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Message-ID: <077935ef-705f-4b8b-a5aa-5673f22af75d@nvidia.com>
Date: Wed, 3 Sep 2025 17:52:39 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Elle Rhumsaa <elle@...thered-steel.dev>
Cc: linux-kernel@...r.kernel.org, Danilo Krummrich <dakr@...nel.org>,
Alexandre Courbot <acourbot@...dia.com>, David Airlie <airlied@...il.com>,
Simona Vetter <simona@...ll.ch>, 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>, John Hubbard <jhubbard@...dia.com>,
Alistair Popple <apopple@...dia.com>, nouveau@...ts.freedesktop.org,
dri-devel@...ts.freedesktop.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 1/2] nova-core: Add a library for bitfields in Rust
structs
On 8/25/2025 7:20 PM, Elle Rhumsaa wrote:
> On Sun, Aug 24, 2025 at 09:59:52AM -0400, Joel Fernandes wrote:
>> Add a minimal bitfield library for defining in Rust structures (called
>> bitstruct), similar in concept to bit fields in C structs. This will be used
>> for defining page table entries and other structures in nova-core.
>>
>> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
>> ---
>> drivers/gpu/nova-core/bitstruct.rs | 149 +++++++++++++++++++++++++++++
>> drivers/gpu/nova-core/nova_core.rs | 1 +
>> 2 files changed, 150 insertions(+)
>> 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..661a75da0a9c
>> --- /dev/null
>> +++ b/drivers/gpu/nova-core/bitstruct.rs
>> @@ -0,0 +1,149 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +//
>> +// bitstruct.rs — C-style library for bitfield-packed Rust structures
>> +//
>> +// A library that provides support for defining bit fields in Rust
>> +// structures to circumvent lack of native language support for this.
>> +//
>> +// Similar usage syntax to the register! macro.
>> +
>> +use kernel::prelude::*;
>> +
>> +/// Macro for defining bitfield-packed structures in Rust.
>> +/// The size of the underlying storage type is specified with #[repr(TYPE)].
>> +///
>> +/// # Example (just for illustration)
>> +/// ```rust
>> +/// bitstruct! {
>> +/// #[repr(u64)]
>> +/// pub struct PageTableEntry {
>> +/// 0:0 present as bool,
>> +/// 1:1 writable as bool,
>> +/// 11:9 available as u8,
>> +/// 51:12 pfn as u64,
>> +/// 62:52 available2 as u16,
>> +/// 63:63 nx as bool,
>> +/// }
>> +/// }
>> +/// ```
>> +///
>> +/// This generates a struct with methods:
>> +/// - Constructor: `default()` sets all bits to zero.
>> +/// - Field accessors: `present()`, `pfn()`, etc.
>> +/// - Field setters: `set_present()`, `set_pfn()`, etc.
>> +/// - Builder methods: `with_present()`, `with_pfn()`, etc.
>> +/// - Raw conversion: `from_raw()`, `into_raw()`
>> +#[allow(unused_macros)]
>> +macro_rules! bitstruct {
>> + (
>> + #[repr($storage:ty)]
>> + $vis:vis struct $name:ident {
>> + $(
>> + $hi:literal : $lo:literal $field:ident as $field_type:tt
>> + ),* $(,)?
>> + }
>> + ) => {
>> + #[repr(transparent)]
>> + #[derive(Copy, Clone, Default)]
>> + $vis struct $name($storage);
>> +
>> + impl $name {
>> + /// Create from raw value
>> + #[inline(always)]
>> + $vis const fn from_raw(val: $storage) -> Self {
>> + Self(val)
>> + }
>> +
>> + /// Get raw value
>> + #[inline(always)]
>> + $vis const fn into_raw(self) -> $storage {
>> + self.0
>> + }
>> + }
>> +
>> + impl core::fmt::Debug for $name {
>> + fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
>> + write!(f, "{}({:#x})", stringify!($name), self.0)
>> + }
>> + }
>> +
>> + // Generate all field methods
>> + $(
>> + bitstruct_field_impl!($vis, $name, $storage, $hi, $lo, $field as $field_type);
>> + )*
>> + };
>> +}
>> +
>> +/// Helper to calculate mask for bit fields
>> +#[allow(unused_macros)]
>> +macro_rules! bitstruct_mask {
>> + ($hi:literal, $lo:literal, $storage:ty) => {{
>> + let width = ($hi - $lo + 1) as usize;
>> + let storage_bits = 8 * core::mem::size_of::<$storage>();
>> + if width >= storage_bits {
>> + <$storage>::MAX
>> + } else {
>> + ((1 as $storage) << width) - 1
>> + }
>> + }};
>> +}
>> +
>> +#[allow(unused_macros)]
>> +macro_rules! bitstruct_field_impl {
>> + ($vis:vis, $struct_name:ident, $storage:ty, $hi:literal, $lo:literal, $field:ident as $field_type:tt) => {
>> + impl $struct_name {
>> + #[inline(always)]
>> + $vis const fn $field(&self) -> $field_type {
>> + let field_val = (self.0 >> $lo) & bitstruct_mask!($hi, $lo, $storage);
>> + bitstruct_cast_value!(field_val, $field_type)
>> + }
>> + }
>> + bitstruct_make_setters!($vis, $struct_name, $storage, $hi, $lo, $field, $field_type);
>> + };
>> +}
>> +
>> +/// Helper macro to convert extracted value to target type
>> +///
>> +/// Special handling for bool types is required because the `as` keyword
>> +/// cannot be used to convert to bool in Rust. For bool fields, we check
>> +/// if the extracted value is non-zero. For all other types, we use the
>> +/// standard `as` conversion.
>> +#[allow(unused_macros)]
>> +macro_rules! bitstruct_cast_value {
>> + ($field_val:expr, bool) => {
>> + $field_val != 0
>> + };
>> + ($field_val:expr, $field_type:tt) => {
>> + $field_val as $field_type
>> + };
>> +}
>> +
>> +#[allow(unused_macros)]
>> +macro_rules! bitstruct_write_bits {
>> + ($raw:expr, $hi:literal, $lo:literal, $val:expr, $storage:ty) => {{
>> + let mask = bitstruct_mask!($hi, $lo, $storage);
>> + ($raw & !(mask << $lo)) | ((($val as $storage) & mask) << $lo)
>> + }};
>> +}
>> +
>> +#[allow(unused_macros)]
>> +macro_rules! bitstruct_make_setters {
>> + ($vis:vis, $struct_name:ident, $storage:ty, $hi:literal, $lo:literal, $field:ident, $field_type:tt) => {
>> + ::kernel::macros::paste! {
>> + impl $struct_name {
>> + #[inline(always)]
>> + #[allow(dead_code)]
>> + $vis fn [<set_ $field>](&mut self, val: $field_type) {
>> + self.0 = bitstruct_write_bits!(self.0, $hi, $lo, val, $storage);
>> + }
>> +
>> + #[inline(always)]
>> + #[allow(dead_code)]
>> + $vis const fn [<with_ $field>](mut self, val: $field_type) -> Self {
>> + self.0 = bitstruct_write_bits!(self.0, $hi, $lo, val, $storage);
>> + self
>> + }
>> + }
>> + }
>> + };
>> +}
>
> This is awesome. Is there a place for this to live outside of
> `nova-core`? I would think this would be extremely useful as a general
> helper for bitfield struct definitions.
About to send v2 which moves it. :)
>
>> diff --git a/drivers/gpu/nova-core/nova_core.rs b/drivers/gpu/nova-core/nova_core.rs
>> index cb2bbb30cba1..54505cad4a73 100644
>> --- a/drivers/gpu/nova-core/nova_core.rs
>> +++ b/drivers/gpu/nova-core/nova_core.rs
>> @@ -2,6 +2,7 @@
>>
>> //! Nova Core GPU Driver
>>
>> +mod bitstruct;
>> mod dma;
>> mod driver;
>> mod falcon;
>> --
>> 2.34.1
>
> Reviewed-by: Elle Rhumsaa <elle@...thered-steel.dev>
Thanks! Since the patches are a bit new, kindly review again.
- Joel
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