[RFC PATCH v7 4/7] time: Add mechanism to recognize clocksource in time_get_snapshot

[Jianyong Wu <jianyong.wu@xxxxxxx>]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
In some scenario like return device time to ptp_kvm guest,
we need identify the current clocksource outside core time code.
A mechanism added to recognize the current clocksource
by export clocksource id in time_get_snapshot.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Jianyong Wu <jianyong.wu@xxxxxxx>
---
 include/linux/clocksource.h     |  6 ++++++
 include/linux/clocksource_ids.h | 13 +++++++++++++
 include/linux/timekeeping.h     | 12 +++++++-----
 kernel/time/clocksource.c       |  3 +++
 kernel/time/timekeeping.c       |  1 +
 5 files changed, 30 insertions(+), 5 deletions(-)
 create mode 100644 include/linux/clocksource_ids.h

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index b21db536fd52..96e85b6f9ca0 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -17,6 +17,7 @@
 #include <linux/timer.h>
 #include <linux/init.h>
 #include <linux/of.h>
+#include <linux/clocksource_ids.h>
 #include <asm/div64.h>
 #include <asm/io.h>
 
@@ -49,6 +50,10 @@ struct module;
  *			400-499: Perfect
  *				The ideal clocksource. A must-use where
  *				available.
+ * @id:			Defaults to CSID_GENERIC. The id value is captured
+ *			in certain snapshot functions to allow callers to
+ *			validate the clocksource from which the snapshot was
+ *			taken.
  * @read:		returns a cycle value, passes clocksource as argument
  * @enable:		optional function to enable the clocksource
  * @disable:		optional function to disable the clocksource
@@ -91,6 +96,7 @@ struct clocksource {
 	const char *name;
 	struct list_head list;
 	int rating;
+	enum clocksource_ids id;
 	int (*enable)(struct clocksource *cs);
 	void (*disable)(struct clocksource *cs);
 	unsigned long flags;
diff --git a/include/linux/clocksource_ids.h b/include/linux/clocksource_ids.h
new file mode 100644
index 000000000000..93bec8426c44
--- /dev/null
+++ b/include/linux/clocksource_ids.h
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_CLOCKSOURCE_IDS_H
+#define _LINUX_CLOCKSOURCE_IDS_H
+
+/* Enum to give clocksources a unique identifier */
+enum clocksource_ids {
+	CSID_GENERIC		= 0,
+	CSID_ARM_ARCH_COUNTER,
+	CSID_MAX,
+};
+
+#endif
+
diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index a8ab0f143ac4..ecce56269a7e 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -2,6 +2,7 @@
 #ifndef _LINUX_TIMEKEEPING_H
 #define _LINUX_TIMEKEEPING_H
 
+#include <linux/clocksource_ids.h>
 #include <linux/errno.h>
 
 /* Included from linux/ktime.h */
@@ -204,11 +205,12 @@ extern void timekeeping_inject_sleeptime64(const struct timespec64 *delta);
  * @cs_was_changed_seq:	The sequence number of clocksource change events
  */
 struct system_time_snapshot {
-	u64		cycles;
-	ktime_t		real;
-	ktime_t		raw;
-	unsigned int	clock_was_set_seq;
-	u8		cs_was_changed_seq;
+	u64			cycles;
+	ktime_t			real;
+	ktime_t			raw;
+	enum clocksource_ids	cs_id;
+	unsigned int		clock_was_set_seq;
+	u8			cs_was_changed_seq;
 };
 
 /*
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 3bcc19ceb073..26a3add61771 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -921,6 +921,9 @@ int __clocksource_register_scale(struct clocksource *cs, u32 scale, u32 freq)
 
 	clocksource_arch_init(cs);
 
+	if (WARN_ON_ONCE((unsigned int)cs->id >= CSID_MAX))
+		cs->id = CSID_GENERIC;
+
 	/* Initialize mult/shift and max_idle_ns */
 	__clocksource_update_freq_scale(cs, scale, freq);
 
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 44b726bab4bd..88078cff7fe2 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -974,6 +974,7 @@ void ktime_get_snapshot(struct system_time_snapshot *systime_snapshot)
 	do {
 		seq = read_seqcount_begin(&tk_core.seq);
 		now = tk_clock_read(&tk->tkr_mono);
+		systime_snapshot->cs_id = tk->tkr_mono.clock->id;
 		systime_snapshot->cs_was_changed_seq = tk->cs_was_changed_seq;
 		systime_snapshot->clock_was_set_seq = tk->clock_was_set_seq;
 		base_real = ktime_add(tk->tkr_mono.base,
-- 
2.17.1