[PATCH v3] rtc: bq32000: add configurable delay between RTC reads

[Adriana Stancu]

When the RTC is used on systems without a interrupt line, userspace
tools like `hwclock` fall back to a frequent polling loop to synchronize
with the edge of the next second.

On the BQ32000, this aggressive polling can temporarly lock the register
refresh cycle, because the continuous transfers prevent the hardware from
updating the buffer. This results in stale data reads or select() timeouts
in userspace.

This patch introduces a delay before reading the RTC registers in order to
provide a sufficient idle time for the hardware to sync with the register
buffer.

Signed-off-by: Adriana Stancu <adriana@xxxxxxxxxx>
---
 drivers/rtc/rtc-bq32k.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/rtc/rtc-bq32k.c b/drivers/rtc/rtc-bq32k.c
index 7ad34539be4d..edce95eb328f 100644
--- a/drivers/rtc/rtc-bq32k.c
+++ b/drivers/rtc/rtc-bq32k.c
@@ -16,6 +16,7 @@
 #include <linux/kstrtox.h>
 #include <linux/errno.h>
 #include <linux/bcd.h>
+#include <linux/delay.h>
 
 #define BQ32K_SECONDS		0x00	/* Seconds register address */
 #define BQ32K_SECONDS_MASK	0x7F	/* Mask over seconds value */
@@ -89,9 +90,17 @@ static int bq32k_write(struct device *dev, void *data, uint8_t off, uint8_t len)
 
 static int bq32k_rtc_read_time(struct device *dev, struct rtc_time *tm)
 {
+	struct i2c_client *client = to_i2c_client(dev);
 	struct bq32k_regs regs;
 	int error;
 
+	/*
+	 * When the device doesn't have the interrupt connected, prevent
+	 * userpace from polling the RTC registers to frequently.
+	 */
+	if (client->irq <= 0)
+		usleep_range(2000, 2500);
+
 	error = bq32k_read(dev, &regs, 0, sizeof(regs));
 	if (error)
 		return error;
-- 
2.51.0