m68k: mac: Fix VIA timer counter accesses

This resolves some bugs that affect VIA timer counter accesses.
Avoid lost interrupts caused by reading the counter low byte register.
Make allowance for the fact that the counter will be decremented to
0xFFFF before being reloaded.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/mac/via.c

@@ -54,16 +54,6 @@ static __u8 rbv_clear;
 
 static int gIER,gIFR,gBufA,gBufB;
 
-/*
- * Timer defs.
- */
-
-#define TICK_SIZE		10000
-#define MAC_CLOCK_TICK		(783300/HZ)		/* ticks per HZ */
-#define MAC_CLOCK_LOW		(MAC_CLOCK_TICK&0xFF)
-#define MAC_CLOCK_HIGH		(MAC_CLOCK_TICK>>8)
-
-
 /*
  * On Macs with a genuine VIA chip there is no way to mask an individual slot
  * interrupt. This limitation also seems to apply to VIA clone logic cores in
@@ -271,22 +261,6 @@ void __init via_init(void)
 	}
 }
 
-/*
- * Start the 100 Hz clock
- */
-
-void __init via_init_clock(irq_handler_t func)
-{
-	via1[vACR] |= 0x40;
-	via1[vT1LL] = MAC_CLOCK_LOW;
-	via1[vT1LH] = MAC_CLOCK_HIGH;
-	via1[vT1CL] = MAC_CLOCK_LOW;
-	via1[vT1CH] = MAC_CLOCK_HIGH;
-
-	if (request_irq(IRQ_MAC_TIMER_1, func, 0, "timer", func))
-		pr_err("Couldn't register %s interrupt\n", "timer");
-}
-
 /*
  * Debugging dump, used in various places to see what's going on.
  */
@@ -314,29 +288,6 @@ void via_debug_dump(void)
 	}
 }
 
-/*
- * This is always executed with interrupts disabled.
- *
- * TBI: get time offset between scheduling timer ticks
- */
-
-u32 mac_gettimeoffset(void)
-{
-	unsigned long ticks, offset = 0;
-
-	/* read VIA1 timer 2 current value */
-	ticks = via1[vT1CL] | (via1[vT1CH] << 8);
-	/* The probability of underflow is less than 2% */
-	if (ticks > MAC_CLOCK_TICK - MAC_CLOCK_TICK / 50)
-		/* Check for pending timer interrupt in VIA1 IFR */
-		if (via1[vIFR] & 0x40) offset = TICK_SIZE;
-
-	ticks = MAC_CLOCK_TICK - ticks;
-	ticks = ticks * 10000L / MAC_CLOCK_TICK;
-
-	return (ticks + offset) * 1000;
-}
-
 /*
  * Flush the L2 cache on Macs that have it by flipping
  * the system into 24-bit mode for an instant.
@@ -622,3 +573,56 @@ int via2_scsi_drq_pending(void)
 	return via2[gIFR] & (1 << IRQ_IDX(IRQ_MAC_SCSIDRQ));
 }
 EXPORT_SYMBOL(via2_scsi_drq_pending);
+
+/* timer and clock source */
+
+#define VIA_CLOCK_FREQ     783360                /* VIA "phase 2" clock in Hz */
+#define VIA_TIMER_INTERVAL (1000000 / HZ)        /* microseconds per jiffy */
+#define VIA_TIMER_CYCLES   (VIA_CLOCK_FREQ / HZ) /* clock cycles per jiffy */
+
+#define VIA_TC             (VIA_TIMER_CYCLES - 2) /* including 0 and -1 */
+#define VIA_TC_LOW         (VIA_TC & 0xFF)
+#define VIA_TC_HIGH        (VIA_TC >> 8)
+
+void __init via_init_clock(irq_handler_t timer_routine)
+{
+	if (request_irq(IRQ_MAC_TIMER_1, timer_routine, 0, "timer", NULL)) {
+		pr_err("Couldn't register %s interrupt\n", "timer");
+		return;
+	}
+
+	via1[vT1LL] = VIA_TC_LOW;
+	via1[vT1LH] = VIA_TC_HIGH;
+	via1[vT1CL] = VIA_TC_LOW;
+	via1[vT1CH] = VIA_TC_HIGH;
+	via1[vACR] |= 0x40;
+}
+
+u32 mac_gettimeoffset(void)
+{
+	unsigned long flags;
+	u8 count_high;
+	u16 count, offset = 0;
+
+	/*
+	 * Timer counter wrap-around is detected with the timer interrupt flag
+	 * but reading the counter low byte (vT1CL) would reset the flag.
+	 * Also, accessing both counter registers is essentially a data race.
+	 * These problems are avoided by ignoring the low byte. Clock accuracy
+	 * is 256 times worse (error can reach 0.327 ms) but CPU overhead is
+	 * reduced by avoiding slow VIA register accesses.
+	 */
+
+	local_irq_save(flags);
+	count_high = via1[vT1CH];
+	if (count_high == 0xFF)
+		count_high = 0;
+	if (count_high > 0 && (via1[vIFR] & VIA_TIMER_1_INT))
+		offset = VIA_TIMER_CYCLES;
+	local_irq_restore(flags);
+
+	count = count_high << 8;
+	count = VIA_TIMER_CYCLES - count + offset;
+
+	return ((count * VIA_TIMER_INTERVAL) / VIA_TIMER_CYCLES) * 1000;
+}