Re: [Buildroot] [RFC PATCH 4/5] linux: split HOSTCC from flags

[Thomas Petazzoni via buildroot <buildroot@buildroot.org>]

Hello Mehdi,

Thanks for your patch!

On Fri, Aug 08, 2025 at 03:31:11PM +0200, El Mehdi YOUNES via buildroot wrote:

> Since Linux v4.19, Kbuild provides explicit variables for host flags, so splitting
> compiler vs flags is supported upstream (see:
>   https://github.com/torvalds/linux/commit/96f14fe738b69dd97a33efe3cc8ab330af5fd1f1
>   https://github.com/torvalds/linux/commit/b90a368000abe5e015e0b045ca2ff7c2173b94c0
>   https://github.com/torvalds/linux/commit/f92d19e0ef9bbbb2984845682e740934ad45473b).
> 
> But with this change we can have problems with older kernel versions
> (< 4.19), as the Buildroot commit below warned when avoiding HOSTCFLAGS
> to not override internal kernel flags:
>   https://gitlab.com/buildroot.org/buildroot/-/commit/dde090c299b0357fdb1a4ec44ad8d332ac57f65e

Since your change as-is would break Linux < 4.19, here is the
suggestion that we have after discussing at the Buildroot Developers
Meeting: make things condition based on
BR2_LINUX_KERNEL_RUST_SUPPORT. Something along the lines of:

ifeq ($(BR2_LINUX_KERNEL_RUST_SUPPORT),y)
LINUX_MAKE_FLAGS += \
	HOSTCC="$(HOSTCC)" \
	HOSTCFLAGS="$(subst -I/,-isystem /,$(subst -I /,-isystem /,$(HOST_CFLAGS)))" \
	HOSTLDFLAGS="$(HOST_LDFLAGS)"
else
LINUX_MAKE_FLAGS += \
	HOSTCC="$(HOSTCC) $(subst -I/,-isystem /,$(subst -I /,-isystem /,$(HOST_CFLAGS))) $(HOST_LDFLAGS)"
endif

Of course, if someone enables BR2_LINUX_KERNEL_RUST_SUPPORT with a
kernel < 4.19 it would fail to build, but that's acceptable as it's
stupid to do this since Linux 4.19 did not have any Rust support.

Thanks!

Thomas
-- 
Thomas Petazzoni, co-owner and CEO, Bootlin
Embedded Linux and Kernel engineering and training
https://bootlin.com
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