Re: [PATCH] rtc: Don't lose a minute-resolution alarm that rounds down into the past

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

On 10/11/2026 07:13:47+0000, Arun Iyengar via B4 Relay wrote:
> From: Arun Iyengar <aiyengar1990@gmail.com>
> 
> For an RTC with RTC_FEATURE_ALARM_RES_MINUTE, rtc_set_alarm() rounds the
> alarm down to the start of its minute, so that it never fires late. When
> the alarm is less than a minute away and in the same minute as the
> current time, the rounded time is already in the past.
> rtc_timer_enqueue() then gets -ETIME from __rtc_set_alarm() and expires
> the timer from the irq work right away, and the hardware alarm is never
> armed.

This is the expected behaviour, userspace has to handle the error
properly: the alarm can't be set, set it for a later time or stop there
and don't suspend.

> 
> That loses every short wake alarm. With a HYM8563, which has minute
> resolution, "rtcwake -m freeze -s 15" sets an alarm that expires in
> software before the system suspends, so nothing wakes it: with the
> alarm set at 17:55:00 for 17:55:17, /proc/driver/rtc reads back
> "alrm_time: 17:55:00, alarm_IRQ: no", and the system stays in
> suspend-to-idle until something else wakes it.
> 
> Round the alarm up to the next minute when rounding down would put an
> alarm that is still in the future into the past. It then fires at most
> 59 seconds late instead of never. Alarms that are already in the past,
> or that round down to a time still in the future, are handled as before.

Never round up, you are going to miss deadlines.


-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com