lib/vdso: Allow the high resolution parts to be compiled out

If the architecture knows at compile time that there is no VDSO capable
clocksource supported it makes sense to optimize the guts of the high
resolution parts of the VDSO out at build time. Add a helper function to
check whether the VDSO should be high resolution capable and provide a stub
which can be overridden by an architecture.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://lkml.kernel.org/r/20200207124402.530143168@linutronix.de

lib/vdso/gettimeofday.c

@@ -38,6 +38,13 @@ u64 vdso_calc_delta(u64 cycles, u64 last, u64 mask, u32 mult)
 }
 #endif
 
+#ifndef __arch_vdso_hres_capable
+static inline bool __arch_vdso_hres_capable(void)
+{
+	return true;
+}
+#endif
+
 #ifdef CONFIG_TIME_NS
 static int do_hres_timens(const struct vdso_data *vdns, clockid_t clk,
 			  struct __kernel_timespec *ts)
@@ -101,6 +108,10 @@ static __always_inline int do_hres(const struct vdso_data *vd, clockid_t clk,
 	u64 cycles, last, sec, ns;
 	u32 seq;
 
+	/* Allows to compile the high resolution parts out */
+	if (!__arch_vdso_hres_capable())
+		return -1;
+
 	do {
 		/*
 		 * Open coded to handle VCLOCK_TIMENS. Time namespace