ARM: 7451/1: arch timer: implement read_current_timer and get_cycles

This patch implements read_current_timer using the architected timers
when they are selected via CONFIG_ARM_ARCH_TIMER. If they are detected
not to be usable at runtime, we return -ENXIO to the caller.

Furthermore, if read_current_timer is exported then we can implement
get_cycles in terms of it for use as both an entropy source and for
implementing __udelay and friends.
Tested-by: NShinya Kuribayashi <shinya.kuribayashi.px@renesas.com>
Reviewed-by: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/arch_timer.h

 #ifndef __ASMARM_ARCH_TIMER_H
 #define __ASMARM_ARCH_TIMER_H
 
+#include <asm/errno.h>
+
 #ifdef CONFIG_ARM_ARCH_TIMER
+#define ARCH_HAS_READ_CURRENT_TIMER
 int arch_timer_of_register(void);
 int arch_timer_sched_clock_init(void);
 #else

arch/arm/include/asm/timex.h

@@ -12,13 +12,15 @@
 #ifndef _ASMARM_TIMEX_H
 #define _ASMARM_TIMEX_H
 
+#include <asm/arch_timer.h>
 #include <mach/timex.h>
 
 typedef unsigned long cycles_t;
 
-static inline cycles_t get_cycles (void)
-{
-	return 0;
-}
+#ifdef ARCH_HAS_READ_CURRENT_TIMER
+#define get_cycles()	({ cycles_t c; read_current_timer(&c) ? 0 : c; })
+#else
+#define get_cycles()	(0)
+#endif
 
 #endif

arch/arm/kernel/arch_timer.c

@@ -223,6 +223,14 @@ static cycle_t arch_counter_read(struct clocksource *cs)
 	return arch_counter_get_cntpct();
 }
 
+int read_current_timer(unsigned long *timer_val)
+{
+	if (!arch_timer_rate)
+		return -ENXIO;
+	*timer_val = arch_counter_get_cntpct();
+	return 0;
+}
+
 static struct clocksource clocksource_counter = {
 	.name	= "arch_sys_counter",
 	.rating	= 400,