arm: Move system_clock_scale to armv7m_systick.h

The system_clock_scale global is used only by the armv7m systick
device; move the extern declaration to the armv7m_systick.h header,
and expand the comment to explain what it is and that it should
ideally be replaced with a different approach.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190516163857.6430-2-peter.maydell@linaro.org

include/hw/arm/arm.h

@@ -167,8 +167,4 @@ void arm_write_secure_board_setup_dummy_smc(ARMCPU *cpu,
                                             const struct arm_boot_info *info,
                                             hwaddr mvbar_addr);
 
-/* Multiplication factor to convert from system clock ticks to qemu timer
-   ticks.  */
-extern int system_clock_scale;
-
 #endif /* HW_ARM_H */

include/hw/timer/armv7m_systick.h

@@ -31,4 +31,26 @@ typedef struct SysTickState {
     qemu_irq irq;
 } SysTickState;
 
+/*
+ * Multiplication factor to convert from system clock ticks to qemu timer
+ * ticks. This should be set (by board code, usually) to a value
+ * equal to NANOSECONDS_PER_SECOND / frq, where frq is the clock frequency
+ * in Hz of the CPU.
+ *
+ * This value is used by the systick device when it is running in
+ * its "use the CPU clock" mode (ie when SYST_CSR.CLKSOURCE == 1) to
+ * set how fast the timer should tick.
+ *
+ * TODO: we should refactor this so that rather than using a global
+ * we use a device property or something similar. This is complicated
+ * because (a) the property would need to be plumbed through from the
+ * board code down through various layers to the systick device
+ * and (b) the property needs to be modifiable after realize, because
+ * the stellaris board uses this to implement the behaviour where the
+ * guest can reprogram the PLL registers to downclock the CPU, and the
+ * systick device needs to react accordingly. Possibly this should
+ * be deferred until we have a good API for modelling clock trees.
+ */
+extern int system_clock_scale;
+
 #endif