linux-kernel - Re: [PATCH v2 1/4] nova-core: bitstruct: Move bitfield-specific code from register! into new macro

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Message-Id: <DCN39JCF1DIJ.3JESSN7N9WJBP@nvidia.com>
Date: Mon, 08 Sep 2025 12:12:20 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Joel Fernandes" <joelagnelf@...dia.com>,
 <linux-kernel@...r.kernel.org>, <dri-devel@...ts.freedesktop.org>,
 <dakr@...nel.org>
Cc: "Alistair Popple" <apopple@...dia.com>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng"
 <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "David Airlie" <airlied@...il.com>,
 "Simona Vetter" <simona@...ll.ch>, "Maarten Lankhorst"
 <maarten.lankhorst@...ux.intel.com>, "Maxime Ripard" <mripard@...nel.org>,
 "Thomas Zimmermann" <tzimmermann@...e.de>, "John Hubbard"
 <jhubbard@...dia.com>, "Timur Tabi" <ttabi@...dia.com>,
 <joel@...lfernandes.org>, "Elle Rhumsaa" <elle@...thered-steel.dev>,
 "Daniel Almeida" <daniel.almeida@...labora.com>,
 <nouveau@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] nova-core: bitstruct: Move bitfield-specific
 code from register! into new macro

On Thu Sep 4, 2025 at 6:54 AM JST, Joel Fernandes wrote:
> The bitfield-specific into new macro. This will be used to define
> structs with bitfields, similar to C language.
>
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> ---
>  drivers/gpu/nova-core/bitstruct.rs   | 271 +++++++++++++++++++++++++++
>  drivers/gpu/nova-core/nova_core.rs   |   3 +
>  drivers/gpu/nova-core/regs/macros.rs | 247 +-----------------------
>  3 files changed, 282 insertions(+), 239 deletions(-)
>  create mode 100644 drivers/gpu/nova-core/bitstruct.rs
>
> diff --git a/drivers/gpu/nova-core/bitstruct.rs b/drivers/gpu/nova-core/bitstruct.rs
> new file mode 100644
> index 000000000000..1dd9edab7d07
> --- /dev/null
> +++ b/drivers/gpu/nova-core/bitstruct.rs
> @@ -0,0 +1,271 @@
> +// SPDX-License-Identifier: GPL-2.0
> +//
> +// bitstruct.rs — Bitfield library for Rust structures
> +//
> +// A library that provides support for defining bit fields in Rust
> +// structures. Also used from things that need bitfields like register macro.
> +///
> +/// # Syntax
> +///
> +/// ```rust
> +/// bitstruct! {
> +///     struct ControlReg {

The `struct` naming here looks a bit confusing to me - as of this patch,
this is a u32, right? And eventually these types will be limited to primitive types,
so why not just `ControlReg: u32 {` ?

> +///         3:0       mode        as u8 ?=> Mode;
> +///         7:4       state       as u8 => State;
> +///     }
> +/// }
> +/// ```

As this will move to the kernel crate, it is particularly important to
make sure that this example can compile and run - so please provide
simple definitions for `Mode` and `State` to make sure the kunit tests
will pass after patch 4 (in the current state I'm pretty sure they won't).

> +///
> +/// This generates a struct with:
> +/// - Field accessors: `mode()`, `state()`, etc.
> +/// - Field setters: `set_mode()`, `set_state()`, etc. (supports builder pattern).
> +/// - Debug and Default implementations
> +///
> +/// The field setters can be used with the builder pattern, example:
> +/// ControlReg::default().set_mode(mode).set_state(state);
> +///
> +/// Fields are defined as follows:
> +///
> +/// - `as <type>` simply returns the field value casted to <type>, typically `u32`, `u16`, `u8` or
> +///   `bool`. Note that `bool` fields must have a range of 1 bit.
> +/// - `as <type> => <into_type>` calls `<into_type>`'s `From::<<type>>` implementation and returns
> +///   the result.
> +/// - `as <type> ?=> <try_into_type>` calls `<try_into_type>`'s `TryFrom::<<type>>` implementation
> +///   and returns the result. This is useful with fields for which not all values are valid.

Can you remove the related documentation from `register!` and replace it
with a sentence like "Please look at the [`bitstruct`] macro for the
complete syntax of fields definitions"? This will ensure we don't have
to update the documentation twice if/when the syntax gets updated.

The rest of the patch is a perfect move (with a few renames) of the
internal rules from one macro to the other, which makes it really easy
to review. Thanks for doing this!