On Wed, 14 Jan 2015 11:44:02 -0800 Sonny Rao <sonnyrao at chromium.org> wrote: > On Wed, Jan 14, 2015 at 10:36 AM, Doug Anderson <dianders at chromium.org> wrote: > > Sonny, > > > >> Chris, it looks like you swapped the set and the clear of this bit, > >> and you're relying on the fact that the i2c transaction takes a > >> certain amount of time after the RTC_GET_TIME BIT is set. I'm not > >> sure how long it actually takes, but why not just put in a usleep() > >> for the minimum wait time? > > > > 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 > > > > Personally I think what Chris has is fine, with the comment. > > Ok, I'm fine with that if we're sure it's slow enough. Comment > explaining would certainly help. Thanks, all. I did this: --- a/drivers/rtc/rtc-rk808.c~rtc-rk808-fix-the-rtc-time-reading-issue-fix +++ a/drivers/rtc/rtc-rk808.c @@ -73,10 +73,11 @@ static int rk808_rtc_readtime(struct dev return ret; } - /* After we set the GET_TIME bit, the rtc time couldn't be read - * immediately, we should wait up to 31.25 us, about one cycle of + /* + * 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. + * 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, _
