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

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Message-ID: <CAD=FV=UDj08jkOCDVJ1Q+EanrBHwP_-88XMXojPjY8iB3i-AQA@mail.gmail.com>
Date:	Wed, 14 Jan 2015 10:36:59 -0800
From:	Doug Anderson <dianders@...omium.org>
To:	Sonny Rao <sonnyrao@...omium.org>
Cc:	Chris Zhong <zyw@...k-chips.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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

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.

-Doug
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