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

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Message-ID: <20150520071152.GP24769@pengutronix.de>
Date:	Wed, 20 May 2015 09:11:52 +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 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.)

> > > +static int mtk_i2c_set_speed(struct mtk_i2c *i2c, unsigned int clk_src_in_hz)
clk_src_in_hz is the module's input rate already divided by clock-div.
This clock-div value is fixed in hardware and unchangeable, right?
Maybe give that divided clock a nice name?

The target frequency is i2c->speed_hz, so among the possible frequencies
we want to pick the highest one that is still less than or equal
i2c->speed_hz, right?

> > > +		/* Set the hign speed mode register */
I just notice s/hign/high/ here.

Best regards
Uwe

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