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

[Nikita Kiryanov <nikita@compulab.co.il>]

On 11/08/2013 11:26 PM, Lubomir Popov wrote:
> Hi Nikita,
>
>> On 11/06/2013 03:19 PM, Lubomir Popov wrote:
>>> 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.
>>>
>>> Regards,
>>> Lubo
>>>
>>>
>>
>> Thanks for the quick reply.
>> The reason I stumbled across this is that this check hasn't been working
>> well on our OMAP3 based devices. Some I2C transactions work fine, but
>> some of them fail this check in the address phase, especially if the I2C
>> transactions happen in quick successions.
> You mean that you occasionally get this error on an otherwise properly
> configured and working bus, right?

Yes.

> Does this happen with particular
> slave devices only, or randomly? And is it happening in the SPL, or in
> regular U-Boot?

It happens in U-Boot when communicating with the PMIC (TPS65930),
but like I said, it worked in the driver's previous version, on the
same module.

>
>
>> We did not have any I2C issues
>> with the previous driver, and while this behavior is symptomatic of
>> timing issues playing around with the delays didn't help.
> Which delays did you modify? Did you try to increase I2C_WAIT/I2C_TIMEOUT?

I tried to increase both I2C_WAIT and I2C_TIMEOUT, and place my own
delays as well.

>
>> I was wondering if you might have some insights as to what may cause the
>> controller to behave this way other than unconfigured pads?
> Unfortunately I don't have any hands-on experience with OMAP3 (apart from
> some very quick testing on a AM3359 derivative), and cannot guarantee that
> the I2C controller IP on OMAP3 is absolutely the same as on OMAP4/5 (most
> probably it isn't; shall check this on Monday). Anyway, if everything else
> is OK (muxmode/pad config, pull-ups, power, etc.), the only reasonable
> explanation would be that there is not enough time for the controller to
> update its status register under certain conditions, and these would be
> either a slower clock rate (that makes byte transmission time come close
> to the timeout), or clock stretching by some slaves; btw, the latter
> seems probable, judging from your words that this happens in the address
> phase, when some devices could take longer to prepare for action, and they
> do this by stretching the clock. That is why I'm asking if you tried to
> increase I2C_TIMEOUT. Did you do any measurements on the bus to see if all
> is OK and the clock rate is right, and if it gets stretched by some slaves?

I believe I found the cause of the problem. In the new version of the
driver the following line was added to the exit sequence of i2c_write:

writew(0, &i2c_base->cnt);

Removing this line solved the problem (module has been doing I2C
transactions for the last 16 hours or so without failing), and I
believe the reason has to do with Advisory 1.2 in the DM3730 errata:

Advisory 1.2		I2C Module Does Not Allow 0-Byte Data Requests
Revision(s) Affected	1.2, 1.1 and 1.0
Details			When configured as the master, the I2C module
			does not allow 0-byte data transfers.
			Programming I2Ci.I2C_CNT[15:0]: DCOUNT = 0 will
			cause undefined behavior.
Workaround(s)		There is no workaround for this issue. Do not
			use 0-byte data requests.

While the mentioned write is done at the end of i2c_write, perhaps the
driver's MO still triggers this issue. It certainly is plausible
that this line was missing from the old i2c_write exit sequence on
purpose, and not by accident (i2c_read, i2c_probe, and i2c_init all
had it in the old driver).

>
> Regards,
> Lubo
>


-- 
Regards,
Nikita.