m68k: mvme147: Handle timer counter overflow

Reading the timer counter races with timer overflow (and the
corresponding interrupt). This is resolved by reading the overflow
register and taking this value into account. The interrupt handler
must clear the overflow register when it eventually executes.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/include/asm/mvme147hw.h

@@ -66,6 +66,7 @@ struct pcc_regs {
 #define PCC_INT_ENAB		0x08
 
 #define PCC_TIMER_INT_CLR	0x80
+#define PCC_TIMER_CLR_OVF	0x04
 
 #define PCC_LEVEL_ABORT		0x07
 #define PCC_LEVEL_SERIAL	0x04

arch/m68k/mvme147/config.c

@@ -118,7 +118,7 @@ static irqreturn_t mvme147_timer_int (int irq, void *dev_id)
 
 	local_irq_save(flags);
 	m147_pcc->t1_int_cntrl = PCC_TIMER_INT_CLR;
-	m147_pcc->t1_int_cntrl = PCC_INT_ENAB|PCC_LEVEL_TIMER1;
+	m147_pcc->t1_cntrl = PCC_TIMER_CLR_OVF;
 	clk_total += PCC_TIMER_CYCLES;
 	timer_routine(0, NULL);
 	local_irq_restore(flags);
@@ -144,21 +144,22 @@ void mvme147_sched_init (irq_handler_t timer_routine)
 	clocksource_register_hz(&mvme147_clk, PCC_TIMER_CLOCK_FREQ);
 }
 
-/* XXX There are race hazards in this code XXX */
 static u64 mvme147_read_clk(struct clocksource *cs)
 {
 	unsigned long flags;
-	volatile unsigned short *cp = (volatile unsigned short *)0xfffe1012;
-	unsigned short n;
+	u8 overflow, tmp;
+	u16 count;
 	u32 ticks;
 
 	local_irq_save(flags);
-	n = *cp;
-	while (n != *cp)
-		n = *cp;
-
-	n -= PCC_TIMER_PRELOAD;
-	ticks = clk_total + n;
+	tmp = m147_pcc->t1_cntrl >> 4;
+	count = m147_pcc->t1_count;
+	overflow = m147_pcc->t1_cntrl >> 4;
+	if (overflow != tmp)
+		count = m147_pcc->t1_count;
+	count -= PCC_TIMER_PRELOAD;
+	ticks = count + overflow * PCC_TIMER_CYCLES;
+	ticks += clk_total;
 	local_irq_restore(flags);
 
 	return ticks;