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

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Message-ID: <b3842ced-c3f6-47b4-ab17-02e7ebdfe7e6@nvidia.com>
Date: Thu, 9 Oct 2025 14:37:32 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Yury Norov <yury.norov@...il.com>, Danilo Krummrich <dakr@...nel.org>
Cc: Alexandre Courbot <acourbot@...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 10/9/2025 2:28 PM, Yury Norov wrote:
[..]
>>>>          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.
> 
> Thanks for the links. I am definitely the very beginning on the
> learning curve for this.
>  
>>>             .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?
> 
> Assuming base is 10-bit,
>         
>         let ret = some_c_wrapper()      // 0..1024 or -EINVAL
>         regs::NV_PFALCON_FALCON_DMATRFBASE1::default()
>              .try_set_base1(ret)
> 
> Or maybe I misunderstood the question, because if there's no possibility
> to overflow a field, what for the .try_set_xxx() is needed at all?

Because 'ret' is a value determined at runtime in this example, there is no way
for the compiler to know that ret will fit into the bounded int, at compile
time. So the "try_" means it is runtime checked and validated (via return of
Result). Sure it may well not fail, but the compiler doesn't know that.

Thanks.