s390/tracing: Add s390-tod clock

In order to allow comparing trace timestamps between different
systems or virtual machines on s390, add a s390-tod trace clock.
This clock just uses the returned TOD clock value from stcke
directly.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

Documentation/trace/ftrace.rst

@@ -570,6 +570,10 @@ of ftrace. Here is a list of some of the key files:
 		to correlate events across hypervisor/guest if
 		tb_offset is known.
 
+	s390-tod:
+		This uses the s390 TOD clock value. This clock is usually in
+		sync across virtual machines and STP-enabled machines.
+
 	mono:
 		This uses the fast monotonic clock (CLOCK_MONOTONIC)
 		which is monotonic and is subject to NTP rate adjustments.

arch/s390/include/asm/trace_clock.h

@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_S390_TRACE_CLOCK_H
+#define _ASM_S390_TRACE_CLOCK_H
+
+#include <linux/compiler.h>
+#include <linux/types.h>
+
+u64 notrace trace_clock_s390_tod(void);
+
+#define ARCH_TRACE_CLOCKS \
+	{ trace_clock_s390_tod, "s390-tod", .in_ns = 0 },
+
+#endif /* _ASM_S390_TRACE_CLOCK_H */

arch/s390/kernel/Makefile

@@ -62,6 +62,7 @@ obj-$(CONFIG_KPROBES)		+= mcount.o
 obj-$(CONFIG_RETHOOK)		+= rethook.o
 obj-$(CONFIG_FUNCTION_TRACER)	+= ftrace.o
 obj-$(CONFIG_FUNCTION_TRACER)	+= mcount.o
+obj-$(CONFIG_TRACING)		+= trace_clock.o
 obj-$(CONFIG_CRASH_DUMP)	+= crash_dump.o
 obj-$(CONFIG_KEXEC_CORE)	+= machine_kexec.o relocate_kernel.o
 obj-$(CONFIG_VMCORE_INFO)	+= vmcore_info.o

arch/s390/kernel/trace_clock.c

@@ -0,0 +1,12 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/trace_clock.h>
+#include <linux/timex.h>
+/*
+ * trace_clock_s390_tod(): trace clock based on the s390 TOD clock
+ *
+ * Unlike the other clocks, this is not in nanoseconds.
+ */
+u64 notrace trace_clock_s390_tod(void)
+{
+	return get_tod_clock();
+}