lkml.org 
[lkml]   [2015]   [Jan]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH] RTC: RK808: fix the rtc time reading issue
From
Andrew,

On Wed, Jan 14, 2015 at 2:53 PM, Andrew Morton
<akpm@linux-foundation.org> wrote:
> On Wed, 14 Jan 2015 11:44:02 -0800 Sonny Rao <sonnyrao@chromium.org> wrote:
>
>> On Wed, Jan 14, 2015 at 10:36 AM, Doug Anderson <dianders@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,

Looks great to me, thanks.

-Doug


\
 
 \ /
  Last update: 2015-01-15 00:21    [W:0.380 / U:0.040 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site