linux-kernel - Re: [RFC PATCH 0/2] rust: pci: add config space accessors (and a small in-tree user)

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Message-Id: <DG2D5ONS18FE.TC7K3O8V8SU1@garyguo.net>
Date: Sat, 31 Jan 2026 00:55:24 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Zijing Zhang" <zijing.zhang@...rs>, <dakr@...nel.org>,
 <ojeda@...nel.org>
Cc: <bhelgaas@...gle.com>, <kwilczynski@...nel.org>, <boqun.feng@...il.com>,
 <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>, <lossin@...nel.org>,
 <a.hindborg@...nel.org>, <aliceryhl@...gle.com>, <tmgross@...ch.edu>,
 <linux-pci@...r.kernel.org>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <lianux.mm@...il.com>,
 <zijing.kernel@...il.com>
Subject: Re: [RFC PATCH 0/2] rust: pci: add config space accessors (and a
 small in-tree user)

On Fri Jan 30, 2026 at 5:10 PM GMT, Zijing Zhang wrote:
> This RFC proposes adding basic PCI config space accessors to
> `rust/kernel/pci`.
> It also includes a tiny update to the existing Rust PCI driver sample to
> exercise the new API.
>
> Motivation
> ----------
> Rust PCI drivers occasionally need to access PCI config space (e.g. for
> capability discovery, SR-IOV queries, or MSI-related setup).
>
> Having a small, reviewed API in `kernel::pci` avoids each Rust driver
> growing its own ad-hoc wrappers and error handling around config space.
>
> This RFC is also motivated by the "PCI MISC APIs" TODO item in
> `Documentation/gpu/nova/core/todo.rst`: config space accessors as a first
> step, with capability/MSI/SR-IOV helpers as follow-ups.
>
> Proposed API
> ------------
> Add the following methods to `pci::Device`:
>
>   - read_config_u8/u16/u32(offset: u16) -> Result<T>
>   - write_config_u8/u16/u32(offset: u16, val: T) -> Result

We already have config space merged.

Patch series: https://lore.kernel.org/rust-for-linux/20260121202212.4438-1-zhiw@nvidia.com/

Code: https://rust.docs.kernel.org/next/src/kernel/pci/io.rs.html

Although, I do noted that the documentation is not having it. It looks like
while we re-export `ConfigSpaceKind` and `ConfigSpaceSize`, we forgot to export
the most important `ConfigSpace` struct :/

Best,
Gary

>
> Notes
> -----
> This is intentionally a thin wrapper: it exposes a safe interface and
> translates errors into `Result`, but it does not try to add policy or extra
> validation.
>
>   - No additional range/alignment checks are performed in Rust. If an
>     argument needs validation beyond what the C PCI accessors already do,
>     it should likely be addressed in the PCI core instead of in a Rust-only
>     wrapper.
>   - The underlying C helpers may return positive PCIBIOS status codes.
>     These are mapped to the corresponding `-errno` values for Rust callers
>     (same mapping as `pcibios_err_to_errno()` in `include/linux/pci.h`).
>
> `pcibios_err_to_errno` mapping helper
> -------------------------------------
> The mapping logic is kept as a private helper in the `kernel::pci` module
> rather than inside `Device`: it is not tied to any particular device
> instance and may be reused by future PCI helpers.
>
> Also, the C `pcibios_err_to_errno()` is a `static inline`, so Rust cannot
> call it directly without adding an exported wrapper.
>
> In-tree user
> ------------
> The `samples/rust/rust_driver_pci` sample is updated to read vendor/device
> IDs from config space (0x00/0x02) and print them during probe using
> `dev_dbg!`.
>
> Note: in the current Rust support, `dev_dbg!` does not hook into dynamic
> debug and is compiled out unless Rust debug assertions are enabled.
>
> For local testing, enable `CONFIG_RUST_DEBUG_ASSERTIONS=y` if you want
> to see the `dev_dbg!` line.
>
> Questions for reviewers
> -----------------------
> 1) Does using `u16` for the config-space offset and returning `Result<T>`
>    look OK?
> 2) Is mapping PCIBIOS status codes to `-errno` acceptable for Rust callers,
>    or would you prefer we add a small exported C wrapper so Rust can reuse
>    the existing `pcibios_err_to_errno()` helper directly?
>
> Testing
> -------
> Build:
>   - x86_64 defconfig-based kernel with Rust enabled.
>   - Out-of-tree build directory (i.e. `make O=...`).
>   - Options enabled for this test:
>       * CONFIG_SAMPLES_RUST=y
>       * CONFIG_SAMPLE_RUST_DRIVER_PCI=y (built-in)
>       * CONFIG_RUST_DEBUG_ASSERTIONS=y
>
> Runtime:
>   - Booted the resulting bzImage under QEMU x86_64 with:
>       * `-device virtio-serial-pci`
>       * `-device pci-testdev`
>   - Kernel command line included:
>       * `loglevel=8`
>   - Observed:
>       * `pci-testdev data-match count: 1`
>       * `Probe Rust PCI driver sample (... cfg: 0x1b36:0x0005).`
>
> Zijing Zhang (2):
>   rust: pci: add config space accessors
>   samples: rust: pci: exercise config space accessors
>
>  rust/kernel/pci.rs              | 86 +++++++++++++++++++++++++++++++++
>  samples/rust/rust_driver_pci.rs |  8 ++-
>  2 files changed, 92 insertions(+), 2 deletions(-)