linux-kernel - Re: [PATCH 2/3] selftests/nolibc: don't pass CC to toplevel Makefile

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Message-ID: <20250817093905.GA14213@1wt.eu>
Date: Sun, 17 Aug 2025 11:39:05 +0200
From: Willy Tarreau <w@....eu>
To: Thomas Weißschuh <linux@...ssschuh.net>
Cc: Shuah Khan <shuah@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
        Bill Wendling <morbo@...gle.com>,
        Justin Stitt <justinstitt@...gle.com>, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH 2/3] selftests/nolibc: don't pass CC to toplevel Makefile

Hi Thomas!

On Sun, Aug 17, 2025 at 10:30:52AM +0200, Thomas Weißschuh wrote:
> Hi Willy,
> 
> On 2025-07-21 04:56:27+0200, Willy Tarreau wrote:
> > On Sat, Jul 19, 2025 at 05:38:28PM +0200, Thomas Weißschuh wrote:
> > > The toplevel Makefile is capable of calculating CC from CROSS_COMPILE
> > > and/or ARCH.
> > > 
> > > Stop passing the unnecessary variable.
> > (...) 
> > >  # Execute the toplevel kernel Makefile
> > > -KBUILD_MAKE = $(MAKE) -C $(srctree) ARCH=$(ARCH) CC=$(CC) CROSS_COMPILE=$(CROSS_COMPILE)
> > > +KBUILD_MAKE = $(MAKE) -C $(srctree) ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE)
> > 
> > Here the goal was not to help the toplevel Makefile figure CC, but rather
> > to permit the user to override it, and it's also listed in "make help",
> > and even used in cc-option.
> > 
> > I understnad that you're trying to avoid forcing CC to clang when
> > building, but in this case, what will CROSS_COMPILE contain ?  My
> > guess is that you intend to make CROSS_COMPILE point to the gcc-based
> > toolchain, and have CC point to clang for userland only. Is this the
> > case ?
> 
> Correct.
> 
> > I think I'd be fine with this, but then we need to make it
> > explicit in the help message and fix the current one, possibly just
> > with this:
> > 
> > -	@echo "  nolibc-test       build the executable (uses \$$CC and \$$CROSS_COMPILE)"
> > +	@echo "  nolibc-test       build the executable (uses \$$CC)"
> 
> I don't think this is correct. $CC itself depends on $CROSS_COMPILE
> through tools/scripts/Makefile.include.

I don't understand what you mean by "depends on" here. CC defaults
to ${CROSS_COMPILE}gcc and may override it if set. So if one sets
CC, CROSS_COMPILE will not be used for it. Or maybe we could change
it to this to indicate a precedence if that's the idea you want to
convey ?

 -	@echo "  nolibc-test       build the executable (uses \$$CC and \$$CROSS_COMPILE)"
 +	@echo "  nolibc-test       build the executable (uses \$$CC or \$$CROSS_COMPILE)"

Willy