[DeviceDrivers] Add more API.

1. Use float type for clock_cpu_getres() return value;
2. Add clock_cpu_microsecond/millisecond APIs.

components/drivers/Kconfig

@@ -7,7 +7,7 @@ config RT_USING_DEVICE_IPC
 config RT_USING_SERIAL
     bool "Using serial device drivers"
     select RT_USING_DEVICE_IPC
-    select RT_USING_DEVICE    
+    select RT_USING_DEVICE
     default y
 
 config RT_USING_CAN
@@ -24,13 +24,17 @@ config RT_USING_CPUTIME
     help
         When enable this option, the BSP should provide a rt_clock_cputime_ops
         for CPU time by:
-        clock_cpu_setops(const struct rt_clock_cputime_ops *ops);
+        const static struct rt_clock_cputime_ops _ops = {...};
+        clock_cpu_setops(&_ops);
 
-        Then developer can use high resolution clock counter with:
+        Then user can use high resolution clock counter with:
 
-        ts = clock_cpu_gettime(); 
-        /* The unit of clock_cpu_gettime() can be returned by */
-        unit = clock_cpu_getres(); /* number for nanosecond */
+        ts1 = clock_cpu_gettime();
+        ts2 = clock_cpu_gettime();
+
+        /* and get the ms of delta tick with API: */
+        ms_tick = clock_cpu_millisecond(t2 - t1);
+        us_tick = clock_cpu_microsecond(t2 - t1);
 
 if RT_USING_CPUTIME 
     config RT_USING_CPUTIME_CORTEXM

components/drivers/cputime/cputime.c

@@ -30,8 +30,10 @@ static const struct rt_clock_cputime_ops *_cputime_ops  = RT_NULL;
 /**
  * The clock_cpu_getres() function shall return the resolution of CPU time, the 
  * number of nanosecond per tick.
+ *
+ * @return the number of nanosecond per tick
  */
-uint32_t clock_cpu_getres(void)
+float clock_cpu_getres(void)
 {
     if (_cputime_ops)
         return _cputime_ops->cputime_getres();
@@ -42,6 +44,8 @@ uint32_t clock_cpu_getres(void)
 
 /**
  * The clock_cpu_gettime() function shall return the current value of cpu time tick.
+ *
+ * @return the cpu tick
  */
 uint32_t clock_cpu_gettime(void)
 {
@@ -52,6 +56,36 @@ uint32_t clock_cpu_gettime(void)
     return 0;
 }
 
+/**
+ * The clock_cpu_microsecond() fucntion shall return the microsecond according to 
+ * cpu_tick parameter.
+ *
+ * @param cpu_tick the cpu tick
+ *
+ * @return the microsecond
+ */
+uint32_t clock_cpu_microsecond(uint32_t cpu_tick)
+{
+    float unit = clock_cpu_getres();
+
+    return (cpu_tick * unit) / 1000;
+}
+
+/**
+ * The clock_cpu_microsecond() fucntion shall return the millisecond according to 
+ * cpu_tick parameter.
+ *
+ * @param cpu_tick the cpu tick
+ *
+ * @return the millisecond
+ */
+uint32_t clock_cpu_millisecond(uint32_t cpu_tick)
+{
+    float unit = clock_cpu_getres();
+
+    return (cpu_tick * unit) / (1000 * 1000);
+}
+
 /**
  * The clock_cpu_seops() function shall set the ops of cpu time.
  * 

components/drivers/cputime/cputime_cortexm.c

@@ -30,9 +30,12 @@
 
 /* Use Cycle counter of Data Watchpoint and Trace Register for CPU time */
 
-static uint32_t cortexm_cputime_getres(void)
+static float cortexm_cputime_getres(void)
 {
-    return (1000 * 1000 * 1000)/SystemCoreClock;
+    float ret = 1000 * 1000 * 1000;
+    
+    ret = ret / SystemCoreClock;
+    return ret;
 }
 
 static uint32_t cortexm_cputime_gettime(void)

components/drivers/include/drivers/cputime.h

@@ -27,13 +27,16 @@
 
 struct rt_clock_cputime_ops
 {
-    uint32_t (*cputime_getres) (void);
+    float    (*cputime_getres) (void);
     uint32_t (*cputime_gettime)(void);
 };
 
-uint32_t clock_cpu_getres(void);
+float    clock_cpu_getres(void);
 uint32_t clock_cpu_gettime(void);
 
+uint32_t clock_cpu_microsecond(uint32_t cpu_tick);
+uint32_t clock_cpu_millisecond(uint32_t cpu_tick);
+
 int clock_cpu_setops(const struct rt_clock_cputime_ops *ops);
 
 #endif