m68k: hp300: Handle timer counter overflow

Because hp300_read_clk() never checks the timer interrupt flag it may
fail to notice that the timer has wrapped, allowing the clock to jump
backwards. This is not a new problem.

This is resolved by checking the interrupt flag and, if need be,
taking wrap-around into account. The interrupt handler clears the flag
when it eventually executes.
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/hp300/time.c

@@ -30,7 +30,7 @@ static struct clocksource hp300_clk = {
 	.flags  = CLOCK_SOURCE_IS_CONTINUOUS,
 };
 
-static u32 clk_total;
+static u32 clk_total, clk_offset;
 
 /* Clock hardware definitions */
 
@@ -41,9 +41,12 @@ static u32 clk_total;
 #define	CLKCR3		CLKCR1
 #define	CLKSR		CLKCR2
 #define	CLKMSB1		0x5
+#define	CLKLSB1		0x7
 #define	CLKMSB2		0x9
 #define	CLKMSB3		0xD
 
+#define	CLKSR_INT1	BIT(0)
+
 /* This is for machines which generate the exact clock. */
 
 #define HP300_TIMER_CLOCK_FREQ 250000
@@ -60,6 +63,7 @@ static irqreturn_t hp300_tick(int irq, void *dev_id)
 	in_8(CLOCKBASE + CLKSR);
 	asm volatile ("movpw %1@(5),%0" : "=d" (tmp) : "a" (CLOCKBASE));
 	clk_total += INTVAL;
+	clk_offset = 0;
 	timer_routine(0, NULL);
 	local_irq_restore(flags);
 
@@ -71,20 +75,24 @@ static irqreturn_t hp300_tick(int irq, void *dev_id)
 static u64 hp300_read_clk(struct clocksource *cs)
 {
 	unsigned long flags;
-  unsigned char lsb, msb1, msb2;
+	unsigned char lsb, msb, msb_new;
 	u32 ticks;
 
 	local_irq_save(flags);
 	/* Read current timer 1 value */
-  msb1 = in_8(CLOCKBASE + 5);
-  lsb = in_8(CLOCKBASE + 7);
-  msb2 = in_8(CLOCKBASE + 5);
-  if (msb1 != msb2)
-    /* A carry happened while we were reading.  Read it again */
-    lsb = in_8(CLOCKBASE + 7);
-
-  ticks = INTVAL - ((msb2 << 8) | lsb);
-  ticks += clk_total;
+	msb = in_8(CLOCKBASE + CLKMSB1);
+again:
+	if ((in_8(CLOCKBASE + CLKSR) & CLKSR_INT1) && msb > 0)
+		clk_offset = INTVAL;
+	lsb = in_8(CLOCKBASE + CLKLSB1);
+	msb_new = in_8(CLOCKBASE + CLKMSB1);
+	if (msb_new != msb) {
+		msb = msb_new;
+		goto again;
+	}
+
+	ticks = INTVAL - ((msb << 8) | lsb);
+	ticks += clk_offset + clk_total;
 	local_irq_restore(flags);
 
 	return ticks;