Documentation: rtc: move iotcl interface documentation to ABI

What:		/dev/rtcX

Date:		April 2005

KernelVersion:	2.6.12

Contact:	linux-rtc@vger.kernel.org

Description:

		The ioctl interface to drivers for real-time clocks (RTCs).

		Following actions are supported:

		* RTC_RD_TIME, RTC_SET_TIME: Read or set the RTC time. Time

		  format is a Gregorian calendar date and 24 hour wall clock

		  time.

		* RTC_AIE_ON, RTC_AIE_OFF: Enable or disable the alarm interrupt

		  for RTCs that support alarms

		* RTC_ALM_READ, RTC_ALM_SET: Read or set the alarm time for

		  RTCs that support alarms. Can be set upto 24 hours in the

		  future. Requires a separate RTC_AIE_ON call to enable the

		  alarm interrupt. (Prefer to use RTC_WKALM_*)

		* RTC_WKALM_RD, RTC_WKALM_SET: For RTCs that support a more

		  powerful interface, which can issue alarms beyond 24 hours and

		  enable IRQs in the same request.

		* RTC_PIE_ON, RTC_PIE_OFF: Enable or disable the periodic

		  interrupt for RTCs that support periodic interrupts.

		* RTC_UIE_ON, RTC_UIE_OFF: Enable or disable the update

		  interrupt for RTCs that support it.

		* RTC_IRQP_READ, RTC_IRQP_SET: Read or set the frequency for

		  periodic interrupts for RTCs that support periodic interrupts.

		  Requires a separate RTC_PIE_ON call to enable the periodic

		  interrupts.

		The ioctl() calls supported by the older /dev/rtc interface are

		also supported by the newer RTC class framework. However,

		because the chips and systems are not standardized, some PC/AT

		functionality might not be provided. And in the same way, some

		newer features -- including those enabled by ACPI -- are exposed

		by the RTC class framework, but can't be supported by the older

		driver.

the system clock from the discrete RTC, but use the integrated one for all

other tasks, because of its greater functionality.

IOCTL interface

---------------

The ioctl() calls supported by /dev/rtc are also supported by the RTC class

framework.  However, because the chips and systems are not standardized,

some PC/AT functionality might not be provided.  And in the same way, some

newer features -- including those enabled by ACPI -- are exposed by the

RTC class framework, but can't be supported by the older driver.

    *	RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading

	time, returning the result as a Gregorian calendar date and 24 hour

	wall clock time.  To be most useful, this time may also be updated.

    *	RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC

	is connected to an IRQ line, it can often issue an alarm IRQ up to

	24 hours in the future.  (Use RTC_WKALM_* by preference.)

    *	RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond

	the next 24 hours use a slightly more powerful API, which supports

	setting the longer alarm time and enabling its IRQ using a single

	request (using the same model as EFI firmware).

    *	RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, the RTC framework

	will emulate this mechanism.

    *	RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... these icotls

	are emulated via a kernel hrtimer.

In many cases, the RTC alarm can be a system wake event, used to force

Linux out of a low power sleep state (or hibernation) back to a fully

operational state.  For example, a system could enter a deep power saving

state until it's time to execute some scheduled tasks.

Note that many of these ioctls are handled by the common rtc-dev interface.

Some common examples:

    *	RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be

	called with appropriate values.

    *	RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: gets or sets

	the alarm rtc_timer. May call the set_alarm driver function.

    *	RTC_IRQP_SET, RTC_IRQP_READ: These are emulated by the generic code.

    *	RTC_PIE_ON, RTC_PIE_OFF: These are also emulated by the generic code.

If all else fails, check out the tools/testing/selftests/timers/rtctest.c test!

Check out tools/testing/selftests/timers/rtctest.c for an example usage of the

ioctl interface.