Errant readings on LM81 with T2080 SoC

[Chris Packham]

On 12/03/21 1:07 pm, Guenter Roeck wrote:
> On 3/11/21 3:47 PM, Chris Packham wrote:
>> On 12/03/21 10:34 am, Guenter Roeck wrote:
>>> On 3/11/21 1:17 PM, Chris Packham wrote:
>>>> On 11/03/21 9:18 pm, Wolfram Sang wrote:
>>>>>> Bummer. What is really weird is that you see clock stretching under
>>>>>> CPU load. Normally clock stretching is triggered by the device, not
>>>>>> by the host.
>>>>> One example: Some hosts need an interrupt per byte to know if they
>>>>> should send ACK or NACK. If that interrupt is delayed, they stretch the
>>>>> clock.
>>>>>
>>>> It feels like something like that is happening. Looking at the T2080
>>>> Reference manual there is an interesting timing diagram (Figure 14-2 if
>>>> someone feels like looking it up). It shows SCL low between the ACK for
>>>> the address and the data byte. I think if we're delayed in sending the
>>>> next byte we could violate Ttimeout or Tlow:mext from the SMBUS spec.
>>>>
>>> I think that really leaves you only two options that I can see:
>>> Rework the driver to handle critical actions (such as setting TXAK,
>>> and everything else that might result in clock stretching) in the
>>> interrupt handler, or rework the driver to handle everything in
>>> a high priority kernel thread.
>> One thing I've found that does seem to avoid the problem is to disable
>> preemption, use polling and replace the schedule() in i2c_wait() with
>> udelay(50). That's kind of like the kernel thread option.
> It is kind of hackish, though, especially since it makes the "loaded system"
> situation even worse by adding even more active wait loops.
No -ish about it :). But it might put out one fire for me while I'm 
looking at doing some kind of interrupt driven state machine.