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

[Arnout Vandecappelle via buildroot <buildroot@buildroot.org>]

On 14/09/2024 20:19, Andreas Düring via buildroot wrote:
> Hello,
> 
> I am currently experimenting with building a minimal image for my
> Raspberry Pi.
> 
> During this process, I realized that ccache, although enabled, is not
> used for compiling the Linux kernel.

  That is very weird, because ccache is added by the toolchain wrapper. It can 
only be disabled by setting BR2_USE_CCACHE=0 in the environment (or on the make 
command line).

  How did you determine that it doesn't use ccache?

> 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)" \

  We explicitly don't use ccache here (HOSTCC doesn't use the wrapper, so it 
needs to be added explicitly there). See commit 71a31b23 for an explanation why.

>          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)"

  This is what we had originally, but it was removed in commit 546a69db66ab 
because it is now done by the wrapper.

  Regards,
  Arnout

> 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?
> https://buildroot.org/downloads/manual/manual.html#ccache
> ("This will automatically build ccache and use it for every host and
> target compilation.")
> 
> With best regards,
> Andreas
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