[U-Boot] A question about unconfigured pads check in omap24xx_i2c

[Lubomir Popov <lpopov@mm-sol.com>]

Hi Heiko,

On 07-Nov-13 7:14, Heiko Schocher wrote:
> Hello Lubomir,
>
> Am 06.11.2013 14:19, schrieb Lubomir Popov:
>> On 06-Nov-13 14:12, Nikita Kiryanov wrote:
>>> In drivers/i2c/omap24xx_i2c.c there are a few checks that attempt to
>>> detect unconfigured pads for the i2c bus in use. These checks are
>>> all in the form of
>>>
>>> if (status == I2C_STAT_XRDY) {
>>> printf("unconfigured pads\n");
>>> return -1;
>>> }
>>>
>>> This check seems peculiar to me since the meaning of I2C_STAT_XRDY is
>>> that new data is requested for transmission. Why is that indication 
>>> that
>>> the bus is not padconf'd for I2C?
>> Hi Nikita,
>>
>> This has been empirically confirmed on OMAP4 and OMAP5. When the pads 
>> are not
>> configured, the I2C controller is actually disconnected from the bus. 
>> The clock
>> input for its state machine has to come from the bus however due to 
>> stretching
>> etc., although it is internally generated. So actually nothing 
>> changes within
>> the controller after a transaction attempt is made, and it keeps its 
>> initial
>> state with XRDY set only (ready to accept transmit data). I use this 
>> as an
>> indicator. Not perfect, but works in most cases.
>
> Thanks for this explanation! Maybe we can document this somewhere in
> the code?
>
> bye,
> Heiko
You are right, it would be good to document it. Unfortunately I have not
been on the U-Boot wave for some months now due to very heavy engagement
with other stuff; have even unsubscribed from the list. I think however
that in order to give definite statements and improve the driver, a new
round of experiments has to be made to cover the two major types of design
flaws (software and/or hardware): incorrect pad configuration, and missing
pullups (internal or external). I wrote this driver more that 6 months ago
with the goal to have something working properly (the then existing one was,
mildly put, not good), and this detection is just a bonus side effect.

In summary, the professional approach would require some more effort, which
I'm not sure when I could afford. Otherwise, if just an explanation for the
current algo is to be given, no problem - I can just add some comments.

What do you think?

Regards,
Lubo