Why this function can't be static in Linux driver - c

Question

I have some assembly codes encapsulated in a static function of my driver code. My codes is like
static int _ARMVAtoPA(void *pvAddr)
{
__asm__ __volatile__(
/* ; INTERRUPTS_OFF" */
" mrs r2, CPSR;\n" /* r2 saves current status */
"CPSID iaf;\n" /* Disable interrupts */
/*In order to handle PAGE OUT scenario, we need do the same operation
twice. In the first time, if PAGE OUT happens for the input address,
translation abort will happen and OS will do PAGE IN operation
Then the second time will succeed.
*/
"mcr p15, 0, r0, c7, c8, 0;\n "
/* ; get VA = <Rn> and run nonsecure translation
; with nonsecure privileged read permission.
; if the selected translation table has privileged
; read permission, the PA is loaded in the PA
; Register, otherwise abort information is loaded
; in the PA Register.
*/
/* read in <Rd> the PA value */
"mrc p15, 0, r1, c7, c4, 0;\n"
/* get VA = <Rn> and run nonsecure translation */
" mcr p15, 0, r0, c7, c8, 0;\n"
/* ; with nonsecure privileged read permission.
; if the selected translation table has privileged
; read permission, the PA is loaded in the PA
; Register, otherwise abort information is loaded
; in the PA Register.
*/
"mrc p15, 0, r0, c7, c4, 0;\n" /* read in <Rd> the PA value */
/* restore INTERRUPTS_ON/OFF status*/
"msr cpsr, r2;\n" /* re-enable interrupts */
"tst r0, #0x1;\n"
"ldr r2, =0xffffffff;\n"
/* if error happens,return INVALID_PHYSICAL_ADDRESS */
"movne r0, r2;\n"
"biceq r0, r0, #0xff;\n"
"biceq r0, r0, #0xf00;" /* if ok, clear the flag bits */
);
}
static unsigned long CpuUmAddrToCpuPAddr(void *pvCpuUmAddr)
{
int phyAdrs;
int mask = 0xFFF; /* low 12bit */
int offset = (int)pvCpuUmAddr & mask;
int phyAdrsReg = _ARMVAtoPA((void *)pvCpuUmAddr);
if (INVALID_PHYSICAL_ADDRESS != phyAdrsReg)
phyAdrs = (phyAdrsReg & (~mask)) + offset;
else
phyAdrs = INVALID_PHYSICAL_ADDRESS;
return phyAdrs;
}
As you can see, I tried to convert a virtual address which from user space to physical address. I'm porting this codes from another project, except I modify the _ARMVAtoPA function to static function.
When I'm using static int _ARMVAtoPA(void *pvAddr):
this convert function (which with bunch of assembly codes in it) is always return fffffff, error case for sure.
When I'm using int _ARMVAtoPA(void *pvAddr):
this convert function would working fine.
Can anyone explain to me, why results are vary when I use static and non-static function.
Thanks

Answer 1

The ASM code doesn't define which register holds the function argument pvAddr and which register holds the return value. It just assumes the compiler follows mips ABI.
But if the function is inlined (where probably static does), the register allocation may change, so the asm code can be totally wrong.
In order to fix the problem, you should use gcc extension to assign registers for function arguments and return value. And also declare which registers it will use w/o restore, so the compiler can restore registers after the call in case the function is inlined.