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authorChris Zhong <zyw@rock-chips.com>2015-02-13 14:40:54 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2015-02-13 21:21:43 -0800
commitc412c6034aaf38ff718296f3b77cb8e76b541985 (patch)
tree8714c27538a638e3a927609b9a6d4bf195aa7cef /drivers/rtc
parent1ef2816f26a1d962d6317f7545218fa2ae3643bb (diff)
downloadlinux-c412c6034aaf38ff718296f3b77cb8e76b541985.tar.bz2
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>
Diffstat (limited to 'drivers/rtc')
-rw-r--r--drivers/rtc/rtc-rk808.c10
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;