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Date: Tue, 10 Mar 2026 17:53:10 +0100
Message-Id: <DGZ9BP8JRU4V.1PYK1RR3U145T@kernel.org>
Subject: Re: [PATCH 1/2] rust: sizes: add u64 variants of SZ_* constants
Cc: "John Hubbard" <jhubbard@nvidia.com>, "Alexandre Courbot"
 <acourbot@nvidia.com>, "Joel Fernandes" <joelagnelf@nvidia.com>, "Timur
 Tabi" <ttabi@nvidia.com>, "Alistair Popple" <apopple@nvidia.com>, "Eliot
 Courtney" <ecourtney@nvidia.com>, "Shashank Sharma" <shashanks@nvidia.com>,
 "Zhi Wang" <zhiw@nvidia.com>, "David Airlie" <airlied@gmail.com>, "Simona
 Vetter" <simona@ffwll.ch>, "Bjorn Helgaas" <bhelgaas@google.com>, "Miguel
 Ojeda" <ojeda@kernel.org>, "Alex Gaynor" <alex.gaynor@gmail.com>, "Boqun
 Feng" <boqun.feng@gmail.com>, "Gary Guo" <gary@garyguo.net>,
 =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, "Benno Lossin"
 <lossin@kernel.org>, "Andreas Hindborg" <a.hindborg@kernel.org>, "Alice
 Ryhl" <aliceryhl@google.com>, "Trevor Gross" <tmgross@umich.edu>,
 <rust-for-linux@vger.kernel.org>, "LKML" <linux-kernel@vger.kernel.org>
To: "Miguel Ojeda" <miguel.ojeda.sandonis@gmail.com>
From: "Danilo Krummrich" <dakr@kernel.org>
References: <20260310023145.120037-1-jhubbard@nvidia.com>
 <20260310023145.120037-2-jhubbard@nvidia.com>
 <CANiq72kiiKkK0Q5w=UKNsPXvuuHMASpRJ7KAMy=2908n_u-DUA@mail.gmail.com>
In-Reply-To: <CANiq72kiiKkK0Q5w=UKNsPXvuuHMASpRJ7KAMy=2908n_u-DUA@mail.gmail.com>

On Tue Mar 10, 2026 at 2:51 PM CET, Miguel Ojeda wrote:
> On Tue, Mar 10, 2026 at 3:31=E2=80=AFAM John Hubbard <jhubbard@nvidia.com=
> wrote:
>>
>> +// CAST: every SZ_* value below fits in u64, so `as u64` is always loss=
less.
>
> Nit:
>
>     // CAST: Every `SZ_*` value below fits in `u64`, so `as u64` is
> always lossless.
>
> One alternative could be something like `sizes::u64::SZ_1M`, but if
> you expect to mix `usize` and `u64` often then it may not be great.

I have a slight preference towards this. It also shoudn't be that bad if we=
 need
both, as we could just keep the prefix, i.e. u64::SZ_1M.

> Regarding adding the constants or not, I am ambivalent. On one hand, I
> see the pain of repeating it; on the other hand, adding these only
> works for particular values (i.e. we still need the safe casts around
> anyway for other values).

In a lot of cases we won't need to go back and forth with those values, as =
all
of the DRM infrastructure works with u64. So, there'd be quite a lot of ove=
rhead
if everytime we want to initialize something in GPUVM, GPU Buddy, TTM, etc.=
 with
one of the SZ_* constants, i.e. we'd need to repeat u64::from_safe_cast(SZ_=
*)
over and over again. This is the main reason why I proposed to have u64 var=
iants
in the first place.

Technically, if they are not needed for anything else, we could also make t=
hem
GPU infrastructure, e.g. gpu::SZ_*. Additionally, we could have GPU specifi=
c
usize and isize types.