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

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Message-ID: <20260130171026.1138617-1-zijing.zhang@ry.rs>
Date: Fri, 30 Jan 2026 17:10:24 +0000
From: Zijing Zhang <zijing.zhang@...rs>
To: 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: [RFC PATCH 0/2] rust: pci: add config space accessors (and a small in-tree user)

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

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(-)

-- 
2.52.0