[RFC PATCH V2 1/9] timekeeping: Expose the conversion information of monotonic raw

[kan . liang]

From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>

The conversion information of monotonic raw is not affected by NTP/PTP
correction. The perf tool can utilize the information to correctly
calculate the monotonic raw via a TSC in each PEBS record in the
post-processing stage.

The current conversion information is hidden in the internal
struct tk_read_base. Add a new external struct ktime_conv to store and
share the conversion information with other subsystems.

Add a new interface ktime_get_fast_mono_raw_conv() to expose the
conversion information of monotonic raw.  The function probably be
invoked in a NMI. Use NMI safe tk_fast_raw to retrieve the conversion
information.

Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
---
 include/linux/timekeeping.h | 18 ++++++++++++++++++
 kernel/time/timekeeping.c   | 24 ++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h
index fe1e467ba046..94ba02e7eb13 100644
--- a/include/linux/timekeeping.h
+++ b/include/linux/timekeeping.h
@@ -253,6 +253,21 @@ struct system_time_snapshot {
 	u8			cs_was_changed_seq;
 };
 
+/**
+ * struct ktime_conv - Timestamp conversion information
+ * @mult:	Multiplier for scaled math conversion
+ * @shift:	Shift value for scaled math conversion
+ * @xtime_nsec: Shifted (fractional) nano seconds offset for readout
+ * @base:	(nanoseconds) base time for readout
+ */
+struct ktime_conv {
+	u64			cycle_last;
+	u32			mult;
+	u32			shift;
+	u64			xtime_nsec;
+	u64			base;
+};
+
 /**
  * struct system_device_crosststamp - system/device cross-timestamp
  *				      (synchronized capture)
@@ -297,6 +312,9 @@ extern void ktime_get_snapshot(struct system_time_snapshot *systime_snapshot);
 /* NMI safe mono/boot/realtime timestamps */
 extern void ktime_get_fast_timestamps(struct ktime_timestamps *snap);
 
+/* NMI safe mono raw conv information */
+extern void ktime_get_fast_mono_raw_conv(struct ktime_conv *conv);
+
 /*
  * Persistent clock related interfaces
  */
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 5579ead449f2..a202b7a0a249 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -505,6 +505,30 @@ u64 notrace ktime_get_raw_fast_ns(void)
 }
 EXPORT_SYMBOL_GPL(ktime_get_raw_fast_ns);
 
+/**
+ * ktime_get_fast_mono_raw_conv - NMI safe access to get the conversion
+ *				  information of clock monotonic raw
+ *
+ * The conversion information is not affected by NTP/PTP correction.
+ */
+void ktime_get_fast_mono_raw_conv(struct ktime_conv *conv)
+{
+	struct tk_fast *tkf = &tk_fast_raw;
+	struct tk_read_base *tkr;
+	unsigned int seq;
+
+	do {
+		seq = raw_read_seqcount_latch(&tkf->seq);
+		tkr = tkf->base + (seq & 0x01);
+		conv->cycle_last = tkr->cycle_last;
+		conv->mult = tkr->mult;
+		conv->shift = tkr->shift;
+		conv->xtime_nsec = tkr->xtime_nsec;
+		conv->base = tkr->base;
+	} while (read_seqcount_latch_retry(&tkf->seq, seq));
+}
+EXPORT_SYMBOL_GPL(ktime_get_fast_mono_raw_conv);
+
 /**
  * ktime_get_boot_fast_ns - NMI safe and fast access to boot clock.
  *
-- 
2.35.1