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

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Date:   Mon, 8 Mar 2021 20:27:30 +0000
From:   Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To:     Guenter Roeck <linux@...ck-us.net>,
        "jdelvare@...e.com" <jdelvare@...e.com>
CC:     "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 8/03/21 5:59 pm, Guenter Roeck wrote:
> On 3/7/21 8:37 PM, Chris Packham wrote:
> [ ... ]
>>> That's from -ENXIO which is used in only one place in i2c-mpc.c. I'll
>>> enable some debug and see what we get.
>> For the errant readings there was nothing abnormal reported by the driver.
>>
>> For the "No such device or address" I saw "mpc-i2c ffe119000.i2c: No
>> RXAK" which matches up with the -ENXIO return.
>>
> Id suggest to check the time until not busy and stop in mpc_xfer().
> Those hot loops are unusual, and may well mess up the code especially
> if preempt is enabled. Also, are you using interrupts or polling in
> your system ?
I'm using interrupts but I see the same issue if I comment out the 
interrupts in the dtsi file (i.e. force it to use polling).
> The interrupt handler looks a bit odd, with "Read again
> to allow register to stabilise".

Yeah that stuck out to me too. The code in question predates git, I went 
spelunking in history.git and the "Read again" seems to be in the 
initial version[0]. I did try to alter the interrupt handler so that it 
only does one read but that didn't seem to change anything.

> Do you have fsl,timeout set in the devicetree properties and, if so,
> have you played with it ?
Haven't got it set but I'll have a go at tweaking it.
> Other than that, the only other real idea I have would be to monitor
> the i2c bus.
I am in the fortunate position of being able to go into the office and 
even happen to have the expensive scope at the moment. Now I just need 
to find a tame HW engineer so I don't burn myself trying to attach the 
probes.

-- 

[0] - https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=11b3235dc04a306f6a9ba14c1ab621b2d54f2c56