[PATCH] RTC: RK808: fix the rtc time reading issue
akpm at linux-foundation.org
Wed Jan 14 14:53:07 PST 2015
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:
@@ -73,10 +73,11 @@ static int rk808_rtc_readtime(struct dev
- /* 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,
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