linux-kernel - Re: [PATCH RFC 1/3] rust: add useful ops for u64

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Message-Id: <D7WFP99SMV3H.26AJWK17S0UPX@nvidia.com>
Date: Wed, 19 Feb 2025 21:51:13 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "John Hubbard" <jhubbard@...dia.com>, "Alexandre Courbot"
 <acourbot@...dia.com>, "Timur Tabi" <ttabi@...dia.com>,
 "daniel.almeida@...labora.com" <daniel.almeida@...labora.com>
Cc: "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 "rust-for-linux@...r.kernel.org" <rust-for-linux@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
 "dakr@...nel.org" <dakr@...nel.org>, "airlied@...il.com"
 <airlied@...il.com>, "Ben Skeggs" <bskeggs@...dia.com>
Subject: Re: [PATCH RFC 1/3] rust: add useful ops for u64

On Wed Feb 19, 2025 at 12:24 PM JST, John Hubbard wrote:
> 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?

Yes, that looks like the optimal way to do this actually. It also
doesn't introduce any overhead as the destructuring was doing both
high_half() and low_half() in sequence, so in some cases it might
even be more efficient.

I'd just like to find a better naming. high() and low() might be enough?
Or are there other suggestions?