[ARM] twd: Allow twd rescaling to match cpu frequency

The clock to the ARM TWD local timer scales with the cpu
frequency.  To allow the cpu frequency to change while
maintaining a constant TWD frequency, pick a lower target
frequency for the TWD and use the prescaler to divide down
to the closest lower frequency.

This patch provides a new initialization function that takes
a target TWD frequency and the relation between the cpu
clock and the TWD clock, required to be an integer divider
>= 2 by the ARM spec.  It also provides a function to be
called from cpufreq drivers to set the prescaler whenever
the cpu frequency changes.

Also fixes a typo in the printk of the calibrated frequency.

Change-Id: I3fa8ef718ff5518170f1b2bab29efe960741853e
Signed-off-by: Colin Cross <ccross@google.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/arch/arm/include/asm/smp_twd.h b/arch/arm/include/asm/smp_twd.h
index 634f357be6bb..79aebe2ad7a8 100644
--- a/arch/arm/include/asm/smp_twd.h
+++ b/arch/arm/include/asm/smp_twd.h
@@ -17,6 +17,7 @@
 #define TWD_TIMER_CONTROL_ONESHOT	(0 << 1)
 #define TWD_TIMER_CONTROL_PERIODIC	(1 << 1)
 #define TWD_TIMER_CONTROL_IT_ENABLE	(1 << 2)
+#define TWD_TIMER_CONTROL_PRESCALE_MASK	(0xFF << 8)
 
 struct clock_event_device;
 
@@ -26,4 +27,22 @@ void twd_timer_stop(void);
 int twd_timer_ack(void);
 void twd_timer_setup(struct clock_event_device *);
 
+/*
+ * Use this setup function on systems where the cpu clock frequency may
+ * change.  periphclk_prescaler is the fixed divider value between the cpu
+ * clock and the PERIPHCLK clock that feeds the TWD.  target_rate should be
+ * low enough that the prescaler can accurately reach the target rate from the
+ * lowest cpu frequency.
+ */
+void twd_timer_setup_scalable(struct clock_event_device *,
+	unsigned long target_rate, unsigned int periphclk_prescaler);
+
+/*
+ * Recalculate the twd prescaler value when the cpu frequency changes. To
+ * prevent early timer interrupts, must be called before changing the cpu
+ * frequency if the frequency is increasing, or after if the frequency is
+ * decreasing.
+ */
+void twd_recalc_prescaler(unsigned long new_rate);
+
 #endif