linux: add BR2_LINUX_KERNEL_NEEDS_HOST_PYTHON3

Kernel commit [1] introduced MSM GPU driver header generation with a
Python script. This commit was first included in v6.10. This driver is
also enabled as a module in the arm64 architecture default
configuration. See [2]. This is a common situation.

This missing dependency is not detected in the Buildroot CI, because
the reference docker image contains a python3 interpreter. See [3].

For information, the Linux latest kernel version was updated in
Buildroot commit 2b6dba00b "linux: bump latest version to 6.10".
See [4].

The issue can be observed by running the following commands on a host
without the python3 interpreter installed:

    cat <<EOF >.config
    BR2_aarch64=y
    BR2_LINUX_KERNEL=y
    BR2_LINUX_KERNEL_USE_ARCH_DEFAULT_CONFIG=y
    BR2_LINUX_KERNEL_NEEDS_HOST_OPENSSL=y
    BR2_TOOLCHAIN_EXTERNAL=y
    EOF
    make olddefconfig
    make linux

The Kernel build fails with output:

      GENHDR  drivers/gpu/drm/msm/generated/a2xx.xml.h
    /bin/sh: 1: python3: not found
    make[7]: *** [drivers/gpu/drm/msm/Makefile:176: drivers/gpu/drm/msm/generated/a2xx.xml.h] Error 127

This commit fixes this issue by introducing a new
BR2_LINUX_KERNEL_NEEDS_HOST_PYTHON3 configuration that will need to be
selected in relevant situations.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0fddd045f88e34d6160785a3a5e506d374566454
[2] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/arch/arm64/configs/defconfig?h=v6.10.8#n868
[3] https://gitlab.com/buildroot.org/buildroot/-/blob/2024.08-rc3/support/docker/Dockerfile?ref_type=tags#L40
[4] https://gitlab.com/buildroot.org/buildroot/-/commit/2b6dba00be27a09bc7e88faf0871f82bf95589cf

Reported-by: Vincent Stehlé <vincent.stehle@arm.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

linux/Config.in

@@ -492,6 +492,17 @@ config BR2_LINUX_KERNEL_NEEDS_HOST_PAHOLE
 	  such as "BTF: .tmp_vmlinux.btf: pahole (pahole) is not
 	  available".
 
+config BR2_LINUX_KERNEL_NEEDS_HOST_PYTHON3
+	bool "Needs host python3"
+	help
+	  Some Linux kernel configuration options (such as the
+	  CONFIG_DRM_MSM in v6.10 and greater) require a host python3
+	  interpreter. Enabling this option will ensure host-python3
+	  gets built before the Linux kernel.
+
+	  Enable this option if you get a Linux kernel build failure
+	  such as "python3: not found".
+
 # Linux extensions
 source "linux/Config.ext.in"
 

linux/linux.mk

@@ -138,6 +138,10 @@ define LINUX_FIXUP_CONFIG_PAHOLE_CHECK
 endef
 endif
 
+ifeq ($(BR2_LINUX_KERNEL_NEEDS_HOST_PYTHON3),y)
+LINUX_DEPENDENCIES += host-python3
+endif
+
 # If host-uboot-tools is selected by the user, assume it is needed to
 # create a custom image
 ifeq ($(BR2_PACKAGE_HOST_UBOOT_TOOLS),y)