linux-kernel - Re: Errant readings on LM81 with T2080 SoC

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Date:   Wed, 17 Mar 2021 21:02:00 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Chris Packham <Chris.Packham@...iedtelesis.co.nz>,
        Wolfram Sang <wsa@...nel.org>
Cc:     "jdelvare@...e.com" <jdelvare@...e.com>,
        "linux-hwmon@...r.kernel.org" <linux-hwmon@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: Errant readings on LM81 with T2080 SoC

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.

Guenter