[ARM] 4646/1: AT91: configurable HZ, default to 128

This makes HZ configurable on AT91, following the model used on OMAP.

It defaults to a power of two on AT91rm9200 chips, avoiding rounding
errors which come from dividing a 32 KiHz clock to generate scheduler
irqs; and uses 100 on AT91sam926x chips, using MCK/16 (multi-MHZ).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Remy Bhmer <linux@bohmer.net>
Acked-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/Kconfig

@@ -657,6 +657,7 @@ config HZ
 	default 128 if ARCH_L7200
 	default 200 if ARCH_EBSA110 || ARCH_S3C2410
 	default OMAP_32K_TIMER_HZ if ARCH_OMAP && OMAP_32K_TIMER
+	default AT91_TIMER_HZ if ARCH_AT91
 	default 100
 
 config AEABI

arch/arm/mach-at91/Kconfig

@@ -219,6 +219,22 @@ config AT91_PROGRAMMABLE_CLOCKS
 	  Select this if you need to program one or more of the PCK0..PCK3
 	  programmable clock outputs.
 
+config AT91_TIMER_HZ
+       int "Kernel HZ (jiffies per second)"
+       range 32 1024
+       depends on ARCH_AT91
+       default "128" if ARCH_AT91RM9200
+       default "100"
+       help
+	  On AT91rm9200 chips where you're using a system clock derived
+	  from the 32768 Hz hardware clock, this tick rate should divide
+	  it exactly: use a power-of-two value, such as 128 or 256, to
+	  reduce timing errors caused by rounding.
+
+	  On AT91sam926x chips, or otherwise when using a higher precision
+	  system clock (of at least several MHz), rounding is less of a
+	  problem so it can be safer to use a decimal values like 100.
+
 endmenu
 
 endif