linux-kernel - Re: [PATCH v3 5/5] rust: Add KUNIT tests for bitfield

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Message-ID: <20250920003916.GA2009525@joelbox2>
Date: Fri, 19 Sep 2025 20:39:16 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Yury Norov <yury.norov@...il.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, Elle Rhumsaa <elle@...thered-steel.dev>,
	Daniel Almeida <daniel.almeida@...labora.com>,
	nouveau@...ts.freedesktop.org
Subject: Re: [PATCH v3 5/5] rust: Add KUNIT tests for bitfield

On Tue, Sep 16, 2025 at 05:59:18AM -0400, Joel Fernandes wrote:
[...]
> > > In C also this is valid. If you passed a higher value than what the
> > > bitfield can hold, the compiler will still just use the bits that it
> > > needs and ignore the rest.
> > 
> > In C we've got FIELD_{PREP,GET,MODIFY}, implementing the checks.
> > So those who want to stay on safe side have a choice.
> 
> Ah ok. We can add these checks then for the accessors, I will do so in v4.

The C checks use BUILD_BUG_ON, in rust-for-linux we have build_assert but it
is fragile and depends on the value being a constant. Since the setter API
accepts a run-time value and not a constant, we cannot use this.

Or, we can fail at runtime, but that requires changing the set_* to try_set_*
and returning a Result instead of Self. Alternatively, we can have a debug
option that panics if the setter API is misued.

Thoughts?

Or for the moment, we can keep it simple and filter out / ignore extra bits
of the larger value passed (which is what nova-core's register macro bitfield
implementation currently does anyway).

thanks,

 - Joel