[U-Boot] [PATCH 1/4] OMAP3 I2C Fix the sampling clock.

[Menon, Nishanth]

> -----Original Message-----
> From: Tom Rix [mailto:Tom.Rix at windriver.com]
> Sent: Wednesday, June 10, 2009 7:54 AM
> 
>  void i2c_init (int speed, int slaveadd)
>  {
> -	u16 scl;
> +	int psc, iclk, scll, sclh;
> +
> +	/* Only handle standard and fast speeds */
> +	if ((speed != OMAP_I2C_STANDARD) &&
> +	    (speed != OMAP_I2C_FAST_MODE)) {
> +		printf("Error : I2C unsupported speed %d\n", speed);
> +		return;
> +	}
> +
> +	/*
> +	 * Calculate the prescalar to go from from the function clock
> +	 * to the internal sampling clock, 12MHz.
> +	 */
> +	psc = I2C_PSC_MAX;
> +	while (psc >= I2C_PSC_MIN) {
> +		iclk = I2C_IP_CLK / (psc + 1);
> +		if (12000000 <= iclk)
> +			break;
> +		psc--;
> +	}
> +	if (psc < I2C_PSC_MIN) {
> +		printf("Error : I2C unsupported prescalar %d\n", psc);
> +		return;
> +	}
> +
> +	/*
> +	 * How the low and high time periods are calculated
> +	 * See the OMAP3xxx Reference Manual for more details
> +	 *
> +	 * tlow + thigh = 1 / speed
> +	 * thigh = tlow, nice square wave..
> +	 *
> +	 * tlow = 1 / (2 * speed) = (scll + 7) / iclk;
> +	 * scll + 7 = iclk / 2 * speed
> +	 * sclh + 5 = iclk / 2 * speed
> +	 */
> +	scll = sclh = iclk / (2 * speed);
> +	scll -= 7;
> +	sclh -= 5;
> +	if ((scll < 0) || (sclh < 0)) {
> +		printf("Error : I2C initializing clock\n");
> +		return;
> +	}
> 
>  	writew(0x2, I2C_SYSC); /* for ES2 after soft reset */
>  	udelay(1000);
> @@ -42,12 +84,10 @@ void i2c_init (int speed, int slaveadd)
>  		udelay (50000);

This is a repeat story of what happened in linux-omap and kernel. We had a similar discussion in [1] and related patch [2] to change equations. I have the same reservations with this patch:
a) using speed as default does not scale for all board combinations.
b) need flexible option to provide scll and sclh on a platform basis.
Regards,
Nishanth Menon
Ref:
[1] http://marc.info/?t=123540865900002&r=1&w=2
[2] http://marc.info/?l=linux-omap&m=122770723311340&w=2