On Wed, Feb 19, 2025 at 08:32:55PM +0100, Peter Hilber wrote:
> This series implements a driver for a virtio-rtc device conforming to spec
> proposal v7 [1]. It includes a PTP clock driver and an RTC class driver
> with alarm.
>
> v5 updates
> ==========
>
> Important changes compared to the previous driver series [3] are:
>
> - Update to spec proposal v7 [1].
>
> - Fix multiple initialization related bugs.
>
> - Drop the RFC tag, since no major outstanding issues are apparent.
>
> Overview
> ========
>
> This patch series adds the virtio_rtc module, and related bugfixes. The
> virtio_rtc module implements a driver compatible with the proposed Virtio
> RTC device specification [1]. The Virtio RTC (Real Time Clock) device
> provides information about current time. The device can provide different
> clocks, e.g. for the UTC or TAI time standards, or for physical time
> elapsed since some past epoch. The driver can read the clocks with simple
> or more accurate methods. Optionally, the driver can set an alarm.
>
> For the Virtio RTC device, there is currently a proprietary implementation,
> which has been used for testing.
>
> PTP clock interface
> ===================
>
> virtio_rtc exposes clocks as PTP clocks to userspace, similar to ptp_kvm.
> If both the Virtio RTC device and this driver have special support for the
> current clocksource, time synchronization programs can use
> cross-timestamping using ioctl PTP_SYS_OFFSET_PRECISE2 aka
> PTP_SYS_OFFSET_PRECISE. Similar to ptp_kvm, system time synchronization
> with single-digit ns precision is possible with a quiescent reference clock
> (from the Virtio RTC device). This works even when the Virtio device
> response is slow compared to ptp_kvm hypercalls.
>
> The following illustrates a test using PTP_SYS_OFFSET_PRECISE, with
> interspersed strace log and chrony [2] refclocks log, on arm64. In the
> example, chrony tracks a virtio_rtc PTP clock ("PHCV", /dev/ptp0). The raw
> offset between the virtio_rtc clock and CLOCK_REALTIME is 0 to 1 ns. At the
> device side, the Virtio RTC device artificially delays both the clock read
> request, and the response, by 50 ms. Cross-timestamp interpolation still
> works with this delay. chrony also monitors a ptp_kvm clock ("PHCK",
> /dev/ptp3) for comparison, which yields a similar offset.
>
> ioctl(5</dev/ptp3>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.000329>
> ===============================================================================
> Date (UTC) Time Refid DP L P Raw offset Cooked offset Disp.
> ===============================================================================
> 2023-06-29 18:49:55.595742 PHCK 0 N 0 1.000000e-09 8.717931e-10 5.500e-08
> 2023-06-29 18:49:55.595742 PHCK - N - - 8.717931e-10 5.500e-08
> ioctl(6</dev/ptp0>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.101545>
> 2023-06-29 18:49:56.147766 PHCV 0 N 0 1.000000e-09 8.801870e-10 5.500e-08
> 2023-06-29 18:49:56.147766 PHCV - N - - 8.801870e-10 5.500e-08
> ioctl(5</dev/ptp3>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.000195>
> 2023-06-29 18:49:56.202446 PHCK 0 N 0 1.000000e-09 7.364180e-10 5.500e-08
> 2023-06-29 18:49:56.202446 PHCK - N - - 7.364180e-10 5.500e-08
> ioctl(6</dev/ptp0>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.101484>
> 2023-06-29 18:49:56.754641 PHCV 0 N 0 0.000000e+00 -2.617368e-10 5.500e-08
> 2023-06-29 18:49:56.754641 PHCV - N - - -2.617368e-10 5.500e-08
> ioctl(5</dev/ptp3>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.000270>
> 2023-06-29 18:49:56.809282 PHCK 0 N 0 1.000000e-09 7.779321e-10 5.500e-08
> 2023-06-29 18:49:56.809282 PHCK - N - - 7.779321e-10 5.500e-08
> ioctl(6</dev/ptp0>, PTP_SYS_OFFSET_PRECISE, 0xffffe86691c8) = 0 <0.101510>
> 2023-06-29 18:49:57.361376 PHCV 0 N 0 0.000000e+00 -2.198794e-10 5.500e-08
> 2023-06-29 18:49:57.361376 PHCV - N - - -2.198794e-10 5.500e-08
>
> This patch series only adds special support for the Arm Generic Timer
> clocksource. At the driver side, it should be easy to support more
> clocksources.
>
> Fallback PTP clock interface
> ----------------------------
>
> Without special support for the current clocksource, time synchronization
> programs can still use ioctl PTP_SYS_OFFSET_EXTENDED2 aka
> PTP_SYS_OFFSET_EXTENDED. In this case, precision will generally be worse
> and will depend on the Virtio device response characteristics.
>
> The following illustrates a test using PTP_SYS_OFFSET_EXTENDED, with
> interspersed strace log and chrony refclocks log, on x86-64 (with `ts'
> values omitted):
>
> ioctl(5, PTP_SYS_OFFSET_EXTENDED, {n_samples=10, ts=OMITTED}) = 0
> ===============================================================================
> Date (UTC) Time Refid DP L P Raw offset Cooked offset Disp.
> ===============================================================================
> 2023-06-28 14:11:26.697782 PHCV 0 N 0 3.318200e-05 3.450891e-05 4.611e-06
> 2023-06-28 14:11:26.697782 PHCV - N - - 3.450891e-05 4.611e-06
> ioctl(5, PTP_SYS_OFFSET_EXTENDED, {n_samples=10, ts=OMITTED}) = 0
> 2023-06-28 14:11:27.208763 PHCV 0 N 0 -3.792800e-05 -4.023965e-05 4.611e-06
> 2023-06-28 14:11:27.208763 PHCV - N - - -4.023965e-05 4.611e-06
> ioctl(5, PTP_SYS_OFFSET_EXTENDED, {n_samples=10, ts=OMITTED}) = 0
> 2023-06-28 14:11:27.722818 PHCV 0 N 0 -3.328600e-05 -3.134404e-05 4.611e-06
> 2023-06-28 14:11:27.722818 PHCV - N - - -3.134404e-05 4.611e-06
> ioctl(5, PTP_SYS_OFFSET_EXTENDED, {n_samples=10, ts=OMITTED}) = 0
> 2023-06-28 14:11:28.233572 PHCV 0 N 0 -4.966900e-05 -4.584331e-05 4.611e-06
> 2023-06-28 14:11:28.233572 PHCV - N - - -4.584331e-05 4.611e-06
> ioctl(5, PTP_SYS_OFFSET_EXTENDED, {n_samples=10, ts=OMITTED}) = 0
> 2023-06-28 14:11:28.742737 PHCV 0 N 0 4.902700e-05 5.361388e-05 4.611e-06
> 2023-06-28 14:11:28.742737 PHCV - N - - 5.361388e-05 4.611e-06
>
> PTP clock setup
> ---------------
>
> The following udev rule can be used to get a symlink /dev/ptp_virtio to the
> UTC clock:
>
> SUBSYSTEM=="ptp", ATTR{clock_name}=="Virtio PTP type 0, variant 0", SYMLINK += "ptp_virtio"
>
> The following chrony configuration directive can then be added in
> /etc/chrony/chrony.conf to synchronize to the Virtio UTC clock:
>
> refclock PHC /dev/ptp_virtio refid PHCV poll -1 dpoll -1
>
> RTC interface
> =============
>
> This patch series adds virtio_rtc as a generic Virtio driver, including
> both a PTP clock driver and an RTC class driver.
>
> Feedback is greatly appreciated.
>
> [1] https://lore.kernel.org/virtio-comment/20250123101616.664-1-quic_philber@xxxxxxxxxxx/
> [2] https://chrony.tuxfamily.org/
> [3] https://lore.kernel.org/lkml/20241219201118.2233-1-quic_philber@xxxxxxxxxxx/
>
> Changelog
> =========
>
> v5:
>
> - Update to virtio-rtc spec v7, essentially removing definitions.
>
> - Fix multiple bugs after readying device during probe and restore.
>
> - Actually initialize Virtio clock id for RTC class device.
>
> - Add freeze/restore ops already in first patch.
>
> - Minor changes:
>
> - Use new APIs devm_device_init_wakeup(), secs_to_jiffies().
>
> - Fix style issues.
>
> - Improve logging types and clarity.
>
> - Drop unnecessary memory barrier pair.
>
> - Return error status from device, whenever available.
>
> v4:
>
> - Update Virtio interface to spec v6.
>
> - Distinguish UTC-like clocks by handling of leap seconds (spec v6).
>
> - Do not create RTC class device for clocks which may step on leap seconds
> (Alexandre Belloni).
>
> - Clear RTC class feature bit instead of defining reduced ops
> (Alexandre Belloni).
>
> - For PTP clock name, always use numeric clock type, and numeric variant.
>
> - Use macros for 64-bit divisions.
>
> - Remove unnecessary memory barriers.
>
> - Cosmetic improvements.
>
> - Drop upstreamed timekeeping bugfixes from series.
>
> v3:
>
> - Update to conform to virtio spec RFC v3 (no significant behavioral
> changes).
>
> - Add RTC class driver with alarm according to virtio spec RFC v3.
>
> - For cross-timestamp corner case fix, switch back to v1 style closed
> interval test (Thomas Gleixner).
>
> v2:
>
> - Depend on patch series "treewide: Use clocksource id for
> get_device_system_crosststamp()" to avoid requiring a clocksource pointer
> with get_device_system_crosststamp().
>
> - Assume Arm Generic Timer will use CP15 virtual counter. Drop
> arm_arch_timer helper functions (Marc Zyngier).
>
> - Improve cross-timestamp fixes problem description and implementation
> (John Stultz).
>
>
> Peter Hilber (4):
> virtio_rtc: Add module and driver core
> virtio_rtc: Add PTP clocks
> virtio_rtc: Add Arm Generic Timer cross-timestamping
> virtio_rtc: Add RTC class driver
>
> MAINTAINERS | 7 +
> drivers/virtio/Kconfig | 64 ++
> drivers/virtio/Makefile | 5 +
> drivers/virtio/virtio_rtc_arm.c | 23 +
> drivers/virtio/virtio_rtc_class.c | 262 +++++
> drivers/virtio/virtio_rtc_driver.c | 1404 ++++++++++++++++++++++++++
> drivers/virtio/virtio_rtc_internal.h | 122 +++
> drivers/virtio/virtio_rtc_ptp.c | 347 +++++++
> include/uapi/linux/virtio_rtc.h | 237 +++++
Also, should this driver live under
./drivers/rtc/virtio/
?
> 9 files changed, 2471 insertions(+)
> create mode 100644 drivers/virtio/virtio_rtc_arm.c
> create mode 100644 drivers/virtio/virtio_rtc_class.c
> create mode 100644 drivers/virtio/virtio_rtc_driver.c
> create mode 100644 drivers/virtio/virtio_rtc_internal.h
> create mode 100644 drivers/virtio/virtio_rtc_ptp.c
> create mode 100644 include/uapi/linux/virtio_rtc.h
>
>
> base-commit: 8936cec5cb6e27649b86fabf383d7ce4113bba49
> --
> 2.43.0
