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

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

Hi,

On 18-03-15 09:20, Hans de Goede wrote:
> 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:

p.s.

This means the kernel currently does the wrong thing on Allwinner, I've put fixing
this on my todo list.

Regards,

Hans