linux-kernel - Re: [PATCH v6 0/5] Introduce bitfield and move register macro to rust/kernel/

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Message-ID: <aOZ0_7YQw81lyMWt@yury>
Date: Wed, 8 Oct 2025 10:28:15 -0400
From: Yury Norov <yury.norov@...il.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Joel Fernandes <joelagnelf@...dia.com>,
	Alexandre Courbot <acourbot@...dia.com>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	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, Elle Rhumsaa <elle@...thered-steel.dev>,
	Daniel Almeida <daniel.almeida@...labora.com>,
	Andrea Righi <arighi@...dia.com>, nouveau@...ts.freedesktop.org
Subject: Re: [PATCH v6 0/5] Introduce bitfield and move register macro to
 rust/kernel/

On Wed, Oct 08, 2025 at 12:08:59AM +0200, Danilo Krummrich wrote:

> Regarding the bitfields for page table management: Are we
> sure that we can use raw bitfields for this?

As per my current understanding we can't. Bitfields are not suitable for
direct I/O and considered as a data storage. In the current scheme:

        regs::NV_PFALCON_FALCON_DMATRFBASE::default()
            .set_base((dma_start >> 8) as u32)
            .write(bar, &E::ID);

we account for endianess in the .write() method, which would be a part
of register API, not bitfields.

FYI: ARM64 is in process of dropping BE, and Linus announced the end
of BE support for RISC-V:

https://lore.kernel.org/all/CAHk-=wgYcOiFvsJzFb+HfB4n6Wj6zM5H5EghUMfpXSCzyQVSfA@mail.gmail.com/

Thanks,
Yury