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>

1 files changed, 8 insertions, 2 deletions

diff --git a/drivers/rtc/rtc-rk808.c b/drivers/rtc/rtc-rk808.c
index df42257668ac..91ca0bc1b484 100644
--- a/drivers/rtc/rtc-rk808.c
+++ b/drivers/rtc/rtc-rk808.c
@@ -67,15 +67,21 @@ static int rk808_rtc_readtime(struct device *dev, struct rtc_time *tm)
         /* 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;