[U-Boot] [PATCH] i2c: mvtwsi: Fix problem with baud rate calculation

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 17-03-15 12:15, Hans de Goede wrote:
> Hi,
>
> On 17-03-15 11:08, Stefan Roese wrote:
>> The current implementation for baudrate calculation is incorrect.
>> This part from the formula:
>>
>> "2 ^ (n + 1)" is not equivalent to (1 << n) but to (2 << n)!
>>
>> This patch fixes this and moves this calculation to a function instead of using a macro.
>
> Hmm, this does not match with what the Allwinner datasheets say:
> https://github.com/allwinner-zh/documents/blob/master/A20/A20%20user%20manual%20v1.3%2020141010.pdf
>
> They say:
>
> Fsamp = F 0 = Fin / 2^CLK_N
> F1 = F0 / (CLK_M + 1)
> Foscl = F1 / 10 = Fin / (2^CLK_N * (CLK_M + 1)*10)
>
> With Foscl being the ultimate i2c speed. Notice that they are talking about
> 2 ^ CLK_N not 2 ^ (CLK_N + 1)
>
> And they have a few examples which match this. Now it could be that a
> register value of 0 means CLK_N = 1, reg 1 CLK_N = 2, etc. this is not clearly
> specified ...
>
>> This new function is taken from the Linux kernel.
>
> Interesting, because on Allwinnner / sunxi devices we are using the kernel
> driver formula unmodified, and things seem to work fine despite this. Could
> be tolerances allowing this, could be the Allwinner datasheet being unclear.
>
> I've send allwinner a mail asking them to clarify this.

I've just gotten an answer from allwinner, and the formula in their datasheet
is correct, with the raw register value == CLK_N so the current (before this patch)
u-boot code is correct for Allwinner devices and we need a #ifdef here to keep the
old formula for sunxi devices, you can use:

#ifdef CONFIG_SUNXI

For this.

Regards,

Hans