linux-kernel - Re: [PATCH v8 2/3] I2C: mediatek: Add driver for MediaTek I2C controller

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Message-ID: <20150520083335.GQ24769@pengutronix.de>
Date:	Wed, 20 May 2015 10:33:35 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Eddie Huang <eddie.huang@...iatek.com>
Cc:	Mark Rutland <mark.rutland@....com>,
	Xudong Chen <xudong.chen@...iatek.com>,
	srv_heupstream@...iatek.com, Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Wolfram Sang <wsa@...-dreams.de>,
	Liguo Zhang <liguo.zhang@...iatek.com>,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	Rob Herring <robh+dt@...nel.org>,
	linux-mediatek@...ts.infradead.org, linux-i2c@...r.kernel.org,
	Sascha Hauer <kernel@...gutronix.de>,
	Kumar Gala <galak@...eaurora.org>,
	Matthias Brugger <matthias.bgg@...il.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v8 2/3] I2C: mediatek: Add driver for MediaTek I2C
 controller

Hello Eddie,

On Wed, May 20, 2015 at 03:59:35PM +0800, Eddie Huang wrote:
> On Wed, 2015-05-20 at 09:11 +0200, Uwe Kleine-König wrote:
> > On Wed, May 20, 2015 at 10:40:11AM +0800, Eddie Huang wrote:
> > > On Mon, 2015-05-18 at 20:43 +0200, Uwe Kleine-König wrote:
> > > > On Tue, May 19, 2015 at 12:40:08AM +0800, Eddie Huang wrote:
> > > > > +/* calculate i2c port speed */
> > > > It would be nice to summarize the clock frequency settings here.
> > > > Something like:
> > > > 
> > > > 	/*
> > > > 	 * The input clock is divided by the value specified in the
> > > > 	 * device tree as clock-div. The actual bus speed is then
> > > > 	 * derived from this frequency by the following formula:
> > > > 	 *  ....
> > > > 
> > > > This would make it possible to verify your calculations below.
> > > 
> > > The comment will be:
> > > /*
> > >  * khz: I2C bus clock
> > >  * hclk: The input clock is divided by the value specified in the 
> > >  *       device tree as clock-div
> > and which one of the two clocks you're writing about is hclk now? I
> > assume the divided one.
> > >  * div = (sample_cnt + 1) * (step_cnt + 1)
> > >  * khz = (hclk / 2) / div
> > khz for the 2nd time.
> > 
> > >  *
> > >  * The calculation is to get div value that let result of 
> > >  * ((hclk / 2) / div) most approach and less than khz
> > >  */
> > I imagined something more hardware related. A list of register (or
> > register bit fields) that influence the frequency and a formula
> 
> > 	i2c_freq = parent_clk / clock-div * (...)
> > 
> > (It seems to be a bit more complicated here as there are two registers
> > involved that are set differently depending on the target frequency.)
> 
> Yes, hardware is a little complicated. I rewrite the comment:
> 
> /*
>  * Calculate i2c port speed
>  *
>  * Hardware design:
>  * i2c_bus_freq = parent_clk / (clock-div * 2 * (sample_cnt) *
> (step_cnt))
>  * clock-div: fixed in hardware, but may be various in different SoCs
... fixed in hardware, but different on different SoCs.

>  *
>  * The calculation want to pick the highest bus frequency that is still 
>  * less than or equal to i2c->speed_hz. and the calculation try to get 
>  * sample_cnt and step_cnt to fill in hardware register.
"The calculation picks the highest bus frequency that is still less than
or equal to i2c->speed_hz." and I'd drop the last sentence.

With that in mind I'll reply once more to the original patch.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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