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

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Date:   Thu, 11 Mar 2021 21:17:43 +0000
From:   Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To:     Wolfram Sang <wsa@...nel.org>, Guenter Roeck <linux@...ck-us.net>
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 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.