linux-kernel - Re: [PATCH v8] rust: add new macro for common bitflag operations

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Message-Id: <FD404241-143B-4B24-B8BF-3D121073F4A9@collabora.com>
Date: Thu, 15 Jan 2026 10:45:14 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Gary Guo <gary@...yguo.net>
Cc: Andreas Hindborg <a.hindborg@...nel.org>,
 Filipe Xavier <felipeaggger@...il.com>,
 Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Boqun Feng <boqun.feng@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>,
 Benno Lossin <benno.lossin@...ton.me>,
 Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>,
 Danilo Krummrich <dakr@...nel.org>,
 rust-for-linux@...r.kernel.org,
 felipe_life@...e.com,
 linux-kernel@...r.kernel.org,
 Lyude Paul <lyude@...hat.com>
Subject: Re: [PATCH v8] rust: add new macro for common bitflag operations



> On 15 Jan 2026, at 10:19, Gary Guo <gary@...yguo.net> wrote:
> 
> On Thu Jan 15, 2026 at 12:14 PM GMT, Andreas Hindborg wrote:
>> Andreas Hindborg <a.hindborg@...nel.org> writes:
>> 
>>> "Filipe Xavier" <felipeaggger@...il.com> writes:
>>> 
>>>> We have seen a proliferation of mod_whatever::foo::Flags
>>>> being defined with essentially the same implementation
>>>> for BitAnd, BitOr, contains and etc.
>>>> 
>>>> This macro aims to bring a solution for this,
>>>> allowing to generate these methods for user-defined structs.
>>>> With some use cases in KMS and upcoming GPU drivers.
>>>> 
>>>> Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/We.20really.20need.20a.20common.20.60Flags.60.20type
>>>> Suggested-by: Daniel Almeida <daniel.almeida@...labora.com>
>>>> Suggested-by: Lyude Paul <lyude@...hat.com>
>>>> Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>
>>>> Reviewed-by: Lyude Paul <lyude@...hat.com>
>>>> Signed-off-by: Filipe Xavier <felipeaggger@...il.com>
>>> 
>>> Tested-by: Andreas Hindborg <a.hindborg@...nel.org>
>>> Reviewed-by: Andreas Hindborg <a.hindborg@...nel.org>
>>> 
>>> 
>>> I think it would be useful to add:
>>> 
>>> impl Flags {
>>>     unsafe from_raw(value: Repr) -> {
>>>     ...
>>>     }
>>> }
>>> 
>>> impl TryFrom<Repr> for Flag {
>>>     // Succeed if `value` is a valid `Flag` enum bit pattern.
>>> }
>>> 
>>> impl TryFrom<Repr> for Flags {
>>>     // Succeed if `value` is the logical OR of valid enum bit patterns.
>>> }
>> 
>> Also bitwise operations on the underlying type such as
>> 
>>  impl BitOrAssign<Flag> for u32 {
>>  ...
>>  }
>> 
>> Not sure if that is possible with the orphan rule, but it would be nice.
>> I was trying to
>> 
>>  lim.features |= request::Feature::Rotational;
> 
> This is allowed by orphan rules, in a similar way that you can implement
> `From<Local> for Foreign`.
> 
> However I don't think this is desirable feature? Why can't a conversion be used
> if you're using raw reprs?
> 
> I think we should discourage the use of raw integers when bitflags can be used,
> adding that (or even `TryFrom`) can be a negative incentive.


The flags may be in your uapi or they may otherwise come from C. In that case
you’re stuck with a TryFrom or a manual conversion.

I think a generated TryFrom shouldn't hurt. Just my 0.02c.

> 
> Best,
> Gary
> 
>> 
>> where `lim: bindings::queue_limits`.
>> 
>> 
>> Best regards,
>> Andreas Hindborg