drivers/rtc/rtc-rk808.c: fix rtc time reading issue
After we set the GET_TIME bit, the rtc time can't be read immediately. We
should wait up to 31.25 us, about one cycle of 32khz. Otherwise reading
RTC time will return a old time. If we clear the GET_TIME bit after
setting, the time of i2c transfer is certainly more than 31.25us.
Doug said:
: I think we are safe. At 400kHz (the max speed of this part) each bit can
: be transferred no faster than 2.5us. In order to do a valid i2c
: transaction we need to _at least_ write the address of the device and the
: data onto the bus, which is 16 bits. 16 * 2.5us = 40us. That's above the
: 31.25us
[akpm@linux-foundation.org: tweak comment per review discussion]
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Cc: Sonny Rao <sonnyrao@chromium.org>
Cc: Heiko Stübner <heiko@sntech.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/rtc/rtc-rk808.c b/drivers/rtc/rtc-rk808.c
index df42257..91ca0bc 100644
--- a/drivers/rtc/rtc-rk808.c
+++ b/drivers/rtc/rtc-rk808.c
@@ -67,15 +67,21 @@
/* Force an update of the shadowed registers right now */
ret = regmap_update_bits(rk808->regmap, RK808_RTC_CTRL_REG,
BIT_RTC_CTRL_REG_RTC_GET_TIME,
- 0);
+ BIT_RTC_CTRL_REG_RTC_GET_TIME);
if (ret) {
dev_err(dev, "Failed to update bits rtc_ctrl: %d\n", ret);
return ret;
}
+ /*
+ * After we set the GET_TIME bit, the rtc time can't be read
+ * immediately. So we should wait up to 31.25 us, about one cycle of
+ * 32khz. If we clear the GET_TIME bit here, the time of i2c transfer
+ * certainly more than 31.25us: 16 * 2.5us at 400kHz bus frequency.
+ */
ret = regmap_update_bits(rk808->regmap, RK808_RTC_CTRL_REG,
BIT_RTC_CTRL_REG_RTC_GET_TIME,
- BIT_RTC_CTRL_REG_RTC_GET_TIME);
+ 0);
if (ret) {
dev_err(dev, "Failed to update bits rtc_ctrl: %d\n", ret);
return ret;
