linux-kernel - Re: [PATCH 0/6] Initial Rust V4L2 support

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Message-ID: <136035a4-26df-1c14-e51e-406b4ee5fe33@xs4all.nl>
Date:   Tue, 11 Apr 2023 09:51:07 +0200
From:   Hans Verkuil <hverkuil@...all.nl>
To:     Daniel Almeida <daniel.almeida@...labora.com>, wedsonaf@...il.com,
        ojeda@...nel.org, mchehab@...nel.org
Cc:     rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-media@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCH 0/6] Initial Rust V4L2 support

Hi Daniel,

On 06/04/2023 23:56, Daniel Almeida wrote:
> Hi all, this is my first attempt at adding Rust support to the
> media subsystem.
> 
> It adds just enough support to write a clone of the virtio-camera
> prototype written by my colleague, Dmitry Osipenko, available at [0].
> 
> Basically, there's support for video_device_register,
> v4l2_device_register and for some ioctls in v4l2_ioctl_ops. There is
> also some initial vb2 support, alongside some wrappers for some types
> found in videodev2.h.
> 
> I wrote a sample Rust driver just to prove that this probes, and
> that you get a message on dmesg whenever an ioctl is called.
> 
> As there is no actual implementation for any of the ioctls, this module
> can misbehave with some programs. I can work around this in a future
> submission.
> 
> Note that this is based on the rust branch, as opposed to rust-next. The
> reasoning is simple: I expect this series to just kickstart some
> discussion around the subject. Actual upstreaming can come up much
> later, at which point I can rebase on the rust branch.
> 
> Lastly, the origins of this series trace back to a v4l2 virtIO driver I
> was writing in Rust. As the project was eventually shelved for other
> reasons, I picked both the virtIO and the v4l2 bindings into their own
> patches which I am now in the process of submitting. This is to say that
> I tested this code with said driver and CrosVM in the past, and it
> worked ok.
> 
> Please let me know your thoughts.

I think this could be a good topic to discuss at the upcoming media summit:

https://lore.kernel.org/linux-media/893a7e34-1d98-23e2-4d27-d25cb3ee5bf0@xs4all.nl/

Please reply to that message with a topic description if you are interested.

One of my main concerns here is time: as subsystem maintainers we can barely
keep up with all the incoming patches. Introducing support for a new language
would add only more pressure. Even though these are mainly bindings (as I
understand it), this would still require that every change to a C kAPI is
duplicated in rust, requiring someone to do that work, and have maintainers
with enough rust knowledge to verify it.

And just reading about a new programming language is not enough to be able to
properly review code: that requires far more experience IMHO. The only way
to acquire that experience is by writing non-trivial drivers in rust.

And that in turn takes a lot of time. Which we do not have.

Regards,

	Hans

> 
> [0]: https://gitlab.collabora.com/dmitry.osipenko/linux-kernel-rd/-/commit/055a2c322e931a8b388f864f1db81bbdfd525602
> 
> Daniel Almeida (6):
>   rust: media: add the media module
>   rust: media: add initial videodev2.h abstractions
>   rust: sync: introduce FfiMutex
>   rust: media: videobuf2: add a videobuf2 abstraction
>   rust: media: add {video|v4l2}_device_register support
>   rust: media: add v4l2 rust sample
> 
>  rust/bindings/bindings_helper.h        |   8 +
>  rust/kernel/lib.rs                     |   2 +
>  rust/kernel/media/mod.rs               |   6 +
>  rust/kernel/media/v4l2/capabilities.rs |  80 ++++
>  rust/kernel/media/v4l2/dev.rs          | 369 +++++++++++++++
>  rust/kernel/media/v4l2/device.rs       | 115 +++++
>  rust/kernel/media/v4l2/enums.rs        | 135 ++++++
>  rust/kernel/media/v4l2/format.rs       | 178 ++++++++
>  rust/kernel/media/v4l2/framesize.rs    | 176 +++++++
>  rust/kernel/media/v4l2/inputs.rs       | 104 +++++
>  rust/kernel/media/v4l2/ioctls.rs       | 608 +++++++++++++++++++++++++
>  rust/kernel/media/v4l2/mmap.rs         |  81 ++++
>  rust/kernel/media/v4l2/mod.rs          |  13 +
>  rust/kernel/media/videobuf2/core.rs    | 552 ++++++++++++++++++++++
>  rust/kernel/media/videobuf2/mod.rs     |   5 +
>  rust/kernel/sync.rs                    |   1 +
>  rust/kernel/sync/ffi_mutex.rs          |  70 +++
>  samples/rust/Kconfig                   |  11 +
>  samples/rust/Makefile                  |   1 +
>  samples/rust/rust_v4l2.rs              | 403 ++++++++++++++++
>  20 files changed, 2918 insertions(+)
>  create mode 100644 rust/kernel/media/mod.rs
>  create mode 100644 rust/kernel/media/v4l2/capabilities.rs
>  create mode 100644 rust/kernel/media/v4l2/dev.rs
>  create mode 100644 rust/kernel/media/v4l2/device.rs
>  create mode 100644 rust/kernel/media/v4l2/enums.rs
>  create mode 100644 rust/kernel/media/v4l2/format.rs
>  create mode 100644 rust/kernel/media/v4l2/framesize.rs
>  create mode 100644 rust/kernel/media/v4l2/inputs.rs
>  create mode 100644 rust/kernel/media/v4l2/ioctls.rs
>  create mode 100644 rust/kernel/media/v4l2/mmap.rs
>  create mode 100644 rust/kernel/media/v4l2/mod.rs
>  create mode 100644 rust/kernel/media/videobuf2/core.rs
>  create mode 100644 rust/kernel/media/videobuf2/mod.rs
>  create mode 100644 rust/kernel/sync/ffi_mutex.rs
>  create mode 100644 samples/rust/rust_v4l2.rs
>