[PATCH v2 0/6] rust: io: turn IoCapable into a functional trait

Alexandre Courbot
2026-02-06T06:00:40Z

`IoCapable<T>` is currently used as a marker trait to signal that the
methods of the `Io` trait corresponding to `T` have been overridden by
the implementor (the default implementations triggering a build-time
error).

This goes against the DRY principle and separates the signaling of the
capability from its implementation, making it possible to forget a step
while implementing a new `Io`.

Another undesirable side-effect is that it makes the implementation of
I/O backends boilerplate-y and convoluted: currently this is done using
two levels of imbricated macros that generate unsafe code.

This patchset fixes these issues by turning `IoCapable` into a
functional trait including the raw implementation of the I/O
accessors for `T` using unsafe methods that work with an arbitrary
address, and making the default methods of `Io` call into these
implementations after checking the bounds.

This makes overriding these accessors on all I/O backends unneeded,
resulting in a net -90 LoCs while avoiding a violation of the DRY
principle and reducing (and simplifying) the use of macros generating
unsafe code.

Patch 1 adds the `io_read` and `io_write` unsafe methods to `IoCapable`,
provides the required implementations for `Mmio` and `pci::ConfigSpace`,
and make the default I/O accessors of `Io` call into them instead of
failing.

Patches 2 to 4 get rid of the `_relaxed` variants we had in `Mmio`,
since these are not usable in code generic against `Io` and makes use of
the macros we want to remove. They are replaced by a `RelaxedMmio`
wrapper type that implements the required `IoCapable`s and is thus
usable in generic code.

Patches 5 and 6 remove the overloaded implementations of the `Io`
methods for `pci::ConfigSpace` and `Mmio`, respectively, while also
deleting the macros that have become unused.

There is more work coming on top of this patchset (notably the
`register!` macro with proper I/O), but I wanted to send this work first
as it stands on its own IMHO and is more digestible from a review
perspective.

The base for this patchset is `driver-core-testing`.

Cc: Zhi Wang <zhiw@nvidia.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Eliot Courtney <ecourtney@nvidia.com>

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
Changes in v2:
- Turn `RelaxedMmio` into an actual wrapper type and make it available
through a `Mmio::relaxed()` method.
- Link to v1: https://patch.msgid.link/20260202-io-v1-0-9bb2177d23be@nvidia.com

---
Alexandre Courbot (6):
rust: io: turn IoCapable into a functional trait
rust: io: mem: use non-relaxed I/O ops in examples
rust: io: provide Mmio relaxed ops through a wrapper type
rust: io: remove legacy relaxed accessors of Mmio
rust: pci: io: remove overloaded Io methods of ConfigSpace
rust: io: remove overloaded Io methods of Mmio

rust/kernel/io.rs | 435 ++++++++++++++++++++++----------------------------
rust/kernel/io/mem.rs | 10 +-
rust/kernel/pci/io.rs | 99 ++++--------
3 files changed, 227 insertions(+), 317 deletions(-)
---
base-commit: f55ae0bfa00e446ea751d09f468daeafc303e03f
change-id: 20260202-io-81fd368f7565

Best regards,
--
Alexandre Courbot <acourbot@nvidia.com>