Message-ID: <CAJ-ks9nF5+m+_bn0Pzi9yU0pw0TyN7Fs4x--mQ4ygyHz4A6hzg@mail.gmail.com>
Date: Thu, 3 Jul 2025 14:55:30 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Benno Lossin <lossin@...nel.org>
Cc: Michal Rostecki <vadorovsky@...tonmail.com>, Miguel Ojeda <ojeda@...nel.org>, 
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Subject: Re: [PATCH v13 2/5] rust: support formatting of foreign types

On Thu, Jul 3, 2025 at 11:08 AM Benno Lossin <lossin@...nel.org> wrote:
>
> On Thu Jul 3, 2025 at 3:55 PM CEST, Tamir Duberstein wrote:
> > On Thu, Jul 3, 2025 at 5:32 AM Benno Lossin <lossin@...nel.org> wrote:
> >> On Tue Jul 1, 2025 at 6:49 PM CEST, Tamir Duberstein wrote:
> >> > Introduce a `fmt!` macro which wraps all arguments in
> >> > `kernel::fmt::Adapter` and a `kernel::fmt::Display` trait. This enables
> >> > formatting of foreign types (like `core::ffi::CStr`) that do not
> >> > implement `core::fmt::Display` due to concerns around lossy conversions which
> >> > do not apply in the kernel.
> >> >
> >> > Replace all direct calls to `format_args!` with `fmt!`.
> >> >
> >> > Replace all implementations of `core::fmt::Display` with implementations
> >> > of `kernel::fmt::Display`.
> >> >
> >> > Suggested-by: Alice Ryhl <aliceryhl@...gle.com>
> >> > Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/Custom.20formatting/with/516476467
> >> > Acked-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> >> > Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> >> > Signed-off-by: Tamir Duberstein <tamird@...il.com>
> >> > ---
> >> >  drivers/block/rnull.rs       |  2 +-
> >> >  drivers/gpu/nova-core/gpu.rs |  4 +-
> >> >  rust/kernel/block/mq.rs      |  2 +-
> >> >  rust/kernel/device.rs        |  2 +-
> >> >  rust/kernel/fmt.rs           | 89 +++++++++++++++++++++++++++++++++++++++
> >> >  rust/kernel/kunit.rs         |  6 +--
> >> >  rust/kernel/lib.rs           |  1 +
> >> >  rust/kernel/prelude.rs       |  3 +-
> >> >  rust/kernel/print.rs         |  4 +-
> >> >  rust/kernel/seq_file.rs      |  2 +-
> >> >  rust/kernel/str.rs           | 22 ++++------
> >> >  rust/macros/fmt.rs           | 99 ++++++++++++++++++++++++++++++++++++++++++++
> >> >  rust/macros/lib.rs           | 19 +++++++++
> >> >  rust/macros/quote.rs         |  7 ++++
> >> >  scripts/rustdoc_test_gen.rs  |  2 +-
> >> >  15 files changed, 236 insertions(+), 28 deletions(-)
> >>
> >> This would be a lot easier to review if he proc-macro and the call
> >> replacement were different patches.
> >>
> >> Also the `kernel/fmt.rs` file should be a different commit.
> >
> > Can you help me understand why? The changes you ask to be separated
> > would all be in different files, so why would separate commits make it
> > easier to review?
>
> It takes less time to go through the entire patch and give a RB. I can
> take smaller time chunks and don't have to get back into the entire
> context of the patch when I don't have 30-60min available.

Ah, I see what you mean. Yeah, the requirement to RB the entire patch
does mean there's a benefit to smaller patches.

> In this patch the biggest problem is the rename & addition of new
> things, maybe just adding 200 lines in those files could be okay to go
> together, see below for more.

After implementing your suggestion of re-exporting things from
`kernel::fmt` the diffstat is

26 files changed, 253 insertions(+), 51 deletions(-)

so I guess I could do all the additions in one patch, but then
*everything* else has to go in a single patch together because the
formatting macros either want core::fmt::Display or
kernel::fmt::Display; they can't work in a halfway state.

>
> > I prefer to keep things in one commit because the changes are highly
> > interdependent. The proc macro doesn't make sense without
> > kernel/fmt.rs and kernel/fmt.rs is useless without the proc macro.
>
> I think that `Adapter`, the custom `Display` and their impl blocks
> don't need to be in the same commit as the proc-macro. They are related,
> but maybe someone is not well-versed in proc-macros and thus doesn't
> want to review that part.

Sure, I guess I will split them. But as noted above: changing the
formatting macros and all the types' trait implementations has to be a
"flag day" change.

>
> >> > diff --git a/rust/kernel/fmt.rs b/rust/kernel/fmt.rs
> >> > new file mode 100644
> >> > index 000000000000..348d16987de6
> >> > --- /dev/null
> >> > +++ b/rust/kernel/fmt.rs
> >> > @@ -0,0 +1,89 @@
> >> > +// SPDX-License-Identifier: GPL-2.0
> >> > +
> >> > +//! Formatting utilities.
> >> > +
> >> > +use core::fmt;
> >>
> >> I think we should pub export all types that we are still using from
> >> `core::fmt`. For example `Result`, `Formatter`, `Debug` etc.
> >>
> >> That way I can still use the same pattern of importing `fmt` and then
> >> writing
> >>
> >>     impl fmt::Display for MyType {
> >>         fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {}
> >>     }
> >
> > Great idea, done for the next spin. It would be nice to be able to
> > lint against references to `core::fmt` outside of kernel/fmt.rs.
>
> I think there was something in clippy that can do that globally and we
> could allow that in this file?

I didn't find anything suitable. Do you have one in mind?

> >> > +
> >> > +/// Internal adapter used to route allow implementations of formatting traits for foreign types.
> >> > +///
> >> > +/// It is inserted automatically by the [`fmt!`] macro and is not meant to be used directly.
> >> > +///
> >> > +/// [`fmt!`]: crate::prelude::fmt!
> >> > +#[doc(hidden)]
> >> > +pub struct Adapter<T>(pub T);
> >> > +
> >> > +macro_rules! impl_fmt_adapter_forward {
> >> > +    ($($trait:ident),* $(,)?) => {
> >> > +        $(
> >> > +            impl<T: fmt::$trait> fmt::$trait for Adapter<T> {
> >> > +                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
> >> > +                    let Self(t) = self;
> >> > +                    fmt::$trait::fmt(t, f)
> >> > +                }
> >> > +            }
> >> > +        )*
> >> > +    };
> >> > +}
> >> > +
> >> > +impl_fmt_adapter_forward!(Debug, LowerHex, UpperHex, Octal, Binary, Pointer, LowerExp, UpperExp);
> >> > +
> >> > +/// A copy of [`fmt::Display`] that allows us to implement it for foreign types.
> >> > +///
> >> > +/// Types should implement this trait rather than [`fmt::Display`]. Together with the [`Adapter`]
> >> > +/// type and [`fmt!`] macro, it allows for formatting foreign types (e.g. types from core) which do
> >> > +/// not implement [`fmt::Display`] directly.
> >> > +///
> >> > +/// [`fmt!`]: crate::prelude::fmt!
> >> > +pub trait Display {
> >> > +    /// Same as [`fmt::Display::fmt`].
> >> > +    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result;
> >> > +}
> >> > +
> >> > +impl<T: ?Sized + Display> Display for &T {
> >> > +    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
> >> > +        Display::fmt(*self, f)
> >> > +    }
> >> > +}
> >> > +
> >> > +impl<T: ?Sized + Display> fmt::Display for Adapter<&T> {
> >> > +    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
> >> > +        let Self(t) = self;
> >> > +        Display::fmt(t, f)
> >>
> >> Why not `Display::fmt(&self.0, f)`?
> >
> > I like destructuring because it shows me that there's only one field.
> > With `self.0` I don't see that.
>
> And what is the benefit here?

In general the benefit is that the method does not ignore some portion
of `Self`. A method that uses `self.0` would not provoke a compiler
error in case another field is added, while this form would.

>
> >> > +
> >> > +    let mut args = TokenStream::from_iter(first_opt);
> >> > +    {
> >> > +        let mut flush = |args: &mut TokenStream, current: &mut TokenStream| {
> >>
> >> You don't need to pass `args` as a closure argument, since you always
> >> call it with `&mut args`.
> >
> > This doesn't work because of the borrow checker. If I wrote what you
> > suggest, then `args` is mutably borrowed by the closure, which
> > prohibits the mutable borrow needed for the .extend() call here:
>
> Ahh right... Well then it's fine.
>
> ---
> Cheers,
> Benno

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