virtio_rtc: Add PTP clocks

Expose the virtio_rtc clocks as PTP clocks to userspace, similar to
ptp_kvm. virtio_rtc can expose multiple clocks, e.g. a UTC clock and a
monotonic clock.

Userspace should distinguish different clocks through the name assigned by
the driver. In particular, UTC-like clocks can also be distinguished by if
and how leap seconds are smeared. udev rules such as the following can be
used to get different symlinks for different clock types:

	SUBSYSTEM=="ptp", ATTR{clock_name}=="Virtio PTP type 0/variant 0", SYMLINK += "ptp_virtio"
	SUBSYSTEM=="ptp", ATTR{clock_name}=="Virtio PTP type 1/variant 0", SYMLINK += "ptp_virtio_tai"
	SUBSYSTEM=="ptp", ATTR{clock_name}=="Virtio PTP type 2/variant 0", SYMLINK += "ptp_virtio_monotonic"
	SUBSYSTEM=="ptp", ATTR{clock_name}=="Virtio PTP type 3/variant 0", SYMLINK += "ptp_virtio_smear_unspecified"
	SUBSYSTEM=="ptp", ATTR{clock_name}=="Virtio PTP type 3/variant 1", SYMLINK += "ptp_virtio_smear_noon_linear"
	SUBSYSTEM=="ptp", ATTR{clock_name}=="Virtio PTP type 3/variant 2", SYMLINK += "ptp_virtio_smear_sls"
	SUBSYSTEM=="ptp", ATTR{clock_name}=="Virtio PTP type 4/variant 0", SYMLINK += "ptp_virtio_maybe_smeared"

The preferred PTP clock reading method is ioctl PTP_SYS_OFFSET_PRECISE2,
through the ptp_clock_info.getcrosststamp() op. For now,
PTP_SYS_OFFSET_PRECISE2 will return -EOPNOTSUPP through a weak function.
PTP_SYS_OFFSET_PRECISE2 requires cross-timestamping support for specific
clocksources, which will be added in the following. If the clocksource
specific code is enabled, check that the Virtio RTC device supports the
respective HW counter before obtaining an actual cross-timestamp from the
Virtio device.

The Virtio RTC device response time may be higher than the timekeeper
seqcount increment interval. Therefore, obtain the cross-timestamp before
calling get_device_system_crosststamp().

As a fallback, support the ioctl PTP_SYS_OFFSET_EXTENDED2 for all
platforms.

Assume that concurrency issues during PTP clock removal are avoided by the
posix_clock framework.

Kconfig recursive dependencies prevent virtio_rtc from implicitly enabling
PTP_1588_CLOCK, therefore just warn the user if PTP_1588_CLOCK is not
available. Since virtio_rtc should in the future also expose clocks as RTC
class devices, do not depend VIRTIO_RTC on PTP_1588_CLOCK.

Signed-off-by: Peter Hilber <quic_philber@quicinc.com>
Message-Id: <20250509160734.1772-3-quic_philber@quicinc.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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