rtc : tps6586x: avoid RTC time is getting slower

To ensure an accurate read of the RTC registers during the required
multi-byte read operation, the PMU RTC is designed with the following
protection scheme

- A circuit detects a write/read and locks the RTC_COUNT4 value
by keeping the RTC in a suspended mode

- During the suspended mode, a secondary counter is used to
keep track of all counts that would have normally incremented the RTC

- After the read is complete, the value of the secondary counter is
added back to the RTC registers and thereby keeping the RTC accurate

- The backup counter allows for a 1ms RTC suspend mode duration
when the RTC prescaler is enabled.

i2c needs to generate a 2 msgs when reading.
  - the address setup(write RTC_COUNT4 operation),
hence start locking the RTC_COUNT4
  - the data transfer (read RTC_COUNT4 operation),release locking it.
this may allow the CPU to execute other portions of code
in between the two operation.

The fix is to start a PMU RTC access by reading the register prior to
the RTC_COUNT4 so that access of the RTC PMU registers will be guaranteed
to always occur within the 1ms time period.
  - the address setup(write RTC_COUNT4-1 operation),
so there is no locking the RTC_COUNT4
  - the data transfer (read RTC_COUNT4 operation),
starting locking the RTC_COUNT4 and release locking the RTC_COUNT4
in one operation, so it will be guaranteed within 1ms

Bug 811075

Original-Change-Id: Ie07472a329f6a0eed11e6a039cd93307bb5276a0
Reviewed-on: http://git-master/r/27537
Reviewed-by: Varun Colbert <vcolbert@nvidia.com>
Tested-by: Varun Colbert <vcolbert@nvidia.com>

Rebase-Id: R0279c3dac03756636596c504e623b7159ee474f7

1 files changed, 4 insertions, 3 deletions

diff --git a/drivers/rtc/rtc-tps6586x.c b/drivers/rtc/rtc-tps6586x.c
index 20bd0d06ce1c..265c57d55ed1 100644
--- a/drivers/rtc/rtc-tps6586x.c
+++ b/drivers/rtc/rtc-tps6586x.c
@@ -39,6 +39,7 @@
 #define CL_SEL_POS	1
 #define RTC_ALARM1_HI	0xc1
 #define RTC_COUNT4	0xc6
+#define RTC_COUNT4_DUMMYREAD 0xc5  /* start a PMU RTC access by reading the register prior to the RTC_COUNT4 */
 
 struct tps6586x_rtc {
 	unsigned long		epoch_start;
@@ -58,17 +59,17 @@ static int tps6586x_rtc_read_time(struct device *dev, struct rtc_time *tm)
 	struct device *tps_dev = to_tps6586x_dev(dev);
 	unsigned long long ticks = 0;
 	unsigned long seconds;
-	u8 buff[5];
+	u8 buff[6];
 	int err;
 	int i;
 
-	err = tps6586x_reads(tps_dev, RTC_COUNT4, sizeof(buff), buff);
+	err = tps6586x_reads(tps_dev, RTC_COUNT4_DUMMYREAD, sizeof(buff), buff);
 	if (err < 0) {
 		dev_err(dev, "failed to read counter\n");
 		return err;
 	}
 
-	for (i = 0; i < sizeof(buff); i++) {
+	for (i = 1; i < sizeof(buff); i++) {
 		ticks <<= 8;
 		ticks |= buff[i];
 	}