[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-Id: <DAKL0KOWUB1G.1DSJPRWFYC43O@nvidia.com>
Date: Thu, 12 Jun 2025 22:17:01 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Benno Lossin" <lossin@...nel.org>, "Miguel Ojeda" <ojeda@...nel.org>,
"Alex Gaynor" <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>,
"Gary Guo" <gary@...yguo.net>, Björn Roy Baron
<bjorn3_gh@...tonmail.com>, "Benno Lossin" <benno.lossin@...ton.me>,
"Andreas Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl"
<aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "Danilo
Krummrich" <dakr@...nel.org>, "David Airlie" <airlied@...il.com>, "Simona
Vetter" <simona@...ll.ch>, "Maarten Lankhorst"
<maarten.lankhorst@...ux.intel.com>, "Maxime Ripard" <mripard@...nel.org>,
"Thomas Zimmermann" <tzimmermann@...e.de>
Cc: "John Hubbard" <jhubbard@...dia.com>, "Ben Skeggs" <bskeggs@...dia.com>,
"Joel Fernandes" <joelagnelf@...dia.com>, "Timur Tabi" <ttabi@...dia.com>,
"Alistair Popple" <apopple@...dia.com>, <linux-kernel@...r.kernel.org>,
<rust-for-linux@...r.kernel.org>, <nouveau@...ts.freedesktop.org>,
<dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH v4 04/20] rust: add new `num` module with useful integer
operations
On Wed Jun 4, 2025 at 4:18 PM JST, Benno Lossin wrote:
> On Wed Jun 4, 2025 at 2:05 AM CEST, Alexandre Courbot wrote:
>> On Wed Jun 4, 2025 at 8:02 AM JST, Benno Lossin wrote:
>>> On Mon Jun 2, 2025 at 3:09 PM CEST, Alexandre Courbot wrote:
>>>> On Thu May 29, 2025 at 4:27 PM JST, Benno Lossin wrote:
>>>>> On Thu May 29, 2025 at 3:18 AM CEST, Alexandre Courbot wrote:
>>>>>> On Thu May 29, 2025 at 5:17 AM JST, Benno Lossin wrote:
>>>>>>> On Wed May 21, 2025 at 8:44 AM CEST, Alexandre Courbot wrote:
>>>>>>>> + /// Align `self` up to `alignment`.
>>>>>>>> + ///
>>>>>>>> + /// `alignment` must be a power of 2 for accurate results.
>>>>>>>> + ///
>>>>>>>> + /// Wraps around to `0` if the requested alignment pushes the result above the type's limits.
>>>>>>>> + ///
>>>>>>>> + /// # Examples
>>>>>>>> + ///
>>>>>>>> + /// ```
>>>>>>>> + /// use kernel::num::NumExt;
>>>>>>>> + ///
>>>>>>>> + /// assert_eq!(0x4fffu32.align_up(0x1000), 0x5000);
>>>>>>>> + /// assert_eq!(0x4000u32.align_up(0x1000), 0x4000);
>>>>>>>> + /// assert_eq!(0x0u32.align_up(0x1000), 0x0);
>>>>>>>> + /// assert_eq!(0xffffu16.align_up(0x100), 0x0);
>>>>>>>> + /// assert_eq!(0x4fffu32.align_up(0x0), 0x0);
>>>>>>>> + /// ```
>>>>>>>> + fn align_up(self, alignment: Self) -> Self;
>>>>>>>
>>>>>>> Isn't this `next_multiple_of` [1] (it also allows non power of 2
>>>>>>> inputs).
>>>>>>>
>>>>>>> [1]: https://doc.rust-lang.org/std/primitive.u32.html#method.next_multiple_of
>>>>>>
>>>>>> It is, however the fact that `next_multiple_of` works with non powers of
>>>>>> two also means it needs to perform a modulo operation. That operation
>>>>>> might well be optimized away by the compiler, but ACAICT we have no way
>>>>>> of proving it will always be the case, hence the always-optimal
>>>>>> implementation here.
>>>>>
>>>>> When you use a power of 2 constant, then I'm very sure that it will get
>>>>> optimized [1]. Even with non-powers of 2, you don't get a division [2].
>>>>> If you find some code that is not optimized, then sure add a custom
>>>>> function.
>>>>>
>>>>> [1]: https://godbolt.org/z/57M9e36T3
>>>>> [2]: https://godbolt.org/z/9P4P8zExh
>>>>
>>>> That's impressive and would definitely work well with a constant. But
>>>> when the value is not known at compile-time, the division does occur
>>>> unfortunately: https://godbolt.org/z/WK1bPMeEx
>>>>
>>>> So I think we will still need a kernel-optimized version of these
>>>> alignment functions.
>>>
>>> Hmm what exactly is the use-case for a variable align amount? Could you
>>> store it in const generics?
>>
>> Say you have an IOMMU with support for different pages sizes, the size
>> of a particular page can be decided at runtime.
>>
>>>
>>> If not, there are also these two variants that are more efficient:
>>>
>>> * option: https://godbolt.org/z/ecnb19zaM
>>> * unsafe: https://godbolt.org/z/EqTaGov71
>>>
>>> So if the compiler can infer it from context it still optimizes it :)
>>
>> I think the `Option` (and subsequent `unwrap`) is something we want to
>> avoid on such a common operation.
>
> Makes sense.
>
>>> But yeah to be extra sure, you need your version. By the way, what
>>> happens if `align` is not a power of 2 in your version?
>>
>> It will just return `(self + (self - 1)) & (alignment - 1)`, which will
>> likely be a value you don't want.
>
> So wouldn't it be better to make users validate that they gave a
> power-of-2 alignment?
>
>> So yes, for this particular operation we would prefer to only use powers
>> of 2 as inputs - if we can ensure that then it solves most of our
>> problems (can use `next_multiple_of`, no `Option`, etc).
>>
>> Maybe we can introduce a new integer type that, similarly to `NonZero`,
>> guarantees that the value it stores is a power of 2? Users with const
>> values (90+% of uses) won't see any difference, and if working with a
>> runtime-generated value we will want to validate it anyway...
>
> I like this idea. But it will mean that we have to have a custom
> function that is either standalone and const or in an extension trait :(
> But for this one we can use the name `align_up` :)
>
> Here is a cool idea for the implementation: https://godbolt.org/z/x6navM5WK
Yeah that's close to what I had in mind. Actually, we can also define
`align_up` and `align_down` within this new type, and these methods can
now be const since they are not implemented via a trait!
Powered by blists - more mailing lists