Re: [Buildroot] Buildroot does not use ccache for compiling Linux kernel?

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

Hello Andreas,

On Sat, 14 Sep 2024 20:19:37 +0200
Andreas Düring via buildroot <buildroot@buildroot.org> wrote:

> During this process, I realized that ccache, although enabled, is not
> used for compiling the Linux kernel. I think this is because of the
> recipe in linux/linux.mk:
> 
> LINUX_MAKE_FLAGS = \
>         HOSTCC="$(HOSTCC) $(subst -I/,-isystem /,$(subst -I /,-isystem /,$(HOST_CFLAGS))) $(HOST_LDFLAGS)" \
>         ARCH=$(KERNEL_ARCH) \
>         INSTALL_MOD_PATH=$(TARGET_DIR) \
>         CROSS_COMPILE="$(TARGET_CROSS)" \
>         WERROR=0 \
>         REGENERATE_PARSERS=1 \
>         DEPMOD=$(HOST_DIR)/sbin/depmod
> 
> The linux kernel seems to assemble the compiler using TARGET_CROSS,
> e.g.
> CC = $(TARGET_CROSS)gcc
> 
> As a very crude solution, I changed the corresponding line to
>         CROSS_COMPILE="ccache $(TARGET_CROSS)"
> but this is probably not the optimal solution. (I didn't carefully
> check where else CROSS_COMPILE is used).
> 
> If this is a known limitation? Should this be added to the
> documentation?

The thing you're missing is that for target builds, calling ccache is
hidden inside our toolchain wrapper.

When we call $(HOST_DIR)/bin/arm-linux-gcc, it doesn't call the
compiler. It calls our toolchain wrapper
(toolchain/toolchain-wrapper.c), which itself calls the compiler. And
as part of that, it does invoke ccache.

So we are using ccache for all target code, as it's directly done by
our compiler wrapper.

Best regards,

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