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

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Message-ID: <6cb6b5f2-52a3-c7c3-9a10-7dd0f86414a8@roeck-us.net>
Date:   Mon, 8 Mar 2021 21:24:59 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Chris Packham <Chris.Packham@...iedtelesis.co.nz>,
        "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 3/8/21 8:36 PM, Chris Packham wrote:
> 
> On 9/03/21 11:10 am, Chris Packham wrote:
>>
>> 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.
>> Reworking those loops seems to have had a positive result. I'll do a 
>> bit more testing and hopefully get a patch out later today.
> D'oh my "fix" was to replace the cond_reshed() with msleep(10) which did 
> "fix" the problem but made every i2c read slow. I didn't notice when 
> testing just the lm81 but as soon as I booted the system with more i2c 
> devices I saw stupidly slow boot times.

msleep() is indeed a bad idea. You'd want something like usleep_range()
with increasing timeout. Like start with a few uS and double the sleep time
with each iteration (eg 4-8 / 8-16 / 16-32 / 32-64 / ...).

Guenter