Re: [PATCH v6 4/5] rust: Move register and bitfield macros out of Nova

["Alexandre Courbot" <acourbot@nvidia.com>]

On Sun Nov 2, 2025 at 3:51 AM JST, Dirk Behme wrote:
> On 09.10.25 13:28, Alexandre Courbot wrote:
>> On Thu Oct 9, 2025 at 8:16 PM JST, Danilo Krummrich wrote:
>>> On Thu Oct 9, 2025 at 8:59 AM CEST, Dirk Behme wrote:
>>>> Assuming that register.rs is supposed to become the "generic" way to 
>>>> access hardware registers I started to have a look to it. Some weeks 
>>>> back testing interrupts I used some quite simple timer with 4 registers 
>>>> [1]. Now, thinking about converting it to register!() I have three 
>>>> understanding / usage questions:
>>>>
>>>> * At the moment register!() is for 32-bit registers, only? So it can't 
>>>> be used for my example having 8-bit and 16-bit registers as well?
>>>
>>> Yes, currently the register!() macro always generates a 32-bit register type
>>> (mainly because nova-core did not need anything else). However, this will of
>>> course be generalized (which should be pretty straight forward).
>>>
>>> Having a brief look at the TMU datasheet it looks like you should be able to
>>> treat TSTR and TCR as 32-bit registers without any issues for testing the
>>> register!() macro today. I.e. you can just define it as:
>>>
>>> 	register!(TSTR @ 0x04, "Timer Start Register" {
>>> 	    2:2    str2 as bool, "Specifies whether TCNT2 is operated or stopped.";
>>> 	    1:1    str1 as bool, "Specifies whether TCNT1 is operated or stopped.";
>>> 	    0:0    str0 as bool, "Specifies whether TCNT0 is operated or stopped.";
>>> 	});
>>>
>>> Same for TCR.
>> 
>> Patch 2 of this series actually adds support for 16 and 8 bit register
>> storage.
>
> Hmm, how to use that with the register!() macro? I mean patch 2 adds
> support for different storage widths for *bitfields*. But looking at
> patch 4 [2] it looks like *register!()* still uses $name(u32)? With
> that it looks like that the register!() macro still just supports 32
> bit registers? Or what have I missed?
>
> What I'm looking for is a way to specify if a register is 8, 16 or 32
> bit. Using the example from above something like
>
> register!(TSTR<u8> @ ....

Errr indeed, you are correct. The `register` macro's syntax has not been
updated to take advantage of `bitfield`'s storage types, and `u32` is
still hardcoded as of this series.

This looks like an oversight - a register is basically a bitfield with
some I/O, so making it support storage types should be trivial. I guess
this hasn't been done yet because Nova is the only user so far, and we
don't need/want to explicitly specify a type for each register since
they are invariably `u32`.

But it wouldn't look good to change the syntax of `register` after
moving it out, so I agree this should take place before the move. The
same applies to the visiblity feature.

One way to avoid a update all the declarations so far would be to give
Nova its own `register` macro that invokes the one in `kernel` with
the relevant parameters hardcoded.