How to do module dependency on out-of-tree kernel modules

example.txt

Export your symbols using EXPORT_GPL_SYMBOL or EXPORT_SYMBOL as normal.

Build your provider module as always - take a look at the Module.symvers file as we're going to depend on what it exports.

Your dependent module will now need a Makefile that uses KBUILD_EXTRA_SYMBOLS in order to reference these additional symbols

-----------------8<------------------
obj-m += dependent.o
KDIR := /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

CC := $(CROSS_COMPILE)gcc

all:
	$(MAKE) -C $(KDIR) M=${shell pwd} modules KBUILD_EXTRA_SYMBOLS=`pwd`/dep/Module.symvers
	
clean:
	-$(MAKE) -C $(KDIR) M=${shell pwd} clean || true
	-rm *.o *.ko *.mod.{c,o} modules.order Module.symvers || true

-----------------8<------------------

Copy both the modules into your kernel modules directory /lib/modules/`uname -r`/ and issue a depmod -a

Once that's done, you should be able to use 
 $ modprobe dependent

which will resolve the dependencies and allow you to insert your module.

How will this work with cross compilation when our kernel source is (typically) not the same as that of the development machine so we won't have '/lib/modules/...' directory??

Change the KDIR to point at the correct location?

That part is understandable. What I'd like to know is, what is the best practice here for copying/installing the .ko file as well as the meta files (generated by the build) into the target file-system which is typically a rootfs built using yocto/buildroot etc?

Changing the KDIR will build the module for said kernel but what happens after that? On native development, we typically use the 'modules_install' make target which the top level makefile in the kernel directory understands and copies the modules into /lib/modules/uname-r but how does this work with cross development is what I am looking for

Ah, if you're using Yocto you're better off creating a bblayer and doing the extra work to have it create them as part of your rootfs build IMO.
The above is purely for building out-of-tree. The kind of modules that you can either copy over manually to the boxes or package them in .deb or .rpm file

I'm actually using buildroot and was looking at a quick and dirty fix to accomplish this task. Looks like there is none ;)

BTW between Yocto and buildroot, which one is easier to learn vs which one is more prevalent in the industry?

yocto / bitbake was pretty much the winner of that battle 👯‍♀️