[PATCH/RFC] arm: arch_timer: Do not set C3STOP in case CPU_IDLE=n

[Magnus Damm <magnus.damm@gmail.com>]

From: Magnus Damm <damm@opensource.se>

Modify the ARM architected timer driver to not set C3STOP
in case CPU_IDLE is disabled. This is a short term fix that
allows use of high resolution timers even though no additional
clock event is registered.

Not-really-Signed-off-by: Magnus Damm <damm@opensource.se>
---

 If someone cares about this case then perhaps it should be
 moved up to the clock event main code. The same issue should
 in theory trigger on all architectures, perhaps x86 people
 hunting for low latency may try to disable CPU_IDLE?

 I propose carrying this patch locally to enable high resolution
 timers until CPU_IDLE and an additional clock event is supported.

 Observed on r8a73a4 and APE6EVM.

 drivers/clocksource/arm_arch_timer.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

--- 0001/drivers/clocksource/arm_arch_timer.c
+++ work/drivers/clocksource/arm_arch_timer.c	2013-06-17 09:03:44.000000000 +0900
@@ -125,7 +125,23 @@ static int arch_timer_set_next_event_phy
 
 static int __cpuinit arch_timer_setup(struct clock_event_device *clk)
 {
-	clk->features = CLOCK_EVT_FEAT_ONESHOT | CLOCK_EVT_FEAT_C3STOP;
+	clk->features = CLOCK_EVT_FEAT_ONESHOT;
+#ifdef CONFIG_CPU_IDLE
+	/* By not setting the C3STOP flag it is possible to let the
+	 * ARM architected timer to be the only clock event installed
+	 * on the system and have working high resolution timers.
+	 *
+	 * If the C3STOP flag is set unconditionally then the kernel
+	 * will always prevent using the high resoultion timer feature
+	 * unless an additional clock event is registered.
+	 *
+	 * In the case where CPU_IDLE is enabled then there is a chance
+	 * that deeper sleep states will be handled by software, but
+	 * if CPU_IDLE is disabled then deep sleep states cannot be
+	 * entered and the feature flagged by C3STOP is not needed.
+	 */
+	clk->features |= CLOCK_EVT_FEAT_C3STOP;
+#endif
 	clk->name = "arch_sys_timer";
 	clk->rating = 450;
 	if (arch_timer_use_virtual) {