linux-kernel - Re: [PATCH RFC v2 3/3] gpu: nova-core: use BoundedInt

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Message-Id: <DDDYOBOZTF7Q.124VJDF4C76B6@kernel.org>
Date: Thu, 09 Oct 2025 19:18:33 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Yury Norov" <yury.norov@...il.com>
Cc: "Alexandre Courbot" <acourbot@...dia.com>, "Joel Fernandes"
 <joelagnelf@...dia.com>, "Jesung Yang" <y.j3ms.n@...il.com>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng"
 <boqun.feong@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <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>,
 <nouveau@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH RFC v2 3/3] gpu: nova-core: use BoundedInt

On Thu Oct 9, 2025 at 6:40 PM CEST, Yury Norov wrote:
> On Thu, Oct 09, 2025 at 09:37:10PM +0900, Alexandre Courbot wrote:
>> Use BoundedInt with the register!() macro and adapt the nova-core code
>> accordingly. This makes it impossible to trim values when setting a
>> register field, because either the value of the field has been inferred
>> at compile-time to fit within the bounds of the field, or the user has
>> been forced to check at runtime that it does indeed fit.
>
> In C23 we've got _BitInt(), which works like:
>
>         unsigned _BitInt(2) a = 5; // compile-time error
>
> Can you consider a similar name and syntax in rust?

Rust is a different language and has its own syntax, I think we should not try
to use C syntax instead.

>>          regs::NV_PFALCON_FALCON_DMATRFBASE1::default()
>> -            .set_base((dma_start >> 40) as u16)
>> +            .try_set_base(dma_start >> 40)?
>>              .write(bar, &E::ID);
>
> Does it mean that something like the following syntax is possible?
>
>         regs::NV_PFALCON_FALCON_DMATRFBASE1::default()
>             .try_set_base1(base1 >> 40)?        // fail here

Note that try_set_base1() returns a Result [1], which is handled immediately by
the question mark operator [2]. I.e. if try_set_base1() returns an error it is
propagated to the caller right away without executing any of the code below.

>             .try_set_base2(base2 >> 40)?        // skip
>             .write(bar, &E::ID) else { pr_err!(); return -EINVAL };
>
> This is my main concern: Rust is advertised a as runtime-safe language
> (at lease safer than C), but current design isn't safe against one of
> the most common errors: type overflow.

Where do you see a potential runtime overflows in the register!() code?

[1] https://rust.docs.kernel.org/kernel/error/type.Result.html
[2] https://doc.rust-lang.org/reference/expressions/operator-expr.html?highlight=question%20mark#the-question-mark-operator