How to build an external linux module which uses a custom library - linux

Question

I am trying to compile an external linux driver with the the line below within an existing platform using many dependencies (such as libraries):
obj-m += mydriver.o
KDIR ?= $(OUT_DIR)
default:
$(MAKE) -C $(KDIR) M=$$PWD
clean:
$(MAKE) -C $(KDIR) M=$$PWD clean
modules:
$(MAKE) -C $(KDIR) M=$$PWD modules
I have noticed that this invokes the kernel Makefile with creates object files and does the link in order to prepare a "module".ko loadable with linux. But what if, I have to use a specific library (eg. my_library.a): how can I prevent the Linux makefile to take into account this extra library when linking all the object files
Appendice:
My_library.a is a c++ source code contains functions that access to FPGA registers in order to report some useful data. Then my_driver (since it is a C code source , I had to create an C- interface from my_library.a ) will prepare basic system calls accessible from a user-space application. Bottom of line, my_driver reads from FPGA with 8khz, thanks to my_library.a via a C-interface and make data readable for user-space APP.
Cheers,
sahbi

Answer 1

Most probably, you cannot use an application-level external library libmy.a or my_library.a in any kernel module (or kernel code). Such libraries are generally built above the C standard library (e.g. because they use <stdio.h> thru fprintf or <stdlib.h> thru malloc), which does not exist for kernel code.
Conceptually kernel modules and kernel code is freestanding (i.e. not above the C standard library).
Notice that the ABI & calling conventions might be different in kernel code and in application code. And certainly, some standard functions (like malloc, printf, snprintf ....) might not exist inside the kernel. BTW kernel code is not allowed to use floating point, etc...
Of course, you could link your own kernel library (out of your own kernel object files), but that is often not worthwhile.
(an improbable exception could be some external library which do not use any standard C function and do not compute with floating point, but that does not happen in pracice)
At last, the conventional wisdom is to avoid needing lot of kernel code. So you should consider having some helper user-mode process. See netlink(7).
You probably should redesign your driver to avoid needing any external library. Since you read the FPGA at only 8KHz, that part of your code could sit in user-land above existing system calls.
Notice that kernel code cannot be written in C++