arm: clarify the use of muldiv64()

muldiv64() is used to convert microseconds into CPU ticks.

But it is not clear and not commented. This patch uses macro
to clearly identify what is used: time, CPU frequency and ticks.
For an elapsed time and a given frequency, we compute how many ticks
 we have.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>

target-arm/helper.c

@@ -12,6 +12,8 @@
 #include <zlib.h> /* For crc32 */
 #include "exec/semihost.h"
 
+#define ARM_CPU_FREQ 1000000000 /* FIXME: 1 GHz, should be configurable */
+
 #ifndef CONFIG_USER_ONLY
 static inline bool get_phys_addr(CPUARMState *env, target_ulong address,
                                  int access_type, ARMMMUIdx mmu_idx,
@@ -706,8 +708,8 @@ void pmccntr_sync(CPUARMState *env)
 {
     uint64_t temp_ticks;
 
-    temp_ticks = muldiv64(qemu_clock_get_us(QEMU_CLOCK_VIRTUAL),
-                          get_ticks_per_sec(), 1000000);
+    temp_ticks = muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
+                          ARM_CPU_FREQ, NANOSECONDS_PER_SECOND);
 
     if (env->cp15.c9_pmcr & PMCRD) {
         /* Increment once every 64 processor clock cycles */
@@ -745,8 +747,8 @@ static uint64_t pmccntr_read(CPUARMState *env, const ARMCPRegInfo *ri)
         return env->cp15.c15_ccnt;
     }
 
-    total_ticks = muldiv64(qemu_clock_get_us(QEMU_CLOCK_VIRTUAL),
-                           get_ticks_per_sec(), 1000000);
+    total_ticks = muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
+                           ARM_CPU_FREQ, NANOSECONDS_PER_SECOND);
 
     if (env->cp15.c9_pmcr & PMCRD) {
         /* Increment once every 64 processor clock cycles */
@@ -766,8 +768,8 @@ static void pmccntr_write(CPUARMState *env, const ARMCPRegInfo *ri,
         return;
     }
 
-    total_ticks = muldiv64(qemu_clock_get_us(QEMU_CLOCK_VIRTUAL),
-                           get_ticks_per_sec(), 1000000);
+    total_ticks = muldiv64(qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL),
+                           ARM_CPU_FREQ, NANOSECONDS_PER_SECOND);
 
     if (env->cp15.c9_pmcr & PMCRD) {
         /* Increment once every 64 processor clock cycles */