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Message-Id: <20250804-num-v2-0-a96b9ca6eb02@nvidia.com>
Date: Mon, 04 Aug 2025 20:45:23 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
Björn Roy Baron <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>,
Danilo Krummrich <dakr@...nel.org>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
nouveau@...ts.freedesktop.org, Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH v2 0/4] rust: add `Alignment` type
After some discussions on the list [1] and the Rust libs team [2], this
patchset undergoes two major changes:
- The `PowerOfTwo` type is replaced by `Alignment`, which is heavily
inspired by the nightly type of the same name in the standard library
[3].
- The `last_set_bit` function is dropped, with the recommendation to use
the standard library's `checked_ilog2` which does essentially the same
thing.
The upstream `Alignment` is more constrained than the `PowerOfTwo` of
the last revision: it uses `usize` internally instead of a generic
value, and does not provide `align_down` or `align_up` methods.
These two shortcomings come together very nicely to gift us with a nice
headache: we need to align values potentially larger than `usize`, thus
need to make `align_down` and `align_up` generic. The generic parameter
needs to be constrained on the operations used to perform the alignment
(e.g. `BitAnd`, `Not`, etc) and there is one essential operation for
which no trait exists in the standard library: `checked_add`. Thus the
first patch of this series introduces a trait for it in the `num` module
and implements it for all integer types. I suspect we will need
something alongside these lines for other purposes anyway, and probably
other traits too.
This generic nature also restricts these methods to being non-const,
unfortunately. I have tried to implement them as macros instead, but
quickly hit a wall due to the inability to convert `Alignment`'s `usize`
into the type of the value to align.
So here it is, not perfect but the need for a standard way to align is
starting to become more pressing.
[1] https://lore.kernel.org/rust-for-linux/DBTGVEJQOUDM.OTGZ6PXLB9JV@nvidia.com/T/#m09e068ecadf5b41099d4c6c55e13fbb3a98c5839
[2] https://github.com/rust-lang/libs-team/issues/631
[3] https://doc.rust-lang.org/std/ptr/struct.Alignment.html
Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
---
Changes in v2:
- Fix indentation of paste! in impl_last_set_bit.
- Link to v1: https://lore.kernel.org/r/20250620-num-v1-0-7ec3d3fb06c9@nvidia.com
Changes since split from the nova-core series:
- Rename `fls` to `last_set_bit`,
- Generate per-type doctests,
- Add invariants section to `PowerOfTwo`.
- Do not use reference to `self` in `PowerOfTwo` methods since it
implements `Copy`,
- Use #[derive] where possible instead of implementing traits
manually,
- Remove `Deref` and `Borrow` implementations.
---
Alexandre Courbot (4):
rust: add `CheckedAdd` trait
rust: add `Alignment` type
gpu: nova-core: use Alignment for alignment-related operations
gpu: nova-core: use `checked_ilog2` to emulate `fls`
Documentation/gpu/nova/core/todo.rst | 15 ---
drivers/gpu/nova-core/falcon/hal/ga102.rs | 4 +-
drivers/gpu/nova-core/fb.rs | 6 +-
drivers/gpu/nova-core/firmware/fwsec.rs | 11 +-
drivers/gpu/nova-core/vbios.rs | 4 +-
rust/kernel/lib.rs | 2 +
rust/kernel/num.rs | 28 ++++
rust/kernel/ptr.rs | 213 ++++++++++++++++++++++++++++++
8 files changed, 255 insertions(+), 28 deletions(-)
---
base-commit: 14ae91a81ec8fa0bc23170d4aa16dd2a20d54105
change-id: 20250620-num-9420281c02c7
Best regards,
--
Alexandre Courbot <acourbot@...dia.com>
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