On Thu Oct 16, 2025 at 9:28 PM CEST, Joel Fernandes wrote:
>> On Oct 16, 2025, at 1:48 PM, Yury Norov <[email protected]> wrote:
>> On Thu, Oct 16, 2025 at 11:13:21AM -0400, Joel Fernandes wrote:
>>> +///
>>> +/// bitfield! {
>>> +/// struct ControlReg {
>>> +/// 3:0 mode as u8 ?=> Mode;
>>> +/// 7:7 state as bool => State;
>>> +/// }
>>> +/// }
>>
>> This notation is really unwelcome this days. It may be OK for a random
>> macro in some local driver, but doesn't really work for a global basic
>> data type:
>>
>> https://lore.kernel.org/all/CAHk-=whoOUsqPKb7OQwhQf9H_3=5sxgpjrdbfqfwlb3bi13...@mail.gmail.com/
>>
>> I've already shared this link with you, and shared my concern.
>>
>> I realize that rust/bitfield derives the GENMASK(hi, lo) notation here,
>> and GENMASK() derives verilog or hardware specs popular notations. But
>> software people prefer lo:hi. I'm probably OK if you choose C-style
>> start:nbits, if you prefer. But let's stop this hi:lo early, please.
>>
>> Let me quote Linus from the link above:
>>
>> It does "high, low", which is often very unintuitive, and in fact the
>> very commit that introduced this thing from hell had to convert the
>> sane "low,high" cases to the other way around.
>
> I agree with Linus but I disagree with comparing it with these macros.
> I agree with Linus it is oddly unreadable when used as function parameters.
> But that is a different syntax. Over here we are using colons with sufficient
> whitespace around hi:lo.
I agree with Joel here.
While I'm not super opinionated for general bitfields, for the register!()
infrastructure I very much prefer the hi:lo notation, as this is the common
notation in datasheets and TRMs.
However, if we use hi:lo, we should use it decending, i.e.:
bitfield! {
struct ControlReg {
7:5 state as u8 => State;
3:0 mode as u8 ?=> Mode;
}
}
- Danilo
