rtc: snvs: Allow a time difference on clock register read

On an iMX6ULL the following message appears when a wakealarm is set:

echo 0 > /sys/class/rtc/rtc1/wakealarm
rtc rtc1: Timeout trying to get valid LPSRT Counter read

This does not always happen but is reproducible quite often (7 out of 10
times). The problem appears because the iMX6ULL is not able to read the
registers within one 32kHz clock cycle which is the base clock of the
RTC. Therefore, this patch allows a difference of up to 320 cycles
(10ms). 10ms was chosen to be big enough even on systems with less cpu
power (e.g. iMX6ULL). According to the reference manual a difference is
fine:
- If the two consecutive reads are similar, the value is correct.
The values have to be similar, not equal.

Fixes: cd7f3a2 ("rtc: snvs: Add timeouts to avoid kernel lockups")
Reviewed-by: Francesco Dolcini <[email protected]>
Signed-off-by: Stefan Eichenberger <[email protected]>
Signed-off-by: Francesco Dolcini <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexandre Belloni <[email protected]>

Author: eichenberger

drivers/rtc/rtc-snvs.c

@@ -32,6 +32,14 @@
 #define SNVS_LPPGDR_INIT	0x41736166
 #define CNTR_TO_SECS_SH		15
 
+/* The maximum RTC clock cycles that are allowed to pass between two
+ * consecutive clock counter register reads. If the values are corrupted a
+ * bigger difference is expected. The RTC frequency is 32kHz. With 320 cycles
+ * we end at 10ms which should be enough for most cases. If it once takes
+ * longer than expected we do a retry.
+ */
+#define MAX_RTC_READ_DIFF_CYCLES	320
+
 struct snvs_rtc_data {
 	struct rtc_device *rtc;
 	struct regmap *regmap;
@@ -56,6 +64,7 @@ static u64 rtc_read_lpsrt(struct snvs_rtc_data *data)
 static u32 rtc_read_lp_counter(struct snvs_rtc_data *data)
 {
 	u64 read1, read2;
+	s64 diff;
 	unsigned int timeout = 100;
 
 	/* As expected, the registers might update between the read of the LSB
@@ -66,7 +75,8 @@ static u32 rtc_read_lp_counter(struct snvs_rtc_data *data)
 	do {
 		read2 = read1;
 		read1 = rtc_read_lpsrt(data);
-	} while (read1 != read2 && --timeout);
+		diff = read1 - read2;
+	} while (((diff < 0) || (diff > MAX_RTC_READ_DIFF_CYCLES)) && --timeout);
 	if (!timeout)
 		dev_err(&data->rtc->dev, "Timeout trying to get valid LPSRT Counter read\n");
 
@@ -78,13 +88,15 @@ static u32 rtc_read_lp_counter(struct snvs_rtc_data *data)
 static int rtc_read_lp_counter_lsb(struct snvs_rtc_data *data, u32 *lsb)
 {
 	u32 count1, count2;
+	s32 diff;
 	unsigned int timeout = 100;
 
 	regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &count1);
 	do {
 		count2 = count1;
 		regmap_read(data->regmap, data->offset + SNVS_LPSRTCLR, &count1);
-	} while (count1 != count2 && --timeout);
+		diff = count1 - count2;
+	} while (((diff < 0) || (diff > MAX_RTC_READ_DIFF_CYCLES)) && --timeout);
 	if (!timeout) {
 		dev_err(&data->rtc->dev, "Timeout trying to get valid LPSRT Counter read\n");
 		return -ETIMEDOUT;