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

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

Hi,

On 17-03-15 12:25, Stefan Roese wrote:
> Hi Hans,
>
> On 17.03.2015 12:15, Hans de Goede wrote:
>> 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.
>
> Good.
>
> Here the complete formula from the A38x manual:
>
> F_SCL = F_TCLK / (10 * (M + 1) * 2 ^ (N + 1))
>
>>> This was detected and tested on the Marvell Armada A38x DB-88F6820-GP
>>> eval board.
>>
>> So I take it you connected a memory oscilloscope to the i2c wires ?
>
> No. I noticed that I2C doesn't work on this board with the current code. And checked for differences to the Linux code. And with this change, I2C does work on this board.

Ok (note neither have I measured the actual speed on Allwinner devices),
lets wait a bit to see what Allwinner has to say.

Regards,

Hans