Message-Id: <DB2PIGAQHCJR.3BF8ZHECYH3KB@kernel.org>
Date: Thu, 03 Jul 2025 22:36:26 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Tamir Duberstein" <tamird@...il.com>
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Subject: Re: [PATCH v13 2/5] rust: support formatting of foreign types

On Thu Jul 3, 2025 at 8:55 PM CEST, Tamir Duberstein wrote:
> 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 don't understand, can't you just do:

* add `rust/kernel/fmt.rs`,
* add `rust/macros/fmt.rs`,
* change all occurrences of `core::fmt` to `kernel::fmt` and
  `format_args!` to `fmt!`.

The last one could be split by subsystem, no? Some subsystems might
interact and thus need simultaneous splitting, but there should be some
independent ones.

>> > 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.

See above.

>> >> > +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.

Yeah, but why would that change happen here? And even if it got another
field, why would that invalidate the impl of `fn fmt`?

---
Cheers,
Benno

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