Nouveau - Feb 2025 - [PATCH RFC 1/3] rust: add useful ops for u64

On Wed Feb 19, 2025 at 5:51 AM JST, Timur Tabi wrote:
> On Tue, 2025-02-18 at 22:16 +0900, Alexandre Courbot wrote:
>> > A proper struct with `high` and `low` might be more verbose, but
>> > it rules out this issue.
>> 
>> Mmm indeed, so we would have client code looking like:
>> 
>> ? let SplitU64 { high, low } = some_u64.into_u32();
>> 
>> instead of 
>> 
>> ? let (high, low) = some_u64.into_u32();
>> 
>> which is correct, and
>> 
>> ? let (low, high) = some_u64.into_u32();
>> 
>> which is incorrect, but is likely to not be caught.
>
> I'm new to Rust, so let me see if I get this right.
>
> struct SplitU64 {
>   high: u32,
>   low: u32
> }
>
> So if you want to extract the upper 32 bits of a u64, you have to do this:
>
> let split = some_u64.into_u32s();
> let some_u32 = split.high;
More likely this would be something like:

  let SplitU64 { high: some_u32, .. } = some_u64;

Which is still a bit verbose, but a single-liner.

Actually. How about adding methods to this trait that return either
component?

  let some_u32 = some_u64.high_half();
  let another_u32 = some_u64.low_half();

These should be used most of the times, and using destructuring/tuple
would only be useful for a few select cases.
>
> as opposed to your original design:
>
> let (some_u32, _) = some_u64.into_u32s();
>
> Personally, I prefer the latter.  The other advantage is that into_u32s and
> from_u32s are reciprocal:
>
> assert_eq!(u64::from_u32s(u64::into_u32s(some_u64)), some_u64);
>
> (or something like that)
Yeah, having symmetry is definitely nice. OTOH there are no safeguards
against mixing up the order and the high and low components, so a
compromise will have to be made one way or the other. But if we also add
the methods I proposed above, that question should matter less.

On 2/18/25 5:21 PM, Alexandre Courbot wrote:
> On Wed Feb 19, 2025 at 5:51 AM JST, Timur Tabi wrote:
>> On Tue, 2025-02-18 at 22:16 +0900, Alexandre Courbot wrote:
...
> More likely this would be something like:
> 
>    let SplitU64 { high: some_u32, .. } = some_u64;
> 
> Which is still a bit verbose, but a single-liner.
> 
> Actually. How about adding methods to this trait that return either
> component?
> 
>    let some_u32 = some_u64.high_half();
>    let another_u32 = some_u64.low_half();
> 
> These should be used most of the times, and using destructuring/tuple
> would only be useful for a few select cases.
I think I like this approach best so far, because that is actually how
drivers tend to use these values: one or the other 32 bits at a time.
Registers are often grouped into 32-bit named registers, and driver code
wants to refer to them one at a time (before breaking some of them down
into smaller named fields)>

The .high_half() and .low_half() approach matches that very closely.
And it's simpler to read than the SplitU64 API, without losing anything
we need, right?

  
thanks,
-- 
John Hubbard

> Actually. How about adding methods to this trait that return either
> component?
>
>   let some_u32 = some_u64.high_half();
>   let another_u32 = some_u64.low_half();
>
> These should be used most of the times, and using destructuring/tuple
> would only be useful for a few select cases.
>
Indeed very nice!