ASM code in KMDF driver

Question

I added asm source to KMDF driver project:

.CODE

read_port PROC port : WORD, result_ptr : PTR BYTE

    mov dx, port
    in al, dx                       ; read port

    mov rbx, result_ptr
    mov BYTE PTR [rbx], al          ; place value at result address

ret

read_port ENDP

END

In Driver.c file I can call read_port like this:

extern void read_port(unsigned short port, unsigned char* result);

//...

unsigned char val;
read_port(0, &val);

However I can't call this function like that from additional source file, for example test.cpp. Complier gives me unresolved external.

test.cpp:

extern void read_port(unsigned short port, unsigned char* result);

//...

unsigned char val;
read_port(0, &val);         // unresolved external

What is the reason?