linux-kernel - Re: [PATCH v3] kbuild: rust: add PROCMACROLDFLAGS

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Message-ID: <CAK7LNAQJUdubjkvV0T0XRU8=336rV88b7+t6BP4xR9v_g38eKQ@mail.gmail.com>
Date: Fri, 6 Dec 2024 10:28:00 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: "Hong, Yifan" <elsk@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Jonathan Corbet <corbet@....net>, Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, 
	linux-kbuild@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	patches@...ts.linux.dev
Subject: Re: [PATCH v3] kbuild: rust: add PROCMACROLDFLAGS

On Fri, Dec 6, 2024 at 8:40 AM Hong, Yifan <elsk@...gle.com> wrote:
>
> On Tue, Nov 12, 2024 at 10:45 AM Miguel Ojeda <ojeda@...nel.org> wrote:
> >
> > From: HONG Yifan <elsk@...gle.com>
> >
> > These are flags to be passed when linking proc macros for the Rust
> > toolchain. If unset, it defaults to $(KBUILD_HOSTLDFLAGS).
> >
> > This is needed because the list of flags to link hostprogs is not
> > necessarily the same as the list of flags used to link libmacros.so.
> > When we build proc macros, we need the latter, not the former (e.g. when
> > using a Rust compiler binary linked to a different C library than host
> > programs).
> >
> > To distinguish between the two, introduce this new variable to stand
> > out from KBUILD_HOSTLDFLAGS used to link other host progs.
> >
> > Signed-off-by: HONG Yifan <elsk@...gle.com>
> > Link: https://lore.kernel.org/r/20241017210430.2401398-2-elsk@google.com
> > [ v3:
> >
> >   - `export`ed the variable. Otherwise it would not be visible in
> >     `rust/Makefile`.
> >
> >   - Removed "additional" from the documentation and commit message,
> >     since this actually replaces the other flags, unlike other cases.
> >
> >   - Added example of use case to documentation and commit message.
> >     Thanks Alice for the details on what Google needs!
> >
> >   - Instead of `HOSTLDFLAGS`, used `KBUILD_HOSTLDFLAGS` as the fallback
> >     to preserve the previous behavior as much as possible, as discussed
> >     with Alice/Yifan. Thus moved the variable down too (currently we
> >     do not modify `KBUILD_HOSTLDFLAGS` elsewhere) and avoided
> >     mentioning `HOSTLDFLAGS` directly in the documentation.
> >
> >   - Fixed documentation header formatting.
> >
> >   - Reworded slightly.
> >
> >          - Miguel ]
> > Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
> > ---
> > Masahiro: if Kbuild wants to pick this up, that is great. Otherwise, I am happy
> > picking this up early next cycle, if you give an `Acked-by` since this is
> > changing the interface for Kbuild users given we are introducing a new
> > environment variable. Thanks!
> >
> > Note that the `or` means if the string is empty, we will use the default rather
> > than nothing. I didn't change that from Yifan's version, but maybe we want to do
> > otherwise. Users can still provide e.g. an empty space to avoid any flag.
>
> I am not sure if I understand the implications here.
> https://www.gnu.org/software/make/manual/html_node/Conditional-Functions.html
> says:
>
> The or function provides a “short-circuiting” OR operation. Each
> argument is expanded, in order. If an argument expands to a non-empty
> string the processing stops and the result of the expansion is that
> string. If, after all arguments are expanded, all of them are false
> (empty), then the result of the expansion is the empty string.
>
> I am assuming that this means:
> - If PROCMACROLDFLAGS is not empty, KBUILD_PROCMACROLDFLAGS evaluates
> to PROCMACROLDFLAGS
> - Otherwise if KBUILD_HOSTLDFLAGS is not empty,
> KBUILD_PROCMACROLDFLAGS evaluates to KBUILD_HOSTLDFLAGS
> - Otherwise KBUILD_PROCMACROLDFLAGS is set to empty.

I think your understanding is correct.

$(or A,B) works like $(if A,A,B)

Commit 5c8166419acf shorten the code.



> What do you mean by "use the default"?


"use the default" means,
"use $(KBUILD_HOSTLDFLAGS)"


-- 
Best Regards
Masahiro Yamada