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

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Message-ID: <569b2f95-fa8d-4bd0-9e90-512b677435fe@nvidia.com>
Date: Fri, 19 Sep 2025 17:52:46 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Joel Fernandes <joelagnelf@...dia.com>, 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>,
 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 9/19/25 5:39 PM, Joel Fernandes wrote:
> 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.

Please no...

> 
> 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).
> 

Yes. Assuming that I'm not completely lost here, you are proposing to
simply truncate to the size of the bitfield--no panics, no warnings. And
that's perfectly fine here IMHO.

thanks,
-- 
John Hubbard