[PATCH 4/4] scripts: allow building USB loader tools for target as well

[Trent Piepho]

On Tue, Sep 14, 2021 at 6:21 AM Ahmad Fatoum <a.fatoum at pengutronix.de> wrote:
>
> We currently build the USB loader tools only for the host (build) system,
> but it can be useful to cross compile them as well for the target.
>
> We already have some target tools, but support for those is easier,
> because they don't link against libraries. We use pkg-config to get
> cc and ld flags, but we always assume that pkg-config is for the host
> system and there is no well-defined way to request pkg-config for the
> target system.
>
> Support this by introducing a new CROSS_PKG_CONFIG. This will be
> consulted only for target tools and default to
> $(CROSS_COMPILE)pkgconfig.
>
> Users can override it as necessary, for example, with Yocto, pkg-config
> will be for the cross environment, so target tools can now be built
> with:

I just added support to Buildroot for building imx-usb-loader from
Barebox, since it's nicer than the standalone version of the program.

Since pkgconfig was only used for host tools, I didn't need to make
both host and target pkgconfig work.  But of course that will no
longer be true after this patch.

There is a problem with only supplying CROSS_PKG_CONFIG.  To get both
host and target pkgconfig to work, I also need to supply the env
variables used by pkgconfig, PKG_CONFIG_SYSROOT and PKG_CONFIG_LIBDIR.

The former makes the paths returned by pkgconfig correct and the
latter controls which set, target or host, of .pc files will be used.

Maybe something like this in the Makefile:

CROSS_PKG_CONFIG ?= $(CROSS_COMPILE)pkg-config
CROSS_PKG_CONFIG_SYSROOT ?= $(PKG_CONFIG_SYSROOT)
CROSS_PKG_CONFIG_LIBDIR ?= $(PKG_CONFIG_LIBDIR)
CROSS_PKG_CONFIG_ENV := \
       PKG_CONFIG_LIBDIR=$(CROSS_PKG_CONFIG_LIBDIR) \
       PKG_CONFIG_SYSROOT=$(CROSS_PKG_CONFIG_SYSROOT)

HOST_LIBUSB_CFLAGS := $(shell $(PKG_CONFIG) --cflags)
CROSS_LIBUSB_CFLAGS := $(shell $(CROSS_PKG_CONFIG_ENV)
$(CROSS_PKG_CONFIG) --cflags)

Then use those everywhere someone wants the libusb cflags.  Repeat for LDFLAGS.

You'll get fewer repeated invocations of pkg-config this way too.