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

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Message-Id: <DCYIX8URVIWM.2ZK3GHH3J82XQ@kernel.org>
Date: Sun, 21 Sep 2025 15:47:55 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Greg KH" <gregkh@...uxfoundation.org>
Cc: "Benno Lossin" <lossin@...nel.org>, "Joel Fernandes"
 <joelagnelf@...dia.com>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <dri-devel@...ts.freedesktop.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>, "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>, "Elle Rhumsaa" <elle@...thered-steel.dev>, "Yury
 Norov" <yury.norov@...il.com>, "Daniel Almeida"
 <daniel.almeida@...labora.com>, <nouveau@...ts.freedesktop.org>
Subject: Re: [PATCH v4 1/6] nova-core: bitfield: Move bitfield-specific code
 from register! into new macro

On Sun Sep 21, 2025 at 2:45 PM CEST, Greg KH wrote:
> Again, regmap handles this all just fine, why not just make bindings to
> that api here instead?

The idea is to use this for the register!() macro, e.g.

	register!(NV_PMC_BOOT_0 @ 0x00000000, "Basic revision information about the GPU" {
	    28:24   architecture_0 as u8, "Lower bits of the architecture";
	    23:20   implementation as u8, "Implementation version of the architecture";
	    8:8     architecture_1 as u8, "MSB of the architecture";
	    7:4     major_revision as u8, "Major revision of the chip";
	    3:0     minor_revision as u8, "Minor revision of the chip";
	});

(More examples in [1].)

This generates a structure with the relevant accessors; we can also implement
additional logic, such as:

	impl NV_PMC_BOOT_0 {
	    /// Combines `architecture_0` and `architecture_1` to obtain the architecture of the chip.
	    pub(crate) fn architecture(self) -> Result<Architecture> {
	        Architecture::try_from(
	            self.architecture_0() | (self.architecture_1() << Self::ARCHITECTURE_0_RANGE.len()),
	        )
	    }

	    /// Combines `architecture` and `implementation` to obtain a code unique to the chipset.
	    pub(crate) fn chipset(self) -> Result<Chipset> {
	        self.architecture()
	            .map(|arch| {
	                ((arch as u32) << Self::IMPLEMENTATION_RANGE.len())
	                    | u32::from(self.implementation())
	            })
	            .and_then(Chipset::try_from)
	    }
	}

This conviniently allows us to read the register with

	let boot0 = regs::NV_PMC_BOOT_0::read(bar);

and obtain an instance of the entire Chipset structure with

	let chipset = boot0.chipset()?;

or pass it to a constructor that creates a Revision instance

	let rev = Revision::from_boot0(boot0);

Analogously it allows us to modify and write registers without having to mess
with error prone shifts, masks and casts, because that code is generated by the
register!() macro. (Of course, unless we have more complicated cases where
multiple fields have to be combined as illustrated above.)

Note that bar is of type pci::Bar<BAR0_SIZE> where BAR0_SIZE in our case is
SZ_16M.

However, the type required by read() as generated by the register!() macro
actually only requires something that implements an I/O backend, i.e
kernel::io::Io<SIZE>.

pci::Bar is a specific implementation of kernel::io::Io.

With this we can let the actual I/O backend handle the endianness of the bus.

(Actually, we could even implement an I/O backend that uses regmap.)

So, I think the register!() stuff is rather orthogonal.

- Danilo

[1] https://gitlab.freedesktop.org/drm/rust/kernel/-/blob/drm-rust-next/drivers/gpu/nova-core/regs.rs