kernel: fix cross-build of Linux modules with meson

When cross-compiling, if no kernel_dir was specified, then the kernel
modules were still being compiled for the build machine. Fix this by
only building modules on cross-compile when we have a kernel_dir value
set. Print out a message indicating why we are skipping kernel
compilation, and in case that the headers for kernel compile are not
found, print a warning instead of erroring out.

Fixes: a52f4574f798 ("igb_uio: build with meson")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Luca Boccassi <bluca@debian.org>

kernel/linux/igb_uio/meson.build

@@ -1,12 +1,6 @@
 # SPDX-License-Identifier: BSD-3-Clause
 # Copyright(c) 2017 Intel Corporation
 
-kernel_dir = get_option('kernel_dir')
-if kernel_dir == ''
-	kernel_version = run_command('uname', '-r').stdout().strip()
-	kernel_dir = '/lib/modules/' + kernel_version + '/build'
-endif
-
 mkfile = custom_target('igb_uio_makefile',
 	output: 'Makefile',
 	command: ['touch', '@OUTPUT@'])

kernel/linux/meson.build

@@ -3,6 +3,40 @@
 
 subdirs = ['igb_uio']
 
-foreach d:subdirs
-	subdir(d)
-endforeach
+WARN_CROSS_COMPILE='Need "kernel_dir" option for kmod compilation when cross-compiling'
+WARN_NO_HEADERS='Cannot compile kernel modules as requested - are kernel headers installed?'
+
+# if we are cross-compiling we need kernel_dir specified
+# NOTE: warning() function only available from version 0.44 onwards
+if get_option('kernel_dir') == '' and meson.is_cross_build()
+	if meson.version().version_compare('>=0.44')
+		warning(WARN_CROSS_COMPILE)
+	else
+		message('WARNING: ' + WARN_CROSS_COMPILE)
+	endif
+else
+
+	kernel_dir = get_option('kernel_dir')
+	if kernel_dir == ''
+		# use default path for native builds
+		kernel_version = run_command('uname', '-r').stdout().strip()
+		kernel_dir = '/lib/modules/' + kernel_version + '/build'
+	endif
+
+	# test running make in kernel directory, using "make kernelversion"
+	make_returncode = run_command('make', '-sC', kernel_dir,
+			'kernelversion').returncode()
+	if make_returncode != 0
+		if meson.version().version_compare('>=0.44')
+			warning(WARN_NO_HEADERS)
+		else
+			message('WARNING: ' + WARN_NO_HEADERS)
+		endif
+	else # returncode == 0
+
+# DO ACTUAL MODULE BUILDING
+		foreach d:subdirs
+			subdir(d)
+		endforeach
+	endif
+endif

meson.build

@@ -26,7 +26,6 @@ subdir('config')
 
 # build libs and drivers
 subdir('lib')
-subdir('kernel')
 subdir('buildtools')
 subdir('drivers')
 
@@ -40,6 +39,11 @@ if get_option('examples') != ''
 	subdir('examples')
 endif
 
+# build kernel modules if enabled
+if get_option('enable_kmods')
+	subdir('kernel')
+endif
+
 # write the build config
 build_cfg = 'rte_build_config.h'
 configure_file(output: build_cfg,