ARM: riscpc: replace gettimeoffset() with clocksource

Replace the old gettimeoffset() interface (which became buggy in
several ways) with a clocksource that atomically reads the count
and status from the timer, and corrects the count as appropriate
ensuring proper resolution of time without time warping backwards.

We keep the original periodic timer non-clock event implementation
to provide the kernel with a regular source of interrupts, which
are required to keep the clocksource properly updated.
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

arch/arm/Kconfig

@@ -528,7 +528,6 @@ config ARCH_RPC
 	select ARCH_ACORN
 	select ARCH_MAY_HAVE_PC_FDC
 	select ARCH_SPARSEMEM_ENABLE
-	select ARCH_USES_GETTIMEOFFSET
 	select CPU_SA110
 	select FIQ
 	select HAVE_IDE

arch/arm/mach-rpc/time.c

@@ -13,7 +13,7 @@
  *   04-Dec-1997	RMK	Updated for new arch/arm/time.c
  *   13=Jun-2004	DS	Moved to arch/arm/common b/c shared w/CLPS7500
  */
-#include <linux/timex.h>
+#include <linux/clocksource.h>
 #include <linux/init.h>
 #include <linux/interrupt.h>
 #include <linux/irq.h>
@@ -27,11 +27,15 @@
 #define RPC_CLOCK_FREQ 2000000
 #define RPC_LATCH DIV_ROUND_CLOSEST(RPC_CLOCK_FREQ, HZ)
 
-static u32 ioc_timer_gettimeoffset(void)
+static u32 ioc_time;
+
+static u64 ioc_timer_read(struct clocksource *cs)
 {
 	unsigned int count1, count2, status;
-	long offset;
+	unsigned long flags;
+	u32 ticks;
 
+	local_irq_save(flags);
 	ioc_writeb (0, IOC_T0LATCH);
 	barrier ();
 	count1 = ioc_readb(IOC_T0CNTL) | (ioc_readb(IOC_T0CNTH) << 8);
@@ -41,27 +45,34 @@ static u32 ioc_timer_gettimeoffset(void)
 	ioc_writeb (0, IOC_T0LATCH);
 	barrier ();
 	count2 = ioc_readb(IOC_T0CNTL) | (ioc_readb(IOC_T0CNTH) << 8);
+	ticks = ioc_time + RPC_LATCH - count2;
+	local_irq_restore(flags);
 
-	offset = count2;
 	if (count2 < count1) {
 		/*
-		 * We have not had an interrupt between reading count1
-		 * and count2.
+		 * The timer has not reloaded between reading count1 and
+		 * count2, check whether an interrupt was actually pending.
 		 */
 		if (status & (1 << 5))
-			offset -= RPC_LATCH;
+			ticks += RPC_LATCH;
 	} else if (count2 > count1) {
 		/*
-		 * We have just had another interrupt between reading
-		 * count1 and count2.
+		 * The timer has reloaded, so count2 indicates the new
+		 * count since the wrap.  The interrupt would not have
+		 * been processed, so add the missed ticks.
 		 */
-		offset -= RPC_LATCH;
+		ticks += RPC_LATCH;
 	}
 
-	offset = (RPC_LATCH - offset) * (tick_nsec / 1000);
-	return DIV_ROUND_CLOSEST(offset, RPC_LATCH) * 1000;
+	return ticks;
 }
 
+static struct clocksource ioctime_clocksource = {
+	.read = ioc_timer_read,
+	.mask = CLOCKSOURCE_MASK(32),
+	.rating = 100,
+};
+
 void __init ioctime_init(void)
 {
 	ioc_writeb(RPC_LATCH & 255, IOC_T0LTCHL);
@@ -72,6 +83,7 @@ void __init ioctime_init(void)
 static irqreturn_t
 ioc_timer_interrupt(int irq, void *dev_id)
 {
+	ioc_time += RPC_LATCH;
 	timer_tick();
 	return IRQ_HANDLED;
 }
@@ -86,7 +98,7 @@ static struct irqaction ioc_timer_irq = {
  */
 void __init ioc_timer_init(void)
 {
-	arch_gettimeoffset = ioc_timer_gettimeoffset;
+	WARN_ON(clocksource_register_hz(&ioctime_clocksource, RPC_CLOCK_FREQ));
 	ioctime_init();
 	setup_irq(IRQ_TIMER0, &ioc_timer_irq);
 }