Re: [dpdk-dev] [RFC PATCH v2] build: kni cross-compilation support

[Bruce Richardson <bruce.richardson@intel.com>]

On Thu, Feb 04, 2021 at 10:51:41AM +0100, Juraj Linkeš wrote:
> The kni linux module is using a custom target for building, which
> doesn't take into account any cross compilation arguments. The arguments
> in question are ARCH, CROSS_COMPILE (for gcc, clang) and CC, LD (for
> clang). Get those from the cross file and pass them to the custom
> target.
> 
> The user supplied path may not contain the 'build' directory, such as
> when using cross-compiled headers, so only append that in the default
> case (when no path is supplied in native builds) and use the unmodified
> path from the user otherwise. Also modify the install path accordingly.
> 
> Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>

Some comments inline below.

/Bruce

> ---
>  kernel/linux/kni/meson.build |  8 +++---
>  kernel/linux/meson.build     | 56 ++++++++++++++++++++++++++++++++----
>  2 files changed, 55 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/linux/kni/meson.build b/kernel/linux/kni/meson.build
> index 07e0c9dae7..bb7123354f 100644
> --- a/kernel/linux/kni/meson.build
> +++ b/kernel/linux/kni/meson.build
> @@ -13,7 +13,7 @@ kni_sources = files(
>  custom_target('rte_kni',
>  	input: kni_sources,
>  	output: 'rte_kni.ko',
> -	command: ['make', '-j4', '-C', kernel_dir + '/build',
> +	command: ['make', '-j4', '-C', kernel_dir,
>  		'M=' + meson.current_build_dir(),
>  		'src=' + meson.current_source_dir(),
>  		'MODULE_CFLAGS=-include ' + meson.source_root() + '/config/rte_config.h' +
> @@ -21,8 +21,8 @@ custom_target('rte_kni',
>  		' -I' + meson.source_root() + '/lib/librte_kni' +
>  		' -I' + meson.build_root() +
>  		' -I' + meson.current_source_dir(),
> -		'modules'],
> +		'modules'] + cross_args,
>  	depends: kni_mkfile,
> -	install: true,
> -	install_dir: kernel_dir + '/extra/dpdk',
> +	install: install,
> +	install_dir: install_dir,
>  	build_by_default: get_option('enable_kmods'))

rather than "kernel_dir" and "install_dir" can we rename these to
"kernel_build_dir" and "kernel_install_dir" for clarity.

> diff --git a/kernel/linux/meson.build b/kernel/linux/meson.build
> index 5c864a4653..74097299bb 100644
> --- a/kernel/linux/meson.build
> +++ b/kernel/linux/meson.build
> @@ -3,20 +3,66 @@
>  
>  subdirs = ['kni']
>  
> +kernel_version = run_command('uname', '-r').stdout().strip()

Rename to "host_kernel_version" and probably should be only queried in the
native build case.

> +cross_args = []
>  # if we are cross-compiling we need kernel_dir specified
> -if get_option('kernel_dir') == '' and meson.is_cross_build()
> -	error('Need "kernel_dir" option for kmod compilation when cross-compiling')
> +if meson.is_cross_build()
> +	if get_option('kernel_dir') == ''
> +		error('Need "kernel_dir" option for kmod compilation when cross-compiling')
> +	else
> +		install_dir = ''
> +		install = false

I think these should be defined and initialized further up the file,
outside the conditional block.

> +		cross_compiler = find_program('c').path()
> +		if cross_compiler.endswith('gcc')
> +			cross_prefix = run_command([py3, '-c', 'print("' + cross_compiler + '"[:-3])']).stdout().strip()
> +		elif cross_compiler.endswith('clang')
> +			cross_prefix = ''
> +			found_target = false
> +			# search for '-target' and use the arg that follows
> +			# (i.e. the value of '-target') as cross_prefix
> +			foreach cross_c_arg : meson.get_cross_property('c_args')
> +				if found_target and cross_prefix == ''
> +					cross_prefix = cross_c_arg
> +				endif
> +				if cross_c_arg == '-target'
> +					found_target = true
> +				endif
> +			endforeach
> +			if cross_prefix == ''
> +				error('Didn\'t find -target and its value in' +
> +				      ' c_args in input cross-file.')
> +			endif
> +			linker = 'lld'
> +			foreach cross_c_link_arg : meson.get_cross_property('c_link_args')
> +				if cross_c_link_arg.startswith('-fuse-ld')
> +					linker = cross_c_link_arg.split('=')[1]
> +				endif
> +			endforeach
> +			cross_args += ['CC=@0@'.format(cross_compiler), 'LD=ld.@0@'.format(linker)]
> +		else
> +			error('Unsupported cross compiler: @0@'.format(cross_compiler))
> +		endif
> +		if host_machine.cpu_family() == 'aarch64'
> +			cross_arch = 'arm64'
> +		else
> +			cross_arch = build_machine.cpu_family()
> +		endif
> +		cross_args += ['ARCH=@0@'.format(cross_arch),
> +			'CROSS_COMPILE=@0@'.format(cross_prefix)]
> +	endif
> +else
> +	install_dir = '/lib/modules/' + kernel_version + '/extra/dpdk'
> +	install = true
>  endif
>  

The block for cross-compiling is fairly large and complex, so I'm wondering
how we can simplify things a bit. If we had multiple kernel modules I'd
suggest splitting thing up into a native and cross-build subdirectories to
get the build info, but that seems like overkill here. Instead I wonder if
it would work better to handle all the native case initially in a hopefully
simpler "if block" and then do subdir_done(), leaving everything else to be
the cross-compilation recipe and saving at least one level of indentation.

Also, throughout the block, anywhere you have an "error()" call you can use
"endif" instead of "else" and save more indentation space.

>  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
> +	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 + '/build',
> +make_returncode = run_command('make', '-sC', kernel_dir,
>  		'kernelversion').returncode()
>  if make_returncode != 0
>  	error('Cannot compile kernel modules as requested - are kernel headers installed?')
> -- 
> 2.20.1
>