calling custom IRQ instructions in C code

[QJTM]

I'm trying to call the instruction "picorv32_timer_insn" in some C code.
Usually, to call assembly instruction I use, for instance, the line asm volatile ("addi x10, x0, 9").
When trying to use the line asm picorv32_timer_insn(x15, x10), and compiling, i get the following error message :

I've included a #incude "custom_ops.S" at the beginning of my file, tried to pass the file custom_ops.S to the assembler as an input file while compiling, but I can't manage to make it work.
Is there a something i'm missing, such as a calling convention ?