virtio_rtc: Add RTC class driver

Expose the virtio-rtc UTC-like clock as an RTC clock to userspace - if it
is present, and if it does not step on leap seconds. The RTC class enables
the virtio-rtc device to resume the system from sleep states on RTC alarm.

Support RTC alarm if the virtio-rtc alarm feature is present. The
virtio-rtc device signals an alarm by marking an alarmq buffer as used.

Peculiarities
-------------

A virtio-rtc clock is a bit special for an RTC clock in that

- the clock may step (also backwards) autonomously at any time and

- the device, and its notification mechanism, will be reset during boot or
  resume from sleep.

The virtio-rtc device avoids that the driver might miss an alarm. The
device signals an alarm whenever the clock has reached or passed the alarm
time, and also when the device is reset (on boot or resume from sleep), if
the alarm time is in the past.

Open Issue
----------

The CLOCK_BOOTTIME_ALARM will use the RTC clock to wake up from sleep, and
implicitly assumes that no RTC clock steps will occur during sleep. The RTC
class driver does not know whether the current alarm is a real-time alarm
or a boot-time alarm.

Perhaps this might be handled by the driver also setting a virtio-rtc
monotonic alarm (which uses a clock similar to CLOCK_BOOTTIME_ALARM). The
virtio-rtc monotonic alarm would just be used to wake up in case it was a
CLOCK_BOOTTIME_ALARM alarm.

Otherwise, the behavior should not differ from other RTC class drivers.

Signed-off-by: Peter Hilber <quic_philber@quicinc.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Message-Id: <20250509160734.1772-5-quic_philber@quicinc.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

0 files changed, 0 insertions, 0 deletions