How to access BCM i2c registers? i2c speed halves on raspbian buster (Zero W, Pi 3 B, Pi 4 B), fine on raspbian stretch

Question

I have a problem with I2C clock in Raspbian Buster (using either SMbus or i2c-dev). It defaults to 50 kHz on RPi 4B and to 62.5 kHz on RPi 3B and Zero W. However, if I set the speed i2c_arm_baudrate=200000 > /boot/config.txt on RPi 4B, then speed is 100 kHz.

But the strange thing that for the first few seconds it is actually 200 kHz, so I2C slave devices that have some kind of initialization only work if I initialize them again after waiting for a couple of seconds in the program. The clock mismatch happens no matter what i2c devices I use, in fact it happens even if there are no devices on the bus (writing 0 to address 9):

Python, SMbus:

import smbus
from time import sleep
bus = smbus.SMBus(1)

while True:
    try:
        bus.write_byte(9, 0)
        sleep(.1)
    except OSError:
        pass

C, i2c-dev:

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <linux/i2c-dev.h>
#include <sys/ioctl.h>
#define delay(A) usleep(A*1000)

char *i2cbus = "/dev/i2c-1";
int addr = 9;
int data[1] = {0};

void loop(void)
{
        int i2cfile = open(i2cbus, O_RDWR);
        ioctl(i2cfile, I2C_SLAVE, addr);
        write(i2cfile, data, 1);
}

int main()
{
        for(;;) {
                loop();
                delay(100);
        }
}

pictures from o-scope:

So, my question is: how to access BCM i2c speed registers? Datasheet says that the register is at offset 0x7e804000, but when I try inline assembly like so:

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

int main()
{
    uint32_t reg = 0;
    __asm(
        "push {r0}\n\t"
        "ldr r0, =0x7e804000\n\t"
        "ldr %[result], [r0]\n\t"
        "pop {r0}\n\t"
        : [result] "=r" (reg)
    );
    printf("%x\n", reg);
}

I get segmentation fault. (my knowledge in ARM assembly is lacking)

Answer 1

Memory locations are a protected resource. If they weren't any Linux user could read the data of another Linux user.

To access memory outside your address space you have to use the mmap command (man mmap).

Here is some example code. It is mainly to show how to access the GPIO registers but the I2C offsets are shown and the same methods apply.

The highlights of the example are

static volatile uint32_t piPeriphBase = 0x20000000;

#define SYST_BASE  (piPeriphBase + 0x003000)
#define DMA_BASE   (piPeriphBase + 0x007000)
#define CLK_BASE   (piPeriphBase + 0x101000)
#define GPIO_BASE  (piPeriphBase + 0x200000)
#define UART0_BASE (piPeriphBase + 0x201000)
#define PCM_BASE   (piPeriphBase + 0x203000)
#define SPI0_BASE  (piPeriphBase + 0x204000)
#define I2C0_BASE  (piPeriphBase + 0x205000)
#define PWM_BASE   (piPeriphBase + 0x20C000)
#define BSCS_BASE  (piPeriphBase + 0x214000)
#define UART1_BASE (piPeriphBase + 0x215000)
#define I2C1_BASE  (piPeriphBase + 0x804000)
#define I2C2_BASE  (piPeriphBase + 0x805000)
#define DMA15_BASE (piPeriphBase + 0xE05000)

#define DMA_LEN   0x1000 /* allow access to all channels */
#define CLK_LEN   0xA8
#define GPIO_LEN  0xF4
#define SYST_LEN  0x1C
#define PCM_LEN   0x24
#define PWM_LEN   0x28
#define I2C_LEN   0x1C
#define BSCS_LEN  0x40

static volatile uint32_t  *gpioReg = MAP_FAILED;
static volatile uint32_t  *systReg = MAP_FAILED;
static volatile uint32_t  *bscsReg = MAP_FAILED;

/* Map in registers. */

static uint32_t * initMapMem(int fd, uint32_t addr, uint32_t len)
{
    return (uint32_t *) mmap(0, len,
       PROT_READ|PROT_WRITE|PROT_EXEC,
       MAP_SHARED|MAP_LOCKED,
       fd, addr);
}

gpioReg  = initMapMem(fd, GPIO_BASE,  GPIO_LEN);
systReg  = initMapMem(fd, SYST_BASE,  SYST_LEN);
bscsReg  = initMapMem(fd, BSCS_BASE,  BSCS_LEN);

I'm not sure if this will be enough to solve your problem. You probably also need to set a stable core clock frequency.