Re: [PATCH v6 0/5] Introduce bitfield and move register macro to rust/kernel/

[Yury Norov <yury.norov@gmail.com>]

On Tue, Oct 07, 2025 at 07:36:21PM +0900, Alexandre Courbot wrote:
> Hi Yuri,
> 
> On Tue Oct 7, 2025 at 7:29 AM JST, Yury Norov wrote:
> <snip>
> > Regardless, I don't think that this is the right path to move the
> > bitfields into the core. The natural path for a feature that has
> > been originally developed on driver side is to mature in there and
> > get merged to core libraries after a while. Resctrl from Intel is one
> > recent example.
> >
> > With that said, I'm OK if you move the bitfields as a whole, like you
> > do in v5, and I'm also OK if you split out the part essential for nova
> > and take it into the driver. In that case the bitfields will stay in 
> > drivers and you'll be able to focus on the features that _you_ need,
> > not on generic considerations.
> >
> > I'm not OK to move bitfields in their current (v6) incomplete form in
> > rust/kernel. We still have no solid understanding on the API and
> > implementation that we've been all agreed on.
> 
> Initially the plan was indeed to give this code some more time to mature
> in nova-core before moving it out.
> 
> The reason for the early move is that we have another driver (Tyr) who
> wants to start using the register macro. Without it, they would be left
> with the option of either reinventing the wheel, or poking at registers
> the old-fashioned way, which I think we can agree is not going to be any
> safer than the current macro. :)
> 
> IIUC your remaining concern is with the possible loss of data when
> setting a field that is smaller than its primitive type? That should be
> addressed by [0], but as it introduces a new core feature I expect some
> discussion to take place before it can be merged. In the meantime, it
> would be great if we can make the register macro available.
> 
> Because letting it fully mature within nova-core also has the drawback
> that we might miss the perspective of other potential users, which may
> make us draw ourselves into a corner that will make the macro less
> useful generally speaking. We are at a stage where we can still make
> design changes if needed, but we need to hear from other users, and
> these won't come as long as the macro is in nova-core.

Hi Alexandre,

Thanks for the broader perspective.

So if there's another user for register!(), then yeah - it's worth to
move it out of the nova earlier. It doesn't mean that we need to split
bitfields out of it immediately.
 
> [0] https://lore.kernel.org/rust-for-linux/20251002-bounded_ints-v1-0-dd60f5804ea4@nvidia.com/

This resembles the _BitInt from C23 standard, and it looks quite
reasonable to me. I'll get back to your RFC shortly.

https://en.cppreference.com/w/c/language/arithmetic_types.html

--

I'm glad that we started this discussion. From my point, what happens now
is inventing the whole new language, and basic bit operations is the heart
of it.

I would really like to avoid adopting an API that will frustrate people
for decades after invention. Please read the following rant to taste
exactly what I mean:

https://lore.kernel.org/all/CAHk-=whoOUsqPKb7OQwhQf9H_3=5sXGPJrDbfQfwLB3Bi13tcQ@mail.gmail.com/

Thanks,
Yury