Re: [PATCH v4 1/6] nova-core: bitfield: Move bitfield-specific code from register! into new macro

Joel Fernandes
2025-09-23T22:24:39Z

Hi Greg,

On Sun, Sep 21, 2025 at 02:45:27PM +0200, Greg KH wrote:
> On Sun, Sep 21, 2025 at 02:33:56PM +0200, Benno Lossin wrote:
> > On Sun Sep 21, 2025 at 11:36 AM CEST, Greg KH wrote:
> > > On Sat, Sep 20, 2025 at 02:22:27PM -0400, Joel Fernandes wrote:
> > >> The bitfield-specific into new macro. This will be used to define
> > >> structs with bitfields, similar to C language.
> > >>
> > >> Reviewed-by: Elle Rhumsaa <elle@weathered-steel.dev>
> > >> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
> > >> ---
> > >> drivers/gpu/nova-core/bitfield.rs | 314 +++++++++++++++++++++++++++
> > >> drivers/gpu/nova-core/nova_core.rs | 3 +
> > >> drivers/gpu/nova-core/regs/macros.rs | 259 +---------------------
> > >> 3 files changed, 327 insertions(+), 249 deletions(-)
> > >> create mode 100644 drivers/gpu/nova-core/bitfield.rs
> > >>
> > >> diff --git a/drivers/gpu/nova-core/bitfield.rs b/drivers/gpu/nova-core/bitfield.rs
> > >> new file mode 100644
> > >> index 000000000000..ba6b7caa05d9
> > >> --- /dev/null
> > >> +++ b/drivers/gpu/nova-core/bitfield.rs
> > >> @@ -0,0 +1,314 @@
> > >> +// SPDX-License-Identifier: GPL-2.0
> > >> +
> > >> +//! Bitfield library for Rust structures
> > >> +//!
> > >> +//! Support for defining bitfields in Rust structures. Also used by the [`register!`] macro.
> > >> +//!
> > >> +//! # Syntax
> > >> +//!
> > >> +//! ```rust
> > >> +//! #[derive(Debug, Clone, Copy)]
> > >> +//! enum Mode {
> > >> +//! Low = 0,
> > >> +//! High = 1,
> > >> +//! Auto = 2,
> > >> +//! }
> > >> +//!
> > >> +//! impl TryFrom<u8> for Mode {
> > >> +//! type Error = u8;
> > >> +//! fn try_from(value: u8) -> Result<Self, Self::Error> {
> > >> +//! match value {
> > >> +//! 0 => Ok(Mode::Low),
> > >> +//! 1 => Ok(Mode::High),
> > >> +//! 2 => Ok(Mode::Auto),
> > >> +//! _ => Err(value),
> > >> +//! }
> > >> +//! }
> > >> +//! }
> > >> +//!
> > >> +//! impl From<Mode> for u32 {
> > >> +//! fn from(mode: Mode) -> u32 {
> > >> +//! mode as u32
> > >> +//! }
> > >> +//! }
> > >> +//!
> > >> +//! #[derive(Debug, Clone, Copy)]
> > >> +//! enum State {
> > >> +//! Inactive = 0,
> > >> +//! Active = 1,
> > >> +//! }
> > >> +//!
> > >> +//! impl From<bool> for State {
> > >> +//! fn from(value: bool) -> Self {
> > >> +//! if value { State::Active } else { State::Inactive }
> > >> +//! }
> > >> +//! }
> > >> +//!
> > >> +//! impl From<State> for u32 {
> > >> +//! fn from(state: State) -> u32 {
> > >> +//! state as u32
> > >> +//! }
> > >> +//! }
> > >> +//!
> > >> +//! bitfield! {
> > >> +//! struct ControlReg {
> > >> +//! 3:0 mode as u8 ?=> Mode;
> > >> +//! 7 state as bool => State;
> > >> +//! }
> > >> +//! }
> > >
> > > As discussed at the conference this week, I do object to this as it
> > > will allow the same mistakes to happen that we used to do in the kernel
> > > for a long time before the regmap() api happened, along with GENMASK().
> >
> > Have you read the following macro arm of the implementation?
> >
> > // Generates the accessor methods for a single field.
> > (
> > @leaf_accessor $name:ident $hi:tt:$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 _SHIFT>]: u32 = Self::[<$field:upper _MASK>].trailing_zeros();
> > );
> >
> > $(
> > #[doc="Returns the value of this field:"]
> > #[doc=$comment]
> > )?
> > #[inline(always)]
> > pub(crate) fn $field(self) -> $res_type {
> > ::kernel::macros::paste!(
> > const MASK: u32 = $name::[<$field:upper _MASK>];
> > const SHIFT: u32 = $name::[<$field:upper _SHIFT>];
> > );
> > let field = ((self.0 & MASK) >> SHIFT);
> >
> > Here you can see that it's just a mask + shift operation internally to
> > access the field.
> >
> > $process(field)
> > }
> >
> > ::kernel::macros::paste!(
> > $(
> > #[doc="Sets the value of this field:"]
> > #[doc=$comment]
> > )?
> > #[inline(always)]
> > pub(crate) fn [<set_ $field>](mut self, value: $to_type) -> Self {
> > const MASK: u32 = $name::[<$field:upper _MASK>];
> > const SHIFT: u32 = $name::[<$field:upper _SHIFT>];
> > let value = (u32::from(value) << SHIFT) & MASK;
> > self.0 = (self.0 & !MASK) | value;
> >
> > self
> > }
> > );
> > };
>
> Yes, that's great, but that is all done in "native cpu" endian, and
> might not actually represent what the hardware does at all, which is
> what I was trying to get at here, sorry for not being more specific.
>
> > Now I too would like to see how exactly this will be used to read data
> > from hardware. But at least in theory if the conversion from hardware
> > endianness to native endianness is done correctly, this will do the
> > right thing :)
>
> That's great, so we are close, but it's not quite correct. How about
> something like:
>
> 0:7 reg_X as __le32
> 8:15 reg_y as __le32

I don't think we should force endianness requirements within specific fields in
the bitfield rust library itself, it is upto the user. bitfields are not only
for registers even in C. If you see on the C side, we have rcu_special union
which uses 'u32' and does not enforce endianness within the fields or bytes
of the struct with respect to the fields. Its all native CPU endian and works
fine. You're basically saying in terms of C that, the designers of the C
bitfield in C standard force the C language to use endianness in the types, no
they can't / shouldn't be forced to.

For the separate issue of enforcing endianness with respect to (across)
multiple fields, I agree with you that if the user's backend (the consumer of
the data) is not doing such conversion, say via regmap, then that becomes a
problem. But that problem is orthogonal/different and cannot be solved here.

> and the like. There has to be a way to actually specify the
> representation here and for C, that is using the "__" types that express
> the endianness (i.e. __le32, __be32, and so on). Then we have type
> checks that enforce accesses to those variables to always turn the data
> into "native" values when the cpu accesses them.

Sure, it is upto the user, they can decide to use whatever types they want.
In fact we also support conversion to enums, not just integer types :)

thanks,

- Joel