Errant readings on LM81 with T2080 SoC

[Guenter Roeck]

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