diff options
| author | Chris Zhong <zyw@rock-chips.com> | 2015-02-13 14:40:54 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2015-02-13 21:21:43 -0800 |
| commit | c412c6034aaf38ff718296f3b77cb8e76b541985 (patch) | |
| tree | 8714c27538a638e3a927609b9a6d4bf195aa7cef /drivers/rtc/rtc-rk808.c | |
| parent | 1ef2816f26a1d962d6317f7545218fa2ae3643bb (diff) | |
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/rtc-rk808.c')
| -rw-r--r-- | drivers/rtc/rtc-rk808.c | 10 |
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; |