From 8a0bdfd7a05f5bb0486fbe7146a2cf775957e95e Mon Sep 17 00:00:00 2001 From: David Brownell Date: Wed, 6 Feb 2008 01:38:45 -0800 Subject: rtc-cmos alarm acts as oneshot Start making the rtc-cmos alarm act more like a oneshot alarm by disabling that alarm after its IRQ fires. (ACPI hooks are also needed.) The Linux RTC framework has previously been a bit vague in this area, but any other behavior is problematic and not very portable. RTCs with full YYYY-MM-DD HH:MM[:SS] alarms won't have a problem here. Only ones with partial match criteria, with the most visible example being the PC RTC, get confused. (Because the criteria will match repeatedly.) Update comments relating to that oneshot behavior and timezone handling. (Timezones are another issue that's mostly visible with rtc-cmos. That's because PCs often dual-boot MS-Windows, which likes its RTC to match local wall-clock time instead of UTC.) Signed-off-by: David Brownell Cc: Alessandro Zummo Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/rtc/rtc-sysfs.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-) (limited to 'drivers/rtc/rtc-sysfs.c') diff --git a/drivers/rtc/rtc-sysfs.c b/drivers/rtc/rtc-sysfs.c index 2ae0e8304d3a..4d27ccc4fc06 100644 --- a/drivers/rtc/rtc-sysfs.c +++ b/drivers/rtc/rtc-sysfs.c @@ -17,6 +17,13 @@ /* device attributes */ +/* + * NOTE: RTC times displayed in sysfs use the RTC's timezone. That's + * ideally UTC. However, PCs that also boot to MS-Windows normally use + * the local time and change to match daylight savings time. That affects + * attributes including date, time, since_epoch, and wakealarm. + */ + static ssize_t rtc_sysfs_show_name(struct device *dev, struct device_attribute *attr, char *buf) @@ -113,13 +120,13 @@ rtc_sysfs_show_wakealarm(struct device *dev, struct device_attribute *attr, 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. + /* Don't show disabled alarms. For uniformity, RTC alarms are + * conceptually one-shot, even though some common RTCs (on PCs) + * don't actually work that way. * - * REVISIT maybe we should require RTC implementations to - * disable the RTC alarm after it triggers, for uniformity. + * NOTE: RTC implementations where the alarm doesn't match an + * exact YYYY-MM-DD HH:MM[:SS] date *must* disable their RTC + * alarms after they trigger, to ensure one-shot semantics. */ retval = rtc_read_alarm(to_rtc_device(dev), &alm); if (retval == 0 && alm.enabled) { -- cgit v1.2.3