Why udelay and ndelay is not accurate in linux kernel?

Question

I make a function like this

trace_printk(\"111111\");
udelay(4000);
trace_printk(\"222222\");

and the log shows it\'s 4.01 ms , it\'OK

Answer 1

You can use ktime_get_ns() to get high precision time since boot. So you can use it not only as high precision delay but also as high precision timer. There is example:

u64 t;
t = ktime_get_ns(); // Get current nanoseconds since boot
for (i = 0; i < 24; i++) // Send 24 1200ns-1300ns pulses via GPIO
{
    gpio_set_value(pin, 1); // Drive GPIO or do something else
    t += 1200; // Now we have absolute time of the next step
    while (ktime_get_ns() < t); // Wait for it
    gpio_set_value(pin, 0); // Do something, again
    t += 1300; // Now we have time of the next step, again
    while (ktime_get_ns() < t);  // Wait for it, again
}