ARM: OMAP: Optional timer modulo code

Adds support for running 32KHz timer at 100HZ using modulo code.

Signed-off-by: Tuukka Tikkanen <>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap/Kconfig

@@ -117,7 +117,9 @@ config OMAP_32K_TIMER_HZ
        default "128"
        help
 	  Kernel internal timer frequency should be a divisor of 32768,
-	  such as 64 or 128.
+	  such as 64 or 128. If ARM Linux default HZ=100 is used, the system
+	  will still work, but the timer interrupt will use less efficient
+	  modulo code.
 
 config OMAP_DM_TIMER
 	bool "Use dual-mode timer"

arch/arm/mach-omap/omap1/time.c

@@ -247,6 +247,13 @@ unsigned long long sched_clock(void)
 #define OMAP_32K_TIMER_TCR		0x04
 
 #define OMAP_32K_TICKS_PER_HZ		(32768 / HZ)
+#if (32768 % HZ) != 0
+/* We cannot ignore modulo.
+ * Potential error can be as high as several percent.
+ */
+#define OMAP_32K_TICK_MODULO		(32768 % HZ)
+static unsigned modulo_count = 0; /* Counts 1/HZ units */
+#endif
 
 /*
  * TRM says 1 / HZ = ( TVR + 1) / 32768, so TRV = (32768 / HZ) - 1
@@ -324,6 +331,19 @@ static irqreturn_t omap_32k_timer_interrupt(int irq, void *dev_id,
 	now = omap_32k_sync_timer_read();
 
 	while (now - omap_32k_last_tick >= OMAP_32K_TICKS_PER_HZ) {
+#ifdef OMAP_32K_TICK_MODULO
+		/* Modulo addition may put omap_32k_last_tick ahead of now
+		 * and cause unwanted repetition of the while loop.
+		 */
+		if (unlikely(now - omap_32k_last_tick == ~0))
+			break;
+
+		modulo_count += OMAP_32K_TICK_MODULO;
+		if (modulo_count > HZ) {
+			++omap_32k_last_tick;
+			modulo_count -= HZ;
+		}
+#endif
 		omap_32k_last_tick += OMAP_32K_TICKS_PER_HZ;
 		timer_tick(regs);
 	}