linux-kernel - Re: Document sysfs interface to RTC system wakeup

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Date:	Fri, 28 Nov 2008 16:49:16 -0500
From:	"Michael Kerrisk" <mtk.manpages@...glemail.com>
To:	"Pavel Machek" <pavel@...e.cz>
Cc:	"kernel list" <linux-kernel@...r.kernel.org>, dl9pf@....de,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	"Andrew Morton" <akpm@...l.org>, "Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: Document sysfs interface to RTC system wakeup

Pavel,

Some comments and suggestions below.

On Wed, Nov 26, 2008 at 11:15 AM, Pavel Machek <pavel@...e.cz> wrote:
> diff --git a/Documentation/rtc.txt b/Documentation/rtc.txt
> index 8deffcd..ac843ab 100644
> --- a/Documentation/rtc.txt
> +++ b/Documentation/rtc.txt
> @@ -187,6 +187,88 @@ driver returns ENOIOCTLCMD.  Some common
>
>  If all else fails, check out the rtc-test.c driver!
>
> +           Even newer /sys interface
> +           ~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +tino.keitel@....de
> +
> +How to use /sys/class/rtc/rtcX/wakealarm
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +This file takes the seconds since epoch to enable a wake event at the
> +specified time.

Better:

This file can be used to view / set the time of a wakeup event.  The
contents of the file are time measured in seconds since the Epoch
(00:00:00h, 1 Jan 1970, UTC).

> +
> +If a '0' is written, the alarm is disabled.

Now -- you started off talking about a "wake event" and now you
switched to talking about an "alarm" -- which is it?  Be consistent.

> +
> +If the alarm was already enabled, a new alarm can only be set after the
> +old alarm is disabled.
> +
> +
> +Migration from /proc/acpi/alarm
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +Users of /proc/acpi/alarm have to change their code to supply the
> +seconds since epoch instead of a date string.

As Jonathan said -- if I'm using  /proc/acpi/alarm, why do I care?
What advantages does the new interface provide for me that would
motivate me to change?

> +For shell scripts, this can be done using the date command, e.g. like
> +this:
> +
> +date -d tomorrow "+%s"
> +
> +This returns the seconds since epoch of the current time on the

s/epoch/the Epoch/

> +following day.
> +
> +Please note that you have to disable the old alarm first, if you want
> +to set a new alarm. Otherwise, you get an error. Example:
> +
> +echo 12345 > /sys/class/rtc/rtc0/wakealarm
> +echo 0 > /sys/class/rtc/rtc0/wakealarm
> +echo 23456 > /sys/class/rtc/rtc0/wakealarm

Here, it might be useful to show an example of an error.  E.g., if one
does these commands instead, show the error from the second command:

echo 12345 > /sys/class/rtc/rtc0/wakealarm
echo 23456 > /sys/class/rtc/rtc0/wakealarm
echo 0 > /sys/class/rtc/rtc0/wakealarm
echo 34567 > /sys/class/rtc/rtc0/wakealarm

> +cd /proc/acpi
> +echo EXP0 > wakeup
> +echo PCI1 > wakeup
> +echo USB0 > wakeup
> +echo USB1 > wakeup
> +echo USB2 > wakeup
> +echo USB7 > wakeup
> +echo HDEF > wakeup
> +cd /sys/class/rtc/rtc0
> +echo $(( $(cat since_epoch) + 20 )) > wakealarm
> +
> +    [PATCH] RTC gets sysfs wakealarm attribute
> +
> +    This adds a new "wakealarm" sysfs attribute to RTC class devices which support
> +    alarm operations and are wakeup-capable:
> +
> +     - It reads as either empty, or the scheduled alarm time as seconds
> +       since the POSIX epoch.  (That time may already have passed, since

s/epoch/Epoch/
(and other instances below)

> +       nothing currently enforces one-shot alarm semantics.)
> +
> +     - It can be written with an alarm time in the future, again seconds
> +       since the POSIX epoch, which enables the alarm.
> +
> +     - It can be written with an alarm time not in the future (such as 0,
> +       the start of the POSIX epoch) to disable the alarm.
> +
> +    Usage examples (some need GNU date) after "cd /sys/class/rtc/rtcN":
> +
> +        alarm after 10 minutes:
> +        # echo $(( $(cat since_epoch) + 10 * 60 )) > wakealarm
> +        alarm tuesday evening 10pm:
> +        # date -d '10pm tuesday' "+%s" > wakealarm
> +        disable alarm:
> +                # echo 0 > wakealarm
> +
> +    This resembles the /proc/acpi/alarm file in that nothing happens when the
> +    alarm triggers ...  except possibly waking the system from sleep.  It's also
> +    like that in a nasty way: not much can be done to prevent one task from
> +    clobbering another task's alarm settings.
> +
> +    It differs from that file in that there's no in-kernel date parser.
> +
> +    Note that a few RTCs ignore rtc_wkalrm.enabled when setting alarms, or aren't
> +    set up correctly, so they won't yet behave with this attribute.

The last sentence is garbled -- bad grammar/missing words -- somewhere
near "or aren't".

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git
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