linux-kernel - Re: [PATCH v2 2/5] gpu: nova-core: add FbRange.len() and use it in boot.rs

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Message-Id: <DEK2VE9DDHUP.SH7LJ2D3TBBG@nvidia.com>
Date: Fri, 28 Nov 2025 14:27:49 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "John Hubbard" <jhubbard@...dia.com>, "Lyude Paul" <lyude@...hat.com>,
 "Danilo Krummrich" <dakr@...nel.org>
Cc: "Alexandre Courbot" <acourbot@...dia.com>, "Joel Fernandes"
 <joelagnelf@...dia.com>, "Timur Tabi" <ttabi@...dia.com>, "Alistair Popple"
 <apopple@...dia.com>, "Edwin Peer" <epeer@...dia.com>, "Zhi Wang"
 <zhiw@...dia.com>, "David Airlie" <airlied@...il.com>, "Simona Vetter"
 <simona@...ll.ch>, "Bjorn Helgaas" <bhelgaas@...gle.com>, "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"
 <lossin@...nel.org>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice
 Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>,
 <nouveau@...ts.freedesktop.org>, <rust-for-linux@...r.kernel.org>, "LKML"
 <linux-kernel@...r.kernel.org>, "Nouveau"
 <nouveau-bounces@...ts.freedesktop.org>
Subject: Re: [PATCH v2 2/5] gpu: nova-core: add FbRange.len() and use it in
 boot.rs

On Thu Nov 27, 2025 at 9:53 AM JST, John Hubbard wrote:
> On 11/26/25 3:43 PM, Lyude Paul wrote:
>> I'm not sure this is necessary - for one, we could just use the .len() method
>> on the Range. As well - if we just implement Deref for FbRange (which I think
>> would be fine here) we could just use .len() through that.
>
> Hi Lyude!
>
> Good idea about the deref coercion. It has a minor type mismatch as-is,
> though: Range<u64>::len() returns usize, but FbRange::len() returns u64,
> which matches the callers that we have so far.

It's even worse than that, `Range<u64>::len()` simply doesn't exist. :)

`len()` is implemented through `ExactSizeIterator`, which specifies the
return type as `usize`. This obviously cannot provide a reliable result
when the range is u64, so the implementation was simply not done. See
[1] for evidence.

But having our own range type lets us slip our own `fn len(&self) ->
u64` implementation.

[1] https://doc.rust-lang.org/std/ops/struct.Range.html