From c412c6034aaf38ff718296f3b77cb8e76b541985 Mon Sep 17 00:00:00 2001 From: Chris Zhong Date: Fri, 13 Feb 2015 14:40:54 -0800 Subject: drivers/rtc/rtc-rk808.c: fix rtc time reading issue MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Doug Anderson Cc: Sonny Rao Cc: Heiko Stübner Cc: Alessandro Zummo Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- drivers/rtc/rtc-rk808.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) (limited to 'drivers') 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; -- cgit v1.2.3-58-ga151