[ARM] 4294/1: ns9xxx: Determine system clock from PLL register settings

The function attribute const is abused here as the PLL register is read.  But I
think this is all right because the PLL register cannot change without a reset.

Note: This patch depends on 4293/1

Signed-off-by: Uwe Kleine-König <ukleinek@informatik.uni-freiburg.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

include/asm-arm/arch-ns9xxx/clock.h

@@ -11,13 +11,43 @@
 #ifndef __ASM_ARCH_CLOCK_H
 #define __ASM_ARCH_CLOCK_H
 
+#include <asm/arch-ns9xxx/regs-sys.h>
+
+#define CRYSTAL 29491200 /* Hz */
+
+/* The HRM calls this value f_vco */
 static inline u32 ns9xxx_systemclock(void) __attribute__((const));
 static inline u32 ns9xxx_systemclock(void)
 {
+	u32 pll = SYS_PLL;
+
 	/*
-	 * This should be a multiple of HZ * TIMERCLOCKSELECT (in time.c)
+	 * The system clock should be a multiple of HZ * TIMERCLOCKSELECT (in
+	 * time.c).
+	 *
+	 * The following values are given:
+	 *   - TIMERCLOCKSELECT == 2^i for an i in {0 .. 6}
+	 *   - CRYSTAL == 29491200 == 2^17 * 3^2 * 5^2
+	 *   - ND in {0 .. 31}
+	 *   - FS in {0 .. 3}
+	 *
+	 * Assuming the worst, we consider:
+	 *   - TIMERCLOCKSELECT == 64
+	 *   - ND == 0
+	 *   - FS == 3
+	 *
+	 * So HZ should be a divisor of:
+	 *      (CRYSTAL * (ND + 1) >> FS) / TIMERCLOCKSELECT
+	 *   == (2^17 * 3^2 * 5^2 * 1 >> 3) / 64
+	 *   == 2^8 * 3^2 * 5^2
+	 *   == 57600
+	 *
+	 * Currently HZ is defined to be 100 for this platform.
+	 *
+	 * Fine.
 	 */
-	return 353894400;
+	return CRYSTAL * (REGGET(pll, SYS_PLL, ND) + 1)
+		>> REGGET(pll, SYS_PLL, FS);
 }
 
 static inline u32 ns9xxx_cpuclock(void) __attribute__((const));