Errant readings on LM81 with T2080 SoC

[Chris Packham]

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.
> Guenter