mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-12-06 02:56:44 +07:00
4a5fd81507
This patch splits rtc.txt into two separate files, one for the documentation itself, and the other for the rtctest.c file. The rtctest file is moved into the kernel tools/testing/selftests/timers directory. This will make automated testing easier. Note that the only difference in the rtc.txt file is that the location of the rtctest.c file has changed. Signed-off-by: Prarit Bhargava <prarit@redhat.com> Acked-by: Jonathan Corbet <corbet@lwn.net> Acked-by: John Stultz <john.stultz@linaro.org> Cc: corbet@lwn.net Cc: rtc-linux@googlegroups.com Cc: linux-doc@vger.kernel.org Cc: a.zummo@towertech.it Cc: prarit@redhat.com Cc: john.stultz@linaro.org Cc: shuahkh@osg.samsung.com Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
208 lines
9.9 KiB
Plaintext
208 lines
9.9 KiB
Plaintext
|
|
Real Time Clock (RTC) Drivers for Linux
|
|
=======================================
|
|
|
|
When Linux developers talk about a "Real Time Clock", they usually mean
|
|
something that tracks wall clock time and is battery backed so that it
|
|
works even with system power off. Such clocks will normally not track
|
|
the local time zone or daylight savings time -- unless they dual boot
|
|
with MS-Windows -- but will instead be set to Coordinated Universal Time
|
|
(UTC, formerly "Greenwich Mean Time").
|
|
|
|
The newest non-PC hardware tends to just count seconds, like the time(2)
|
|
system call reports, but RTCs also very commonly represent time using
|
|
the Gregorian calendar and 24 hour time, as reported by gmtime(3).
|
|
|
|
Linux has two largely-compatible userspace RTC API families you may
|
|
need to know about:
|
|
|
|
* /dev/rtc ... is the RTC provided by PC compatible systems,
|
|
so it's not very portable to non-x86 systems.
|
|
|
|
* /dev/rtc0, /dev/rtc1 ... are part of a framework that's
|
|
supported by a wide variety of RTC chips on all systems.
|
|
|
|
Programmers need to understand that the PC/AT functionality is not
|
|
always available, and some systems can do much more. That is, the
|
|
RTCs use the same API to make requests in both RTC frameworks (using
|
|
different filenames of course), but the hardware may not offer the
|
|
same functionality. For example, not every RTC is hooked up to an
|
|
IRQ, so they can't all issue alarms; and where standard PC RTCs can
|
|
only issue an alarm up to 24 hours in the future, other hardware may
|
|
be able to schedule one any time in the upcoming century.
|
|
|
|
|
|
Old PC/AT-Compatible driver: /dev/rtc
|
|
--------------------------------------
|
|
|
|
All PCs (even Alpha machines) have a Real Time Clock built into them.
|
|
Usually they are built into the chipset of the computer, but some may
|
|
actually have a Motorola MC146818 (or clone) on the board. This is the
|
|
clock that keeps the date and time while your computer is turned off.
|
|
|
|
ACPI has standardized that MC146818 functionality, and extended it in
|
|
a few ways (enabling longer alarm periods, and wake-from-hibernate).
|
|
That functionality is NOT exposed in the old driver.
|
|
|
|
However it can also be used to generate signals from a slow 2Hz to a
|
|
relatively fast 8192Hz, in increments of powers of two. These signals
|
|
are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is
|
|
for...) It can also function as a 24hr alarm, raising IRQ 8 when the
|
|
alarm goes off. The alarm can also be programmed to only check any
|
|
subset of the three programmable values, meaning that it could be set to
|
|
ring on the 30th second of the 30th minute of every hour, for example.
|
|
The clock can also be set to generate an interrupt upon every clock
|
|
update, thus generating a 1Hz signal.
|
|
|
|
The interrupts are reported via /dev/rtc (major 10, minor 135, read only
|
|
character device) in the form of an unsigned long. The low byte contains
|
|
the type of interrupt (update-done, alarm-rang, or periodic) that was
|
|
raised, and the remaining bytes contain the number of interrupts since
|
|
the last read. Status information is reported through the pseudo-file
|
|
/proc/driver/rtc if the /proc filesystem was enabled. The driver has
|
|
built in locking so that only one process is allowed to have the /dev/rtc
|
|
interface open at a time.
|
|
|
|
A user process can monitor these interrupts by doing a read(2) or a
|
|
select(2) on /dev/rtc -- either will block/stop the user process until
|
|
the next interrupt is received. This is useful for things like
|
|
reasonably high frequency data acquisition where one doesn't want to
|
|
burn up 100% CPU by polling gettimeofday etc. etc.
|
|
|
|
At high frequencies, or under high loads, the user process should check
|
|
the number of interrupts received since the last read to determine if
|
|
there has been any interrupt "pileup" so to speak. Just for reference, a
|
|
typical 486-33 running a tight read loop on /dev/rtc will start to suffer
|
|
occasional interrupt pileup (i.e. > 1 IRQ event since last read) for
|
|
frequencies above 1024Hz. So you really should check the high bytes
|
|
of the value you read, especially at frequencies above that of the
|
|
normal timer interrupt, which is 100Hz.
|
|
|
|
Programming and/or enabling interrupt frequencies greater than 64Hz is
|
|
only allowed by root. This is perhaps a bit conservative, but we don't want
|
|
an evil user generating lots of IRQs on a slow 386sx-16, where it might have
|
|
a negative impact on performance. This 64Hz limit can be changed by writing
|
|
a different value to /proc/sys/dev/rtc/max-user-freq. Note that the
|
|
interrupt handler is only a few lines of code to minimize any possibility
|
|
of this effect.
|
|
|
|
Also, if the kernel time is synchronized with an external source, the
|
|
kernel will write the time back to the CMOS clock every 11 minutes. In
|
|
the process of doing this, the kernel briefly turns off RTC periodic
|
|
interrupts, so be aware of this if you are doing serious work. If you
|
|
don't synchronize the kernel time with an external source (via ntp or
|
|
whatever) then the kernel will keep its hands off the RTC, allowing you
|
|
exclusive access to the device for your applications.
|
|
|
|
The alarm and/or interrupt frequency are programmed into the RTC via
|
|
various ioctl(2) calls as listed in ./include/linux/rtc.h
|
|
Rather than write 50 pages describing the ioctl() and so on, it is
|
|
perhaps more useful to include a small test program that demonstrates
|
|
how to use them, and demonstrates the features of the driver. This is
|
|
probably a lot more useful to people interested in writing applications
|
|
that will be using this driver. See the code at the end of this document.
|
|
|
|
(The original /dev/rtc driver was written by Paul Gortmaker.)
|
|
|
|
|
|
New portable "RTC Class" drivers: /dev/rtcN
|
|
--------------------------------------------
|
|
|
|
Because Linux supports many non-ACPI and non-PC platforms, some of which
|
|
have more than one RTC style clock, it needed a more portable solution
|
|
than expecting a single battery-backed MC146818 clone on every system.
|
|
Accordingly, a new "RTC Class" framework has been defined. It offers
|
|
three different userspace interfaces:
|
|
|
|
* /dev/rtcN ... much the same as the older /dev/rtc interface
|
|
|
|
* /sys/class/rtc/rtcN ... sysfs attributes support readonly
|
|
access to some RTC attributes.
|
|
|
|
* /proc/driver/rtc ... the system clock RTC may expose itself
|
|
using a procfs interface. If there is no RTC for the system clock,
|
|
rtc0 is used by default. More information is (currently) shown
|
|
here than through sysfs.
|
|
|
|
The RTC Class framework supports a wide variety of RTCs, ranging from those
|
|
integrated into embeddable system-on-chip (SOC) processors to discrete chips
|
|
using I2C, SPI, or some other bus to communicate with the host CPU. There's
|
|
even support for PC-style RTCs ... including the features exposed on newer PCs
|
|
through ACPI.
|
|
|
|
The new framework also removes the "one RTC per system" restriction. For
|
|
example, maybe the low-power battery-backed RTC is a discrete I2C chip, but
|
|
a high functionality RTC is integrated into the SOC. That system might read
|
|
the system clock from the discrete RTC, but use the integrated one for all
|
|
other tasks, because of its greater functionality.
|
|
|
|
SYSFS INTERFACE
|
|
---------------
|
|
|
|
The sysfs interface under /sys/class/rtc/rtcN provides access to various
|
|
rtc attributes without requiring the use of ioctls. All dates and times
|
|
are in the RTC's timezone, rather than in system time.
|
|
|
|
date: RTC-provided date
|
|
hctosys: 1 if the RTC provided the system time at boot via the
|
|
CONFIG_RTC_HCTOSYS kernel option, 0 otherwise
|
|
max_user_freq: The maximum interrupt rate an unprivileged user may request
|
|
from this RTC.
|
|
name: The name of the RTC corresponding to this sysfs directory
|
|
since_epoch: The number of seconds since the epoch according to the RTC
|
|
time: RTC-provided time
|
|
wakealarm: The time at which the clock will generate a system wakeup
|
|
event. This is a one shot wakeup event, so must be reset
|
|
after wake if a daily wakeup is required. Format is seconds since
|
|
the epoch by default, or if there's a leading +, seconds in the
|
|
future, or if there is a leading +=, seconds ahead of the current
|
|
alarm.
|
|
|
|
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!
|