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Message-ID: <aLtiN9B8ckCF4hSa@archiso>
Date: Fri, 5 Sep 2025 22:21:34 +0000
From: Elle Rhumsaa <elle@...thered-steel.dev>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
dakr@...nel.org, acourbot@...dia.com,
Alistair Popple <apopple@...dia.com>,
Miguel Ojeda <ojeda@...nel.org>,
Alex Gaynor <alex.gaynor@...il.com>,
Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
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>,
David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
Maxime Ripard <mripard@...nel.org>,
Thomas Zimmermann <tzimmermann@...e.de>,
John Hubbard <jhubbard@...dia.com>, Timur Tabi <ttabi@...dia.com>,
joel@...lfernandes.org,
Daniel Almeida <daniel.almeida@...labora.com>,
nouveau@...ts.freedesktop.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v2 2/4] nova-core: bitstruct: Add support for different
storage widths
On Wed, Sep 03, 2025 at 05:54:26PM -0400, Joel Fernandes wrote:
> Previously, bitstructs were hardcoded to use u32 as the underlying
> storage type. Add support for different storage types (u8, u16, u32,
> u64) to the bitstruct macro.
>
> New syntax is: struct Name: <type ex., u32> { ... }
>
> Signed-off-by: Joel Fernandes <joelagnelf@...dia.com>
> ---
> drivers/gpu/nova-core/bitstruct.rs | 71 ++++++++++++++++------------
> drivers/gpu/nova-core/regs/macros.rs | 16 +++----
> 2 files changed, 48 insertions(+), 39 deletions(-)
>
> diff --git a/drivers/gpu/nova-core/bitstruct.rs b/drivers/gpu/nova-core/bitstruct.rs
> index 1dd9edab7d07..068334c86981 100644
> --- a/drivers/gpu/nova-core/bitstruct.rs
> +++ b/drivers/gpu/nova-core/bitstruct.rs
> @@ -9,7 +9,7 @@
> ///
> /// ```rust
> /// bitstruct! {
> -/// struct ControlReg {
> +/// struct ControlReg: u32 {
> /// 3:0 mode as u8 ?=> Mode;
> /// 7:4 state as u8 => State;
> /// }
> @@ -34,21 +34,21 @@
> /// and returns the result. This is useful with fields for which not all values are valid.
> macro_rules! bitstruct {
> // Main entry point - defines the bitfield struct with fields
> - (struct $name:ident $(, $comment:literal)? { $($fields:tt)* }) => {
> - bitstruct!(@core $name $(, $comment)? { $($fields)* });
> + (struct $name:ident : $storage:ty $(, $comment:literal)? { $($fields:tt)* }) => {
> + bitstruct!(@core $name $storage $(, $comment)? { $($fields)* });
> };
>
> // All rules below are helpers.
>
> // Defines the wrapper `$name` type, as well as its relevant implementations (`Debug`,
> // `Default`, `BitOr`, and conversion to the value type) and field accessor methods.
> - (@core $name:ident $(, $comment:literal)? { $($fields:tt)* }) => {
> + (@core $name:ident $storage:ty $(, $comment:literal)? { $($fields:tt)* }) => {
> $(
> #[doc=$comment]
> )?
> #[repr(transparent)]
> #[derive(Clone, Copy)]
> - pub(crate) struct $name(u32);
> + pub(crate) struct $name($storage);
>
> impl ::core::ops::BitOr for $name {
> type Output = Self;
> @@ -58,20 +58,20 @@ fn bitor(self, rhs: Self) -> Self::Output {
> }
> }
>
> - impl ::core::convert::From<$name> for u32 {
> - fn from(val: $name) -> u32 {
> + impl ::core::convert::From<$name> for $storage {
> + fn from(val: $name) -> $storage {
> val.0
> }
> }
>
> - bitstruct!(@fields_dispatcher $name { $($fields)* });
> + bitstruct!(@fields_dispatcher $name $storage { $($fields)* });
> };
>
> // Captures the fields and passes them to all the implementers that require field information.
> //
> // Used to simplify the matching rules for implementers, so they don't need to match the entire
> // complex fields rule even though they only make use of part of it.
> - (@fields_dispatcher $name:ident {
> + (@fields_dispatcher $name:ident $storage:ty {
> $($hi:tt:$lo:tt $field:ident as $type:tt
> $(?=> $try_into_type:ty)?
> $(=> $into_type:ty)?
> @@ -80,7 +80,7 @@ fn from(val: $name) -> u32 {
> )*
> }
> ) => {
> - bitstruct!(@field_accessors $name {
> + bitstruct!(@field_accessors $name $storage {
> $(
> $hi:$lo $field as $type
> $(?=> $try_into_type)?
> @@ -89,13 +89,13 @@ fn from(val: $name) -> u32 {
> ;
> )*
> });
> - bitstruct!(@debug $name { $($field;)* });
> - bitstruct!(@default $name { $($field;)* });
> + bitstruct!(@debug $name $storage { $($field;)* });
> + bitstruct!(@default $name $storage { $($field;)* });
> };
>
> // Defines all the field getter/setter methods for `$name`.
> (
> - @field_accessors $name:ident {
> + @field_accessors $name:ident $storage:ty {
> $($hi:tt:$lo:tt $field:ident as $type:tt
> $(?=> $try_into_type:ty)?
> $(=> $into_type:ty)?
> @@ -111,7 +111,7 @@ fn from(val: $name) -> u32 {
> #[allow(dead_code)]
> impl $name {
> $(
> - bitstruct!(@field_accessor $name $hi:$lo $field as $type
> + bitstruct!(@field_accessor $name $storage, $hi:$lo $field as $type
> $(?=> $try_into_type)?
> $(=> $into_type)?
> $(, $comment)?
> @@ -145,11 +145,11 @@ impl $name {
>
> // Catches fields defined as `bool` and convert them into a boolean value.
> (
> - @field_accessor $name:ident $hi:tt:$lo:tt $field:ident as bool => $into_type:ty
> + @field_accessor $name:ident $storage:ty, $hi:tt:$lo:tt $field:ident as bool => $into_type:ty
> $(, $comment:literal)?;
> ) => {
> bitstruct!(
> - @leaf_accessor $name $hi:$lo $field
> + @leaf_accessor $name $storage, $hi:$lo $field
> { |f| <$into_type>::from(if f != 0 { true } else { false }) }
> $into_type => $into_type $(, $comment)?;
> );
> @@ -157,17 +157,17 @@ impl $name {
>
> // Shortcut for fields defined as `bool` without the `=>` syntax.
> (
> - @field_accessor $name:ident $hi:tt:$lo:tt $field:ident as bool $(, $comment:literal)?;
> + @field_accessor $name:ident $storage:ty, $hi:tt:$lo:tt $field:ident as bool $(, $comment:literal)?;
> ) => {
> - bitstruct!(@field_accessor $name $hi:$lo $field as bool => bool $(, $comment)?;);
> + bitstruct!(@field_accessor $name $storage, $hi:$lo $field as bool => bool $(, $comment)?;);
> };
>
> // Catches the `?=>` syntax for non-boolean fields.
> (
> - @field_accessor $name:ident $hi:tt:$lo:tt $field:ident as $type:tt ?=> $try_into_type:ty
> + @field_accessor $name:ident $storage:ty, $hi:tt:$lo:tt $field:ident as $type:tt ?=> $try_into_type:ty
> $(, $comment:literal)?;
> ) => {
> - bitstruct!(@leaf_accessor $name $hi:$lo $field
> + bitstruct!(@leaf_accessor $name $storage, $hi:$lo $field
> { |f| <$try_into_type>::try_from(f as $type) } $try_into_type =>
> ::core::result::Result<
> $try_into_type,
> @@ -178,29 +178,38 @@ impl $name {
>
> // Catches the `=>` syntax for non-boolean fields.
> (
> - @field_accessor $name:ident $hi:tt:$lo:tt $field:ident as $type:tt => $into_type:ty
> + @field_accessor $name:ident $storage:ty, $hi:tt:$lo:tt $field:ident as $type:tt => $into_type:ty
> $(, $comment:literal)?;
> ) => {
> - bitstruct!(@leaf_accessor $name $hi:$lo $field
> + bitstruct!(@leaf_accessor $name $storage, $hi:$lo $field
> { |f| <$into_type>::from(f as $type) } $into_type => $into_type $(, $comment)?;);
> };
>
> // Shortcut for non-boolean fields defined without the `=>` or `?=>` syntax.
> (
> - @field_accessor $name:ident $hi:tt:$lo:tt $field:ident as $type:tt
> + @field_accessor $name:ident $storage:ty, $hi:tt:$lo:tt $field:ident as $type:tt
> $(, $comment:literal)?;
> ) => {
> - bitstruct!(@field_accessor $name $hi:$lo $field as $type => $type $(, $comment)?;);
> + bitstruct!(@field_accessor $name $storage, $hi:$lo $field as $type => $type $(, $comment)?;);
> };
>
> // Generates the accessor methods for a single field.
> (
> - @leaf_accessor $name:ident $hi:tt:$lo:tt $field:ident
> + @leaf_accessor $name:ident $storage:ty, $hi:tt:$lo:tt $field:ident
> { $process:expr } $to_type:ty => $res_type:ty $(, $comment:literal)?;
> ) => {
> ::kernel::macros::paste!(
> const [<$field:upper _RANGE>]: ::core::ops::RangeInclusive<u8> = $lo..=$hi;
> - const [<$field:upper _MASK>]: u32 = ((((1 << $hi) - 1) << 1) + 1) - ((1 << $lo) - 1);
> + const [<$field:upper _MASK>]: $storage = {
> + // Generate mask for shifting
> + match ::core::mem::size_of::<$storage>() {
> + 1 => ::kernel::bits::genmask_u8($lo..=$hi) as $storage,
> + 2 => ::kernel::bits::genmask_u16($lo..=$hi) as $storage,
> + 4 => ::kernel::bits::genmask_u32($lo..=$hi) as $storage,
> + 8 => ::kernel::bits::genmask_u64($lo..=$hi) as $storage,
Not really important for current users, but something to think about for
the future is fields that are represented using byte arrays (e.g. a
`u24` represented as `[u8; 3]`).
I think your approach here could be fairly easily modified to handle
those use-cases, just something to think about.
For the byte array storage types, you would also need to think about
endianness (MSB v. LSB).
> + _ => <$storage>::MAX
> + }
> + };
> const [<$field:upper _SHIFT>]: u32 = Self::[<$field:upper _MASK>].trailing_zeros();
> );
>
> @@ -211,7 +220,7 @@ impl $name {
> #[inline(always)]
> pub(crate) fn $field(self) -> $res_type {
> ::kernel::macros::paste!(
> - const MASK: u32 = $name::[<$field:upper _MASK>];
> + const MASK: $storage = $name::[<$field:upper _MASK>];
> const SHIFT: u32 = $name::[<$field:upper _SHIFT>];
> );
> let field = ((self.0 & MASK) >> SHIFT);
> @@ -226,9 +235,9 @@ pub(crate) fn $field(self) -> $res_type {
> )?
> #[inline(always)]
> pub(crate) fn [<set_ $field>](mut self, value: $to_type) -> Self {
> - const MASK: u32 = $name::[<$field:upper _MASK>];
> + const MASK: $storage = $name::[<$field:upper _MASK>];
> const SHIFT: u32 = $name::[<$field:upper _SHIFT>];
> - let value = (u32::from(value) << SHIFT) & MASK;
> + let value = (<$storage>::from(value) << SHIFT) & MASK;
> self.0 = (self.0 & !MASK) | value;
>
> self
> @@ -237,7 +246,7 @@ pub(crate) fn [<set_ $field>](mut self, value: $to_type) -> Self {
> };
>
> // Generates the `Debug` implementation for `$name`.
> - (@debug $name:ident { $($field:ident;)* }) => {
> + (@debug $name:ident $storage:ty { $($field:ident;)* }) => {
> impl ::core::fmt::Debug for $name {
> fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
> f.debug_struct(stringify!($name))
> @@ -251,7 +260,7 @@ fn fmt(&self, f: &mut ::core::fmt::Formatter<'_>) -> ::core::fmt::Result {
> };
>
> // Generates the `Default` implementation for `$name`.
> - (@default $name:ident { $($field:ident;)* }) => {
> + (@default $name:ident $storage:ty { $($field:ident;)* }) => {
> /// Returns a value for the bitstruct where all fields are set to their default value.
> impl ::core::default::Default for $name {
> fn default() -> Self {
> diff --git a/drivers/gpu/nova-core/regs/macros.rs b/drivers/gpu/nova-core/regs/macros.rs
> index 3fb6852dff04..bbfeab147c9f 100644
> --- a/drivers/gpu/nova-core/regs/macros.rs
> +++ b/drivers/gpu/nova-core/regs/macros.rs
> @@ -284,25 +284,25 @@ pub(crate) trait RegisterBase<T> {
> macro_rules! register {
> // Creates a register at a fixed offset of the MMIO space.
> ($name:ident @ $offset:literal $(, $comment:literal)? { $($fields:tt)* } ) => {
> - bitstruct!(struct $name $(, $comment)? { $($fields)* } );
> + bitstruct!(struct $name: u32 $(, $comment)? { $($fields)* } );
> register!(@io_fixed $name @ $offset);
> };
>
> // Creates an alias register of fixed offset register `alias` with its own fields.
> ($name:ident => $alias:ident $(, $comment:literal)? { $($fields:tt)* } ) => {
> - bitstruct!(struct $name $(, $comment)? { $($fields)* } );
> + bitstruct!(struct $name: u32 $(, $comment)? { $($fields)* } );
> register!(@io_fixed $name @ $alias::OFFSET);
> };
>
> // Creates a register at a relative offset from a base address provider.
> ($name:ident @ $base:ty [ $offset:literal ] $(, $comment:literal)? { $($fields:tt)* } ) => {
> - bitstruct!(struct $name $(, $comment)? { $($fields)* } );
> + bitstruct!(struct $name: u32 $(, $comment)? { $($fields)* } );
> register!(@io_relative $name @ $base [ $offset ]);
> };
>
> // Creates an alias register of relative offset register `alias` with its own fields.
> ($name:ident => $base:ty [ $alias:ident ] $(, $comment:literal)? { $($fields:tt)* }) => {
> - bitstruct!(struct $name $(, $comment)? { $($fields)* } );
> + bitstruct!(struct $name: u32 $(, $comment)? { $($fields)* } );
> register!(@io_relative $name @ $base [ $alias::OFFSET ]);
> };
>
> @@ -313,7 +313,7 @@ macro_rules! register {
> }
> ) => {
> static_assert!(::core::mem::size_of::<u32>() <= $stride);
> - bitstruct!(struct $name $(, $comment)? { $($fields)* } );
> + bitstruct!(struct $name: u32 $(, $comment)? { $($fields)* } );
> register!(@io_array $name @ $offset [ $size ; $stride ]);
> };
>
> @@ -334,7 +334,7 @@ macro_rules! register {
> $(, $comment:literal)? { $($fields:tt)* }
> ) => {
> static_assert!(::core::mem::size_of::<u32>() <= $stride);
> - bitstruct!(struct $name $(, $comment)? { $($fields)* } );
> + bitstruct!(struct $name: u32 $(, $comment)? { $($fields)* } );
> register!(@io_relative_array $name @ $base [ $offset [ $size ; $stride ] ]);
> };
>
> @@ -356,7 +356,7 @@ macro_rules! register {
> }
> ) => {
> static_assert!($idx < $alias::SIZE);
> - bitstruct!(struct $name $(, $comment)? { $($fields)* } );
> + bitstruct!(struct $name: u32 $(, $comment)? { $($fields)* } );
> register!(@io_relative $name @ $base [ $alias::OFFSET + $idx * $alias::STRIDE ] );
> };
>
> @@ -365,7 +365,7 @@ macro_rules! register {
> // to avoid it being interpreted in place of the relative register array alias rule.
> ($name:ident => $alias:ident [ $idx:expr ] $(, $comment:literal)? { $($fields:tt)* }) => {
> static_assert!($idx < $alias::SIZE);
> - bitstruct!(struct $name $(, $comment)? { $($fields)* } );
> + bitstruct!(struct $name: u32 $(, $comment)? { $($fields)* } );
> register!(@io_fixed $name @ $alias::OFFSET + $idx * $alias::STRIDE );
> };
>
> --
> 2.34.1
>
>
Reviewed-by: Elle Rhumsaa <elle@...thered-steel.dev>
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