linux-kernel - Re: [PATCH v2 5/7] gpu: nova-core: add extra integer conversion functions and traits

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Message-ID: <d6b29e56-9ac2-40ee-8994-8517adc73f34@nvidia.com>
Date: Mon, 27 Oct 2025 15:11:01 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Alexandre Courbot <acourbot@...dia.com>, Alice Ryhl
 <aliceryhl@...gle.com>, David Airlie <airlied@...il.com>,
 Simona Vetter <simona@...ll.ch>, 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>, Trevor Gross <tmgross@...ch.edu>
Cc: John Hubbard <jhubbard@...dia.com>, Alistair Popple <apopple@...dia.com>,
 Timur Tabi <ttabi@...dia.com>, Edwin Peer <epeer@...dia.com>,
 nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
 linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
 Danilo Krummrich <dakr@...nel.org>
Subject: Re: [PATCH v2 5/7] gpu: nova-core: add extra integer conversion
 functions and traits

On 10/27/2025 3:09 PM, Joel Fernandes wrote:
[...]
>> +/// Extension trait providing guaranteed lossless conversion to `Self` from `T`.
>> +///
>> +/// The standard library's `From` implementations do not cover conversions that are not portable or
>> +/// future-proof. For instance, even though it is safe today, `From<usize>` is not implemented for
>> +/// [`u64`] because of the possibility to support larger-than-64bit architectures in the future.
>> +///
>> +/// The workaround is to either deal with the error handling of [`TryFrom`] for an operation that
>> +/// technically cannot fail, or to use the `as` keyword, which can silently strip data if the
>> +/// destination type is smaller than the source.
>> +///
>> +/// Both options are hardly acceptable for the kernel. It is also a much more architecture
>> +/// dependent environment, supporting only 32 and 64 bit architectures, with some modules
>> +/// explicitly depending on a specific bus width that could greatly benefit from infallible
>> +/// conversion operations.
>> +///
>> +/// Thus this extension trait that provides, for the architecture the kernel is built for, safe
>> +/// conversion between types for which such conversion is lossless.
>> +///
>> +/// In other words, this trait is implemented if, for the current build target and with `t: T`, the
>> +/// `t as Self` operation is completely lossless.
>> +///
>> +/// Prefer this over the `as` keyword to ensure no lossy conversions are performed.
>> +///
>> +/// If you need to perform a conversion in `const` context, use [`u64_as_usize`],
>> +/// [`u32_as_usize`], [`usize_as_u64`], etc.
>> +///
>> +/// # Examples
>> +///
>> +/// ```
>> +/// use crate::num::FromAs;
>> +///
>> +/// assert_eq!(usize::from_as(0xf00u32), 0xf00u32 as usize);
> 
> This `from_as()` syntax will be very confusing for users IMO, honestly we should
> just keep it as `from()`, otherwise there is confusion and ambiguity around
> whether someone should use `::from()` or `::from_as()`. Please let us just keep
> all infallible conversions to use `from()`/`into()` and all infallible ones to

s/infallible ones/fallible ones/.

Sorry, thanks,

 - Joel