virtio_rtc: Add module and driver core

Add the virtio_rtc module and driver core. The virtio_rtc module implements
a driver compatible with the proposed Virtio RTC device specification.
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.

Implement the core, which interacts with the Virtio RTC device. Apart from
this, the core does not expose functionality outside of the virtio_rtc
module. Follow-up patches will expose PTP clocks and an RTC Class device.

Provide synchronous messaging, which is enough for the expected time
synchronization use cases through PTP clocks (similar to ptp_kvm) or RTC
Class device.

Signed-off-by: Peter Hilber <quic_philber@quicinc.com>
Message-Id: <20250509160734.1772-2-quic_philber@quicinc.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Author: quic-philber

MAINTAINERS

@@ -25827,6 +25827,13 @@ S:	Maintained
 F:	drivers/nvdimm/nd_virtio.c
 F:	drivers/nvdimm/virtio_pmem.c
 
+VIRTIO RTC DRIVER
+M:	Peter Hilber <quic_philber@quicinc.com>
+L:	virtualization@lists.linux.dev
+S:	Maintained
+F:	drivers/virtio/virtio_rtc_*
+F:	include/uapi/linux/virtio_rtc.h
+
 VIRTIO SOUND DRIVER
 M:	Anton Yakovlev <anton.yakovlev@opensynergy.com>
 M:	"Michael S. Tsirkin" <mst@redhat.com>

drivers/virtio/Kconfig

@@ -188,4 +188,17 @@ config VIRTIO_DEBUG
 
 	  If unsure, say N.
 
+config VIRTIO_RTC
+	tristate "Virtio RTC driver"
+	depends on VIRTIO
+	depends on PTP_1588_CLOCK_OPTIONAL
+	help
+	 This driver provides current time from a Virtio RTC device. The driver
+	 provides the time through one or more clocks.
+
+	 To compile this code as a module, choose M here: the module will be
+	 called virtio_rtc.
+
+	 If unsure, say M.
+
 endif # VIRTIO_MENU

drivers/virtio/Makefile

@@ -14,3 +14,5 @@ obj-$(CONFIG_VIRTIO_VDPA) += virtio_vdpa.o
 obj-$(CONFIG_VIRTIO_MEM) += virtio_mem.o
 obj-$(CONFIG_VIRTIO_DMA_SHARED_BUFFER) += virtio_dma_buf.o
 obj-$(CONFIG_VIRTIO_DEBUG) += virtio_debug.o
+obj-$(CONFIG_VIRTIO_RTC) += virtio_rtc.o
+virtio_rtc-y := virtio_rtc_driver.o