From: Chris Packham <Chris.Packham@alliedtelesis.co.nz>
To: Guenter Roeck <linux@roeck-us.net>,
"jdelvare@suse.com" <jdelvare@suse.com>
Cc: "linux-hwmon@vger.kernel.org" <linux-hwmon@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-i2c@vger.kernel.org" <linux-i2c@vger.kernel.org>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
Subject: Re: Errant readings on LM81 with T2080 SoC
Date: Mon, 8 Mar 2021 02:27:08 +0000 [thread overview]
Message-ID: <4a1b1494-df96-2d8c-9323-beb2c2ba706b@alliedtelesis.co.nz> (raw)
In-Reply-To: <96d660bc-17ab-4e0e-9a94-bce1737a8da1@roeck-us.net>
On 8/03/21 1:31 pm, Guenter Roeck wrote:
> On 3/7/21 2:52 PM, Chris Packham wrote:
>> Hi,
>>
>> I've got a system using a PowerPC T2080 SoC and among other things has
>> an LM81 hwmon chip.
>>
>> Under a high CPU load we see errant readings from the LM81 as well as
>> actual failures. It's the errant readings that cause the most concern
>> since we can easily ignore the read errors in our monitoring application
>> (although it would be better if they weren't there at all).
>>
>> I'm able to reproduce this with a test application[0] that artificially
>> creates a high CPU load then by repeatedly checking for the all-1s
>> values from the LM81 datasheet[1](page 17). The all-1s readings stick
>> out as they are obviously higher than the voltage rails that are
>> connected and disagree with measurements taken with a multimeter.
>>
>> Here's the output from my device
>>
>> [root@linuxbox ~]# cpuload 90&
>> [root@linuxbox ~]# (while true; do cat /sys/class/hwmon/hwmon0/in*_input
>> | grep '3320\|4383\|6641\|15930\|3586'; sleep 1; done)&
>> 3586
>> 3586
>> cat: read error: No such device or address
>> cat: read error: No such device or address
>> 3320
>> 3320
>> 3586
>> 3586
>> 6641
>> 6641
>> 4383
>> 4383
>>
>> Fundamentally I think this is a problem with the fact that the LM81 is
>> an SMBus device but the T2080 (and other Freescale SoCs) uses i2c and we
>> emulate SMBus. I suspect the errant readings are when we don't get round
>> to completing the read within the timeout specified by the SMBus
>> specification. Depending on when that happens we either fail the
>> transfer or interpret the result as all-1s.
>>
> That is quite unlikely. Many sensor chips are SMBus chips connected to
> i2c busses. It is much more likely that there is a bug in the T2080 i2c driver,
> that the chip doesn't like the bulk read command issued through regmap, that
> the chip has problems with the i2c bus speed, or that the i2c bus is noisy.
Perhaps something gets upset when interrupt processing is delayed
because of CPU load. I don't see the problem when there isn't a CPU load
so I think that eliminates board issues.
> In this context, the "No such device or address" responses are very suspicious.
> Those are reported by the i2c driver, not by the hwmon driver, and suggest
> that the chip did not respond to a read request. Maybe it helps to enable
> debugging to the i2c driver to see if it reports anything useful. Even
> better might be to connect an i2c bus analyzer to the i2c bus and check
> what is going on.
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.
>
> Guenter
next prev parent reply other threads:[~2021-03-08 2:27 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-07 22:52 Errant readings on LM81 with T2080 SoC Chris Packham
2021-03-08 0:31 ` Guenter Roeck
2021-03-08 2:27 ` Chris Packham [this message]
2021-03-08 4:37 ` Chris Packham
2021-03-08 4:59 ` Guenter Roeck
2021-03-08 20:27 ` Chris Packham
2021-03-08 22:39 ` Guenter Roeck
2021-03-10 2:19 ` Chris Packham
2021-03-10 5:06 ` Guenter Roeck
2021-03-10 21:48 ` Chris Packham
2021-03-11 7:41 ` Guenter Roeck
2021-03-11 8:18 ` Wolfram Sang
2021-03-11 15:19 ` Guenter Roeck
2021-03-11 21:17 ` Chris Packham
2021-03-11 21:34 ` Guenter Roeck
2021-03-11 23:47 ` Chris Packham
2021-03-12 0:07 ` Guenter Roeck
2021-03-12 0:19 ` Chris Packham
2021-03-12 9:25 ` David Laight
2021-03-14 21:26 ` Chris Packham
2021-03-15 9:46 ` David Laight
2021-03-18 5:44 ` Wolfram Sang
2021-03-18 3:46 ` Chris Packham
2021-03-18 4:02 ` Guenter Roeck
2021-03-18 5:39 ` Wolfram Sang
2021-03-08 22:10 ` Chris Packham
2021-03-09 4:36 ` Chris Packham
2021-03-09 5:24 ` Guenter Roeck
2021-03-09 23:35 ` Chris Packham
2021-03-10 3:29 ` Guenter Roeck
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