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Date: Thu, 31 Jul 2008 15:35:36 -0500
From: Timur Tabi <timur@freescale.com>
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To: Jon Smirl <jonsmirl@gmail.com>
Subject: Re: [PATCH] powerpc: i2c-mpc: make speed registers configurable via
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Cc: Scott Wood <scottwood@freescale.com>, Linux I2C <i2c@lm-sensors.org>,
	Linuxppc-dev@ozlabs.org
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Jon Smirl wrote:

> For mpc5200 it is easy, mpc52xx_find_ipb_freq()  already exists to get
> the source clock for the i2c devices. Each i2c node in the device tree
> can then specify "clock-frequency = 400000;" or let it default. You

400KHz is the *speed* of the I2C bus, so let's be sure to use "speed" in the
property name.  Reserve the word "bus" for the input clock to the I2C device.

> #if defined(CONFIG_MPC8540) || defined(CONFIG_MPC8541) || \
>        defined(CONFIG_MPC8560) || defined(CONFIG_MPC8555)
>        gd->i2c1_clk = sys_info.freqSystemBus;
> #elif defined(CONFIG_MPC8544)
>        /*
>         * On the 8544, the I2C clock is the same as the SEC clock.  This can be
>         * either CCB/2 or CCB/3, depending on the value of cfg_sec_freq. See
>         * 4.4.3.3 of the 8544 RM.  Note that this might actually work for all
>         * 85xx, but only the 8544 has cfg_sec_freq, so it's unknown if the
>         * PORDEVSR2_SEC_CFG bit is 0 on all 85xx boards that are not an 8544.
>         */
>        if (gur->pordevsr2 & MPC85xx_PORDEVSR2_SEC_CFG)
>                gd->i2c1_clk = sys_info.freqSystemBus / 3;
>        else
>                gd->i2c1_clk = sys_info.freqSystemBus / 2;
> #else
>        /* Most 85xx SOCs use CCB/2, so this is the default behavior. */
>        gd->i2c1_clk = sys_info.freqSystemBus / 2;
> #endif
>        gd->i2c2_clk = gd->i2c1_clk;

I think the whole point is to eliminate duplicating this code in the Linux
driver.  It's hideously ugly, so it should be limited as much as possible.

-- 
Timur Tabi
Linux kernel developer at Freescale