linux-kernel - Re: [PATCH] RTC: RK808: fix the rtc time reading issue

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Date:	Wed, 14 Jan 2015 14:53:07 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Sonny Rao <sonnyrao@...omium.org>
Cc:	Doug Anderson <dianders@...omium.org>,
	Chris Zhong <zyw@...k-chips.com>,
	Heiko Stübner <heiko@...ech.de>,
	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	rtc-linux@...glegroups.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] RTC: RK808: fix the rtc time reading issue

On Wed, 14 Jan 2015 11:44:02 -0800 Sonny Rao <sonnyrao@...omium.org> wrote:

> On Wed, Jan 14, 2015 at 10:36 AM, Doug Anderson <dianders@...omium.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,
_

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