Re: Errant readings on LM81 with T2080 SoC

From: Guenter Roeck
Date: Thu Mar 18 2021 - 00:04:39 EST

On 3/17/21 8:46 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.
> I've made some reasonable progress on making i2c-mpc more interrupt
> driven. Assuming it works out for my use-case is there an opinion on
> making interrupt support mandatory? Looking at all the in-tree dts files
> that use one of the compatible strings from i2c-mpc.c they all have
> interrupt properties so in theory nothing is using the polling mode. But
> there may be some out-of-tree boards or boards using an old dtb that
> would be affected?

The polling code is from pre-git times. Like 2005 and earlier.
I'd say it is about time to get rid of it. Any out-of-tree users
had more than 15 years to upstream their code, after all.