OPAL Real Time Clock (RTC) APIs


Read the Real Time Clock.


uint32_t* year_month_day

the year, month and day formatted as follows:

  • bits 0-15 is bcd formatted year (0100-9999)
  • bits 16-23 is bcd formatted month (01-12)
  • bits 24-31 is bcd formatted day (01-31)
uint64_t* hour_minute_second_millisecond

the hour, minute, second and millisecond formatted as follows:

  • bits 0-16 is reserved
  • bits 17-24 is bcd formatted hour (00-23)
  • bits 25-31 is bcd formatted minute (00-59)
  • bits 32-39 is bcd formatted second (00-60)
  • bits 40-63 is bcd formatted milliseconds (000000-999999)


Since RTC calls can be pretty slow, OPAL_RTC_READ is likely to first return OPAL_BUSY_EVENT, requiring the caller to wait until the OPAL_EVENT_RTC = 0x4 event has been signaled. Once the event has been signaled, a subsequent OPAL_RTC_READ call will retrieve the time. Since the OPAL_EVENT_RTC = 0x4 event is used for both reading and writing the RTC, callers must be able to handle the event being signaled for a concurrent in flight OPAL_RTC_WRITE rather than this read request.

The following code is one way to correctly issue and then wait for a response:

while (rc == OPAL_BUSY_EVENT) {
      rc = opal_rtc_read(&y_m_d, &h_m_s_ms);
      if (rc == OPAL_BUSY_EVENT)

Although as of writing all OPAL_RTC_READ backends are asynchronous, there is no requirement for them to be - it is valid for OPAL_RTC_READ to immediately return the retreived value rather than OPAL_BUSY_EVENT.

TODO: describe/document format of arguments.

Return codes

parameters now contain the current time, or one read from cache.
error in retrieving the time. May be transient error, may be permanent.
year_month_day or hour_minute_second_millisecond parameters are NULL
something went wrong, Possibly reported in error log. This can be a transient error
request is in flight
request may be in flight


OPAL_RTC_WRITE is much like OPAL_RTC_READ in that it can be asynchronous.

If multiple WRITES are issued before the first one completes, subsequent writes are ignored. There can only be one write in flight at any one time.

Format of the time is the same as for OPAL_RTC_READ.