Message-ID: <c94dd17e-0e81-47cc-9482-e8743d3bc68f@nvidia.com>
Date: Mon, 27 Oct 2025 15:09:37 -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

Hello Alex,

On 10/27/2025 8:54 AM, Alexandre Courbot wrote:
> The core 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.
> 
> However, the kernel supports a narrower set of architectures, and in the
> case of Nova we only support 64-bit. This makes it helpful and desirable
> to provide more infallible conversions, lest we need to rely on the `as`
> keyword and carry the risk of silently losing data.
> 
> Thus, introduce a new module `num` that provides safe const functions
> performing more conversions allowed by the build target, as well as
> `FromAs` and `IntoAs` traits that are just extensions of `From` and
> `Into` to conversions that are known to be lossless.

Why not just implement `From` and `Into` for the types missing it then, with
adequate comments about why such conversions are Ok for the kernel, instead of
introducing a new trait? This is exactly what `From`/`Into` is for right?
> Suggested-by: Danilo Krummrich <dakr@...nel.org>
> Link: https://lore.kernel.org/rust-for-linux/DDK4KADWJHMG.1FUPL3SDR26XF@kernel.org/
> Acked-by: Danilo Krummrich <dakr@...nel.org>
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
>  drivers/gpu/nova-core/nova_core.rs |   1 +
>  drivers/gpu/nova-core/num.rs       | 158 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 159 insertions(+)
> 
> diff --git a/drivers/gpu/nova-core/nova_core.rs b/drivers/gpu/nova-core/nova_core.rs
> index e130166c1086..9180ec9c27ef 100644
> --- a/drivers/gpu/nova-core/nova_core.rs
> +++ b/drivers/gpu/nova-core/nova_core.rs
> @@ -13,6 +13,7 @@
>  mod gfw;
>  mod gpu;
>  mod gsp;
> +mod num;
>  mod regs;
>  mod vbios;
>  
> diff --git a/drivers/gpu/nova-core/num.rs b/drivers/gpu/nova-core/num.rs
> new file mode 100644
> index 000000000000..adb5a92f0d51
> --- /dev/null
> +++ b/drivers/gpu/nova-core/num.rs
> @@ -0,0 +1,158 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Numerical helpers functions and traits.
> +//!
> +//! This is essentially a staging module for code to mature until it can be moved to the `kernel`
> +//! crate.
> +
> +use kernel::{build_error, macros::paste};
> +
> +macro_rules! impl_lossless_as {
> +    ($from:ty as { $($into:ty),* }) => {
> +        $(
> +        paste! {
> +            #[doc = ::core::concat!(
> +                "Losslessly converts a [`",
> +                ::core::stringify!($from),
> +                "`] into a [`",
> +                ::core::stringify!($into),
> +                "`].")]
> +            ///
> +            /// This conversion is allowed as it is always lossless. Prefer this over the `as`
> +            /// keyword to ensure no lossy conversions are performed.
> +            ///
> +            /// This is for use from a `const` context. For non `const` use, prefer the [`FromAs`]
> +            /// and [`IntoAs`] traits.
> +            ///
> +            /// # Examples
> +            ///
> +            /// ```
> +            /// use crate::num;
> +            ///
> +            #[doc = ::core::concat!(
> +                "assert_eq!(num::",
> +                ::core::stringify!($from),
> +                "_as_",
> +                ::core::stringify!($into),
> +                "(1",
> +                ::core::stringify!($from),
> +                "), 1",
> +                ::core::stringify!($into),
> +                ");")]
> +            /// ```
> +            #[allow(unused)]
> +            pub(crate) const fn [<$from _as_ $into>](value: $from) -> $into {
> +                kernel::static_assert!(size_of::<$into>() >= size_of::<$from>());
> +
> +                value as $into
> +            }
> +        }
> +        )*
> +    };
> +}
> +
> +impl_lossless_as!(u8 as { u16, u32, u64, usize });
> +impl_lossless_as!(u16 as { u32, u64, usize });
> +impl_lossless_as!(u32 as { u64, usize } );

I prefer consistency with the notation in num.rs in the rust standard library:
impl_from!(u16 => usize)
But I am Ok with your notation as well, which is concise.

> +// `u64` and `usize` have the same size on 64-bit platforms.
> +#[cfg(CONFIG_64BIT)]
> +impl_lossless_as!(u64 as { usize } );
> +
> +// A `usize` fits into a `u64` on 32 and 64-bit platforms.
> +#[cfg(any(CONFIG_32BIT, CONFIG_64BIT))]
> +impl_lossless_as!(usize as { u64 });
> +
> +// A `usize` fits into a `u32` on 32-bit platforms.
> +#[cfg(CONFIG_32BIT)]
> +impl_lossless_as!(usize as { u32 });
> +
> +/// 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
use `try_from()`/`try_into()`. No need to additional `_as()` prefix, as it
creates confusion. In the end of the day, the fact the conversion is lossless is
not relevant, all conversions that don't use the `as` keyword should be expected
to be lossless right?

thanks,

 - Joel

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