linux/ktime.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 ktime.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/20200320145351.32292-15-vincenzo.frascino@arm.com

include/linux/ktime.h

@@ -253,14 +253,7 @@ static inline __must_check bool ktime_to_timespec64_cond(const ktime_t kt,
 	}
 }
 
-/*
- * The resolution of the clocks. The resolution value is returned in
- * the clock_getres() system call to give application programmers an
- * idea of the (in)accuracy of timers. Timer values are rounded up to
- * this resolution values.
- */
-#define LOW_RES_NSEC		TICK_NSEC
-#define KTIME_LOW_RES		(LOW_RES_NSEC)
+#include <vdso/ktime.h>
 
 static inline ktime_t ns_to_ktime(u64 ns)
 {

include/vdso/ktime.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __VDSO_KTIME_H
+#define __VDSO_KTIME_H
+
+#include <vdso/jiffies.h>
+
+/*
+ * The resolution of the clocks. The resolution value is returned in
+ * the clock_getres() system call to give application programmers an
+ * idea of the (in)accuracy of timers. Timer values are rounded up to
+ * this resolution values.
+ */
+#define LOW_RES_NSEC		TICK_NSEC
+#define KTIME_LOW_RES		(LOW_RES_NSEC)
+
+#endif /* __VDSO_KTIME_H */