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Delay (delay)

Busy-wait delay functions based on the clock source (clocksource), providing nanosecond, microsecond, and millisecond precision delays, suitable for short and precise wait scenarios.

How It Works​

The delay functions obtain the current time via ktime_get(), compute the target timeout, then busy-wait poll until the timeout:

void udelay(uint32_t us)
{
    ktime_t timeout = ktime_add_us(ktime_get(), us);
    while(ktime_before(ktime_get(), timeout));
}

All three delay functions share the same implementation, differing only in time unit:

  • ndelay() uses ktime_add_ns() to add nanoseconds
  • udelay() uses ktime_add_us() to add microseconds
  • mdelay() uses ktime_add_ms() to add milliseconds

API​

FunctionDescription
ndelay(ns)Nanosecond busy-wait delay
udelay(us)Microsecond busy-wait delay
mdelay(ms)Millisecond busy-wait delay

Usage Examples​

#include <kernel/time/delay.h>

/* Wait 10 microseconds after hardware reset */
udelay(10);

/* I2C timing delay */
ndelay(500);

/* Short wait of 5 milliseconds */
mdelay(5);

Notes​

  • Delays are busy-waits; the CPU is not released during the wait, making them unsuitable for long waits
  • Delay precision depends on the clock source device resolution; nanosecond-level timestamps are obtained via ktime_get()
  • For long waits, use the timer system (timer) or thread sleep (xos_thread_msleep)
  • Safe to use in interrupt context since no scheduling is involved