <ankita@nvidia.com>,
 <aniketa@nvidia.com>, <kwankhede@nvidia.com>, <targupta@nvidia.com>,
 <acourbot@nvidia.com>, <jhubbard@nvidia.com>, <zhiwang@kernel.org>,
 <daniel.almeida@collabora.com>
To: "Jason Gunthorpe" <jgg@nvidia.com>
From: "Danilo Krummrich" <dakr@kernel.org>
Subject: Re: [PATCH v3 1/1] rust: introduce abstractions for fwctl
References: <20260217204909.211793-1-zhiw@nvidia.com>
 <20260217204909.211793-2-zhiw@nvidia.com>
 <20260303201552.GF972761@nvidia.com>
In-Reply-To: <20260303201552.GF972761@nvidia.com>

On Tue Mar 3, 2026 at 9:15 PM CET, Jason Gunthorpe wrote:
> Anyhow is someone waiting for me to do something with it?

I still have this on my list for review, I can probably do a pass tomorrow.=
 :)

>> +impl<T: Operations> Device<T> {
>> +    /// Allocate a new fwctl device with embedded driver data.
>> +    ///
>> +    /// Returns an [`ARef`] that can be passed to [`Registration::new()=
`]
>> +    /// to make the device visible to userspace. The caller may inspect=
 or
>> +    /// configure the device between allocation and registration.
>> +    pub fn new(
>> +        parent: &device::Device<device::Bound>,
>> +        data: impl PinInit<T::DeviceData, Error>,
>> +    ) -> Result<ARef<Self>> {
>> +        let ops =3D core::ptr::from_ref::<bindings::fwctl_ops>(&VTable:=
:<T>::VTABLE).cast_mut();
>> +
>> +        // SAFETY: `_fwctl_alloc_device` allocates `size` bytes via kza=
lloc and
>> +        // initialises the embedded fwctl_device. `ops` points to a sta=
tic vtable
>> +        // that outlives the device. `parent` is bound.
>> +        let raw =3D unsafe {
>> +            bindings::_fwctl_alloc_device(parent.as_raw(), ops, core::m=
em::size_of::<Self>())
>> +        };
>> +
>> +        if raw.is_null() {
>> +            return Err(ENOMEM);
>> +        }
>> +
>> +        // CAST: Device<T> is repr(C) with fwctl_device at offset 0.
>> +        let this =3D raw as *mut Self;
>
> Should this have some helper? It looks a bit fragile, in C we have a
> static_assert on offsetof to prevent errors.

We could add a helper with something like this:

	    const {
	        assert!(
	            core::mem::offset_of!(Self, dev) =3D=3D 0,
	            "struct fwctl_device must be at offset 0"
	        )
	    };

But we could also just include this in this constructor.