clocksource: arch_timer: Fix code to use physical timers when requested

commit 0b46b8a7 upstream.

This is a bug fix for using physical arch timers when
the arch_timer_use_virtual boolean is false.  It restores the
arch_counter_get_cntpct() function after removal in

0d651e4e "clocksource: arch_timer: use virtual counters"

We need this on certain ARMv7 systems which are architected like this:

* The firmware doesn't know and doesn't care about hypervisor mode and
  we don't want to add the complexity of hypervisor there.

* The firmware isn't involved in SMP bringup or resume.

* The ARCH timer come up with an uninitialized offset between the
  virtual and physical counters.  Each core gets a different random
  offset.

* The device boots in "Secure SVC" mode.

* Nothing has touched the reset value of CNTHCTL.PL1PCEN or
  CNTHCTL.PL1PCTEN (both default to 1 at reset)

One example of such as system is RK3288 where it is much simpler to
use the physical counter since there's nobody managing the offset and
each time a core goes down and comes back up it will get reinitialized
to some other random value.

Fixes: 0d651e4e ("clocksource: arch_timer: use virtual counters")
Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

arch/arm/include/asm/arch_timer.h

@@ -78,6 +78,15 @@ static inline u32 arch_timer_get_cntfrq(void)
 	return val;
 }
 
+static inline u64 arch_counter_get_cntpct(void)
+{
+	u64 cval;
+
+	isb();
+	asm volatile("mrrc p15, 0, %Q0, %R0, c14" : "=r" (cval));
+	return cval;
+}
+
 static inline u64 arch_counter_get_cntvct(void)
 {
 	u64 cval;

arch/arm64/include/asm/arch_timer.h

@@ -135,6 +135,15 @@ static inline void arch_timer_evtstrm_enable(int divider)
 #endif
 }
 
+static inline u64 arch_counter_get_cntpct(void)
+{
+	/*
+	 * AArch64 kernel and user space mandate the use of CNTVCT.
+	 */
+	BUG();
+	return 0;
+}
+
 static inline u64 arch_counter_get_cntvct(void)
 {
 	u64 cval;

drivers/clocksource/arm_arch_timer.c

@@ -430,7 +430,10 @@ static void __init arch_counter_register(unsigned type)
 
 	/* Register the CP15 based counter if we have one */
 	if (type & ARCH_CP15_TIMER) {
-		arch_timer_read_counter = arch_counter_get_cntvct;
+		if (arch_timer_use_virtual)
+			arch_timer_read_counter = arch_counter_get_cntvct;
+		else
+			arch_timer_read_counter = arch_counter_get_cntpct;
 	} else {
 		arch_timer_read_counter = arch_counter_get_cntvct_mem;