Reading from register of Allwinner H3 ARM processor

Question

#include <stdint.h>
#include <stddef.h>
#include <stdio.h>


static void foo(void){
    volatile  uint32_t *temp_addr;
    temp_addr = (uint32_t*)(0x01C20C00);
    *temp_addr =0;
}
int main(){
    tinit();

};

It compiles but returns Segmentation fault message as a result. I just want to reset all bits in register 0x01c200c00 .

Answer 1

Your program does not work because 0x01C20C00 is a physical address, but your program uses virtual addresses. For experimentation, you can access GPIO, timers, or other peripherals without writing a kernel driver. For this, you need to create a memory mapping, like this:

#define ALLWINNER_TIMER_BASE 0x01C20C00

struct allwinner_timer
{
  volatile uint32_t IRQ_EN_REG;
  // add other registers from the datasheet, or find a kernel driver source with these definitions
};

int fd = open("/dev/mem", O_RDWR);
struct allwinner_timer *t = (struct allwinner_timer *)mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, fd, ALLWINNER_TIMER_BASE);
// TODO: check for errors
t->IRQ_EN_REG = 0;

Note that:

• It may not work if kernel restricts access to /dev/mem . This can be found out by looking at kernel options, eg CONFIG_STRICT_DEVMEM and with dmesg ;
• It has to run under superuser, obviously;
• Your code may interfere with other parts of the system, eg kernel drivers accessing the same peripheral, leading to unpredictable results.