linux-kernel - Re: [PATCH v9] rust: kernel: add support for bits/genmask macros

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Message-Id: <1DCA3EA2-5CBB-4193-B1D3-7E286C344A14@collabora.com>
Date: Wed, 16 Jul 2025 16:44:29 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...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>,
 linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org,
 Alexandre Courbot <acourbot@...dia.com>
Subject: Re: [PATCH v9] rust: kernel: add support for bits/genmask macros



> On 16 Jul 2025, at 16:32, Danilo Krummrich <dakr@...nel.org> wrote:
> 
> On Wed Jul 16, 2025 at 9:11 PM CEST, Daniel Almeida wrote:
>> Let’s transfer this discussion to this patch.
>> 
>>> I also quickly tried genmask and I have a few questions:
>>> 
>>> (1) Why does genmask not use a const generic? I think this makes it more
>>>     obvious that it's only intended to be used from const context.
>> 
>> I guess none of us thought about it, since the current version also works.
> 
> I think using a const generic would be a bit better for the mentioned reason.

Btw, how does monomorphization work here? Would we have to codegen all the
versions? Also, I don't think that you can take a range as a const generic
argument, i.e., I don't recall ever seeing this syntax:

genmask_u64::<0..=63>();

So we'd have to go back to taking two arguments, and it's less ergonomic than
a..=b (which was discussed a bit at length during the previous iterations).
Also it would stand out against the non-const (i.e. checked) versions, which would
still take a range as argument.

> 
>>> 
>>> (2) Why is there no build_assert() when the range exceeds the number of bits
>>>     of the target type? I would expect genmask_u64(0..100) to fail.
>> 
>> Doesn’t it?
>> 
>> There is a build_assert in the underlying bit implementation. It was redundant
>> to have it both in bit_* and in genmask, because genmask calls bit().
>> 
>> In your example, bit_u64(100) hits that assert, and so it shouldn't compile.
> 
> Indeed, and it also works, except from doc-tests for some reason, which is what
> I tried real quick. :)
> 

Wait, this was a bit confusing :)
You’re confirming that it doesn’t compile, correct?

> I feel like usize would be a better fit, but not a strong opinion.

I guess this is the same problem as u64: drivers will usually have either
i32s/u32s and this would require a cast.

— Daniel