Re: [linux-pm] [PATCH 3/6] [-mm]: ACPI: duplicate ACPI sleep "alarm" attribute in sysfs

[David Brownell <david-b@pacbell.net>]

On Sunday 07 January 2007 3:19 am, Pavel Machek wrote:
> 
> > Create /sys/power/alarm.
> > The way it works is exactly the same as /proc/acpi/alarm.
> > I.e. "#echo yyyy-mm-dd hh-mm-ss >/sys/power/alarm" supports existing absolute time.
> > And "#echo +yyyy-mm-dd hh-mm-ss >/sys/power/alarm" supports a duration.
> 
> NAK. /proc/acpi/alarm is a mess, and this just moves it to /sysfs.
> 'One value per file', please.

Sort of like the appended patch, instead ... which doesn't need to know a
thing about ACPI.  This is what I suggested in response to an earlier patch
from Paul Sokolovsky.

- Dave


================	CUT HERE
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
   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, after "cd /sys/class/rtc/rtcN":

    alarm after 45 minutes:
	# echo $(( $(cat since_epoch) + 45 * 60 )) > wakealarm
    alarm next tuesday evening (using GNU date):
	# 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.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

Index: g26/drivers/rtc/rtc-sysfs.c
===================================================================
--- g26.orig/drivers/rtc/rtc-sysfs.c	2007-01-07 19:34:50.000000000 -0800
+++ g26/drivers/rtc/rtc-sysfs.c	2007-01-07 19:34:52.000000000 -0800
@@ -78,6 +78,92 @@ static struct attribute_group rtc_attr_g
 	.attrs = rtc_attrs,
 };
 
+
+static ssize_t
+rtc_sysfs_show_wakealarm(struct class_device *dev, char *buf)
+{
+	ssize_t retval;
+	unsigned long alarm;
+	struct rtc_wkalrm alm;
+
+	/* Don't show disabled alarms; but the RTC could leave the
+	 * alarm enabled after it's already triggered.  Alarms are
+	 * conceptually one-shot, even though some common hardware
+	 * (PCs) doesn't actually work that way.
+	 *
+	 * REVISIT maybe we should require RTC implementations to
+	 * disable the RTC alarm after it triggers, for uniformity.
+	 */
+	retval = rtc_read_alarm(dev, &alm);
+	if (retval == 0 && alm.enabled) {
+		rtc_tm_to_time(&alm.time, &alarm);
+		retval = sprintf(buf, "%lu\n", alarm);
+	}
+
+	return retval;
+}
+
+static ssize_t
+rtc_sysfs_set_wakealarm(struct class_device *dev, const char *buf, size_t n)
+{
+	ssize_t retval;
+	unsigned long now, alarm;
+	struct rtc_wkalrm alm;
+
+	/* Only request alarms that trigger in the future.  Disable them
+	 * by writing another time, e.g. 0 meaning Jan 1 1970 UTC.
+	 */
+	retval = rtc_read_time(dev, &alm.time);
+	if (retval < 0)
+		return retval;
+	rtc_tm_to_time(&alm.time, &now);
+
+	alarm = simple_strtoul(buf, NULL, 0);
+	if (alarm > now) {
+		/* Avoid accidentally clobbering active alarms; we can't
+		 * entirely prevent that here, without even the minimal
+		 * locking from the /dev/rtcN api.
+		 */
+		retval = rtc_read_alarm(dev, &alm);
+		if (retval < 0)
+			return retval;
+		if (alm.enabled)
+			return -EBUSY;
+
+		alm.enabled = 1;
+	} else {
+		alm.enabled = 0;
+
+		/* Provide a valid future alarm time.  Linux isn't EFI,
+		 * this time won't be ignored when disabling the alarm.
+		 */
+		alarm = now + 300;
+	}
+	rtc_time_to_tm(alarm, &alm.time);
+
+	retval = rtc_set_alarm(dev, &alm);
+	return (retval < 0) ? retval : n;
+}
+static const CLASS_DEVICE_ATTR(wakealarm, S_IRUGO | S_IWUSR,
+		rtc_sysfs_show_wakealarm, rtc_sysfs_set_wakealarm);
+
+
+/* The reason to trigger an alarm with no process watching it (via sysfs)
+ * is its side effect:  waking from a system state like suspend-to-RAM or
+ * suspend-to-disk.  So: no attribute unless that side effect is possible.
+ * (Userspace may disable that mechanism later.)
+ */
+static inline int rtc_does_wakealarm(struct class_device *class_dev)
+{
+	struct rtc_device *rtc;
+
+	if (!device_can_wakeup(class_dev->dev))
+		return 0;
+	rtc = to_rtc_device(class_dev);
+	return rtc->ops->set_alarm != NULL;
+}
+
+
 static int __devinit rtc_sysfs_add_device(struct class_device *class_dev,
 					struct class_interface *class_intf)
 {
@@ -87,8 +173,18 @@ static int __devinit rtc_sysfs_add_devic
 
 	err = sysfs_create_group(&class_dev->kobj, &rtc_attr_group);
 	if (err)
-		dev_err(class_dev->dev,
-			"failed to create sysfs attributes\n");
+		dev_err(class_dev->dev, "failed to create %s\n",
+				"sysfs attributes");
+	else if (rtc_does_wakealarm(class_dev)) {
+		/* not all RTCs support both alarms and wakeup */
+		err = class_device_create_file(class_dev,
+					&class_device_attr_wakealarm);
+		if (err) {
+			dev_err(class_dev->dev, "failed to create %s\n",
+					"alarm attribute");
+			sysfs_remove_group(&class_dev->kobj, &rtc_attr_group);
+		}
+	}
 
 	return err;
 }
@@ -96,6 +192,9 @@ static int __devinit rtc_sysfs_add_devic
 static void rtc_sysfs_remove_device(struct class_device *class_dev,
 				struct class_interface *class_intf)
 {
+	if (rtc_does_wakealarm(class_dev))
+		class_device_remove_file(class_dev,
+				&class_device_attr_wakealarm);
 	sysfs_remove_group(&class_dev->kobj, &rtc_attr_group);
 }