Greg KH
2025-Sep-21 09:36 UTC
[PATCH v4 1/6] nova-core: bitfield: Move bitfield-specific code from register! into new macro
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 at weathered-steel.dev> > Signed-off-by: Joel Fernandes <joelagnelf at 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(). The issue is that you are going to want to take these bitfields as part of a larger structure, and attempt to "lay it over" a chunk of memory that came from, or is going to, hardware. When that happens, all of the endian issues of mis-matches between hardware and cpus come into play, which is not able to be properly expressed here at all, unless you attempt to either resolve it all later on in something like the regmap api, or you have #ifdef stuff to attempt to capture all of the possible combinations and deal with it at build time (which is strongly never recommended, but is what we used to do in previous decades.) Your example code using this is nice, and it shows how to set up, and query these bits, but that's not anything anyone actually does in the kernel, what they want to do is read/write from hardware with this. So, how does that work? Where does this "drop down" to the native bus/memory transactions and swizzle the bits properly to work correctly? And where does this allow us to define things like BIT(2) for values? (ok, that's kind of not the point of this patch series, but it will come up over time...) Ideally, this would map to our existing regmap api, which does handle all of this properly, but I know that's not usually used by PCI drivers like where this code is coming from, as they "just assume" endian formats are all little and can get away with it due to the limited nature of different hardware types for their hardware. Also, a larger meta-comment, why doesn't rust have bit types? Why does everyone either have to roll their own or rely on an external crate? Is anyone working to provide native bit support to the language? I'm sure the embedded people would love it as I imagine it's what they reach for first when using the language on their hardware. thanks, greg k-h
Benno Lossin
2025-Sep-21 12:33 UTC
[PATCH v4 1/6] nova-core: bitfield: Move bitfield-specific code from register! into new macro
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 at weathered-steel.dev> >> Signed-off-by: Joel Fernandes <joelagnelf at 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 } ); }; 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 :)> The issue is that you are going to want to take these bitfields as part > of a larger structure, and attempt to "lay it over" a chunk of memory > that came from, or is going to, hardware. When that happens, all of the > endian issues of mis-matches between hardware and cpus come into play, > which is not able to be properly expressed here at all, unless you > attempt to either resolve it all later on in something like the regmap > api, or you have #ifdef stuff to attempt to capture all of the possible > combinations and deal with it at build time (which is strongly never > recommended, but is what we used to do in previous decades.)The "laying over part" requires a cast or transmute in Rust which is `unsafe`, so I'd say we will definitely notice it in the code if a user would be trying to do it. --- Cheers, Benno
Danilo Krummrich
2025-Sep-21 13:49 UTC
[PATCH v4 1/6] nova-core: bitfield: Move bitfield-specific code from register! into new macro
On Sun Sep 21, 2025 at 11:36 AM CEST, Greg KH wrote:> Your example code using this is nice, and it shows how to set up, and > query these bits, but that's not anything anyone actually does in the > kernel, what they want to do is read/write from hardware with this. > > So, how does that work? Where does this "drop down" to the native > bus/memory transactions and swizzle the bits properly to work correctly?Please see reply in [1], let's continue discussing it there. :) [1] https://lore.kernel.org/lkml/DCYIX8URVIWM.2ZK3GHH3J82XQ at kernel.org/