BACKPORT: linux/clocksource.h: Extract common header for vDSO

The vDSO library should only include the necessary headers required for
a userspace library (UAPI and a minimal set of kernel headers). To make
this possible it is necessary to isolate from the kernel headers the
common parts that are strictly necessary to build the library.

Split clocksource.h into linux and common headers to make the latter
suitable for inclusion in the vDSO library.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200320145351.32292-9-vincenzo.frascino@arm.com
(cherry picked from commit 14ee2ac618e49e767c9af4aa007f951eb9e35153)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: I685711da05a1fe1414b93064e9be8b7f9888d8c0

include/linux/clocksource.h

@@ -27,6 +27,8 @@ struct module;
 #include <asm/clocksource.h>
 #endif
 
+#include <vdso/clocksource.h>
+
 /**
  * struct clocksource - hardware abstraction for a free running counter
  *	Provides mostly state-free accessors to the underlying hardware.

include/vdso/clocksource.h

@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __VDSO_CLOCKSOURCE_H
+#define __VDSO_CLOCKSOURCE_H
+
+#include <vdso/limits.h>
+
+#if defined(CONFIG_ARCH_CLOCKSOURCE_DATA) || \
+	defined(CONFIG_GENERIC_GETTIMEOFDAY)
+#include <asm/vdso/clocksource.h>
+#endif /* CONFIG_ARCH_CLOCKSOURCE_DATA || CONFIG_GENERIC_GETTIMEOFDAY */
+
+enum vdso_clock_mode {
+	VDSO_CLOCKMODE_NONE,
+#ifdef CONFIG_GENERIC_GETTIMEOFDAY
+	VDSO_ARCH_CLOCKMODES,
+#endif
+	VDSO_CLOCKMODE_MAX,
+
+	/* Indicator for time namespace VDSO */
+	VDSO_CLOCKMODE_TIMENS = INT_MAX
+};
+
+#endif /* __VDSO_CLOCKSOURCE_H */