Re: [PATCH v2 1/2] rust: introduce abstractions for fwctl

["Danilo Krummrich" <dakr@kernel.org>]

On Wed Jan 28, 2026 at 12:36 PM CET, Zhi Wang wrote:
> On Tue, 27 Jan 2026 21:07:37 +0100
> "Danilo Krummrich" <dakr@kernel.org> wrote:
>
>> On Tue Jan 27, 2026 at 8:57 PM CET, Zhi Wang wrote:
>> > The fwctl_alloc_device() helper allocates a raw struct fwctl_device
>> > without private driver data here. The Rust driver object should be
>> > already allocated and initialized separately before reaching this
>> > point.
>> >
>> > We rely on the standard dev->parent chain to access the rust driver
>> > object from the fwctl callbacks.
>> 
>> (I will go for a thorough review soon, but for now a quick drive-by
>> comment.)
>> 
>> IIUC, you are saying that the user is supposed to use the private data
>> of the parent device in fwctl callbacks. Let's not make this a design
>> choice please. Instead, allow the user pass in separate private data for
>> the fwctl device as well.
>> 
>> This serves the purpose of clear ownership and lifetime of the data.
>> E.g. the fwctl device does not necessarily exist as long as the parent
>> device is bound.
>> 
>> It is a good thing if driver authors are forced to take a decision about
>> which object owns the data and what's the scope of the data.
>
> I wrote a version like this before. My initial concern of mixing Rust
> objects together with C objecs within C-allocated memory was about
> potential memory alignment issues when rust side doing CAST on the memory.
>
> I agree that providing a way to attach private data directly to the
> fwctl_device also has quite some benetifs.
>
> IMO, if we go this way, the private data from rust side needs to have
> #[repr(C)] to address the above issue all the time?

No that's not necessary. Please have a look at what I did in drm::Device::new()
[1], this should be the exact same case.

[1] https://rust.docs.kernel.org/src/kernel/drm/device.rs.html#98