Re: [PATCH] gpu: nova-core: bitfield: use &mut self setters instead of builder pattern

[Joel Fernandes <joelagnelf@nvidia.com>]

> On Dec 31, 2025, at 5:47 PM, John Hubbard <jhubbard@nvidia.com> wrote:
> 
> On 12/31/25 2:33 PM, Timur Tabi wrote:
>>> On Wed, 2025-12-31 at 13:47 -0800, John Hubbard wrote:
>>> The builder-pattern setters (self -> Self) enabled method chaining like:
>>> 
>>>     reg.set_foo(x).set_sec(y).write(bar);
>>> 
>>> This made separate operations appear as a single expression, obscuring
>>> that each setter is a distinct mutation. 
>> 
>> So you're concerned about the fact that the compiler is not merging the set_foo(x) and the
>> set_sec(y) into a single read-modify-write?
> 
> No, I don't care about that aspect.
> 
>> 
>>> These setters are infallible,
>>> so the chaining provides no error-propagation benefit—it just obscures
>>> what are simple, independent assignments.
>>> 
>>> Change the bitfield!() macro to generate `&mut self` setters, so each
>>> operation is a distinct statement:
>>> 
>>>     reg.set_foo(x);
>>>     reg.set_sec(y);
>>>     reg.write(bar);
>> 
>> Are you sure about this?  It just seems like you're throwing out a neat little feature of Rust and
>> replacing it with something that's very C-like.  This breaks compatible with all users of the regs
>> macros.  Seems really disruptive for what seems to me like a cosmetic change.
>> 
> 
> It's only a neat feature if it *does* something. In this case, it *looks*
> like a neat Rust feature, but under the covers it really is just handing
> around copies unnecessarily, when really, it *is* doing the C-like thing
> in the end.
> 
> I object to the fake Rust-ness that's being done here. It's like putting
> hubcabs on a car.

But IMO there is only one operation here, the IO write. The setter is just mutations. Builder pattern chaining is idiomatic to Rust.

I would not call it fake Rustness since it is Rustness in the Rust language. Afair, similar arguments were made before and conclusion was, well, this is Rust.

Regarding the copies, I am intrigued - have you verified that the code generation actually results in copies? I would be surprised if the compiler did not optimize.

 - Joel





> 
> thanks,
> --
> John Hubbard
>