sys_time: limit the number of retries for reading rtc time

For the rtc device, it is possible to read the time and date while an
update is in progress and get inconsistent values.

We retry for up to 500 times for the RTC to be ready which takes about
100ms.

Signed-off-by: Jiaqi Gao <jiaqi.gao@intel.com>

src/std-support/sys_time/src/lib.rs

@@ -13,7 +13,7 @@ use time::{Date, Month, PrimitiveDateTime, Time};
 pub mod rtc;
 
 pub fn get_sys_time() -> Option<i64> {
-    let data_time = read_rtc();
+    let data_time = read_rtc()?;
 
     let date_time = PrimitiveDateTime::new(
         Date::from_calendar_date(

src/std-support/sys_time/src/rtc.rs

@@ -49,27 +49,35 @@ pub struct DateTime {
     pub year: u16,
 }
 
-pub fn read_rtc() -> DateTime {
+pub fn read_rtc() -> Option<DateTime> {
     // It is possible to read the time and date while an update is in progress and get inconsistent
     // values, for example, at 9:00 o'clock we might get 8:59, or 8:60, or 8:00, or 9:00.
     // The solution here is to read all the values we need twice, and make sure the update_in_progress
     // flag is clear before each reading. If the two values are the same, then we get a correct value.
-    loop {
+
+    // Retry for up to 500 times for the RTC to be ready, it takes about 100ms.
+    let mut retries = 500;
+    while retries > 0 {
         // Wait until RTC finishes its update
         while get_update_in_progress_flag() > 0 {}
         let first_read = read_date_time();
 
         // If the flag is set before our second reading, we can't get the same value
         if get_update_in_progress_flag() > 0 {
+            retries -= 1;
             continue;
         }
         let second_read = read_date_time();
 
         // Compare the values read out twice. If they are equal, then we get a correct value
         if first_read == second_read {
-            return first_read;
+            return Some(first_read);
+        } else {
+            retries -= 1;
         }
     }
+
+    None
 }
 
 /// Convert a value in BCD format into binary mode