rtc: Check update-in-progress bit

While the real-time clock updates its count, values may not be correctly
read or written.  On reads, ensure the UIP bit is clear which guarantees
a minimum of 244 microseconds exists before the update begins.  Writes
already avoid the problem by disabling the RTC count via the SET bit.

Change-Id: I39e34493113015d32582f1c280fafa9e97f43a40
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/17369
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/drivers/pc80/rtc/mc146818rtc.c

@@ -360,6 +360,7 @@ void cmos_check_update_date(void)
 	u8 year, century;
 
 	/* Assume hardware always supports RTC_CLK_ALTCENTURY. */
+	wait_uip();
 	century = cmos_read(RTC_CLK_ALTCENTURY);
 	year = cmos_read(RTC_CLK_YEAR);
 
@@ -388,6 +389,7 @@ int rtc_set(const struct rtc_time *time)
 
 int rtc_get(struct rtc_time *time)
 {
+	wait_uip();
 	time->sec = bcd2bin(cmos_read(RTC_CLK_SECOND));
 	time->min = bcd2bin(cmos_read(RTC_CLK_MINUTE));
 	time->hour = bcd2bin(cmos_read(RTC_CLK_HOUR));

src/include/pc80/mc146818rtc.h

@@ -112,6 +112,16 @@ static inline unsigned char cmos_read(unsigned char addr)
 	return inb(RTC_BASE_PORT + offs + 1);
 }
 
+/* Upon return the caller is guaranteed 244 microseconds to complete any
+ * RTC operations.  wait_uip may be called a single time prior to multiple
+ * accesses, but sequences requiring more time should call wait_uip again.
+ */
+static inline void wait_uip(void)
+{
+	while (cmos_read(RTC_REG_A) & RTC_UIP)
+		;
+}
+
 static inline void cmos_write_inner(unsigned char val, unsigned char addr)
 {
 	int offs = 0;