[RFC PATCH v4 3/4] virtio_rtc: Add Arm Generic Timer cross-timestamping

[Peter Hilber]

Add PTP_SYS_OFFSET_PRECISE2 support on platforms using the Arm Generic
Timer.

Always report the CP15 virtual counter as the HW counter in use by
arm_arch_timer, since the Linux kernel's usage of the Arm Generic Timer
should always be compatible with this.

Signed-off-by: Peter Hilber <quic_philber at quicinc.com>
---

Notes:
    v4:
    
    - Update names and types to spec v6.
    
    v2:
    
    - Depend on prerequisite patch series "treewide: Use clocksource id for
      get_device_system_crosststamp()".
    
    - Return clocksource id instead of calling dropped arm_arch_timer helpers.
    
    - Always report the CP15 virtual counter to be in use by arm_arch_timer,
      since distinction of Arm physical and virtual counter appears unneeded
      after discussion with Marc Zyngier.

 drivers/virtio/Kconfig          | 13 +++++++++++++
 drivers/virtio/Makefile         |  1 +
 drivers/virtio/virtio_rtc_arm.c | 23 +++++++++++++++++++++++
 3 files changed, 37 insertions(+)
 create mode 100644 drivers/virtio/virtio_rtc_arm.c

diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
index a14a2b77e142..3d8b366c0625 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -221,6 +221,19 @@ config VIRTIO_RTC_PTP
 
 	 If unsure, say Y.
 
+config VIRTIO_RTC_ARM
+	bool "Virtio RTC cross-timestamping using Arm Generic Timer"
+	default y
+	depends on VIRTIO_RTC_PTP && ARM_ARCH_TIMER
+	help
+	 This enables Virtio RTC cross-timestamping using the Arm Generic Timer.
+	 It only has an effect if the Virtio RTC device also supports this. The
+	 cross-timestamp is available through the PTP clock driver precise
+	 cross-timestamp ioctl (PTP_SYS_OFFSET_PRECISE2 aka
+	 PTP_SYS_OFFSET_PRECISE).
+
+	 If unsure, say Y.
+
 endif # VIRTIO_RTC
 
 endif # VIRTIO_MENU
diff --git a/drivers/virtio/Makefile b/drivers/virtio/Makefile
index 88d6fb8d4731..dbd77f124ba9 100644
--- a/drivers/virtio/Makefile
+++ b/drivers/virtio/Makefile
@@ -17,3 +17,4 @@ obj-$(CONFIG_VIRTIO_DEBUG) += virtio_debug.o
 obj-$(CONFIG_VIRTIO_RTC) += virtio_rtc.o
 virtio_rtc-y := virtio_rtc_driver.o
 virtio_rtc-$(CONFIG_VIRTIO_RTC_PTP) += virtio_rtc_ptp.o
+virtio_rtc-$(CONFIG_VIRTIO_RTC_ARM) += virtio_rtc_arm.o
diff --git a/drivers/virtio/virtio_rtc_arm.c b/drivers/virtio/virtio_rtc_arm.c
new file mode 100644
index 000000000000..211299d72870
--- /dev/null
+++ b/drivers/virtio/virtio_rtc_arm.c
@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Provides cross-timestamp params for Arm.
+ *
+ * Copyright (C) 2022-2023 OpenSynergy GmbH
+ * Copyright (c) 2024 Qualcomm Innovation Center, Inc. All rights reserved.
+ */
+
+#include <linux/clocksource_ids.h>
+
+#include <uapi/linux/virtio_rtc.h>
+
+#include "virtio_rtc_internal.h"
+
+/* see header for doc */
+
+int viortc_hw_xtstamp_params(u8 *hw_counter, enum clocksource_ids *cs_id)
+{
+	*hw_counter = VIRTIO_RTC_COUNTER_ARM_VCT;
+	*cs_id = CSID_ARM_ARCH_COUNTER;
+
+	return 0;
+}
-- 
2.43.0