On 3/8/21 8:36 PM, Chris Packham wrote:

On 9/03/21 11:10 am, Chris Packham wrote:

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On 8/03/21 5:59 pm, Guenter Roeck wrote:

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On 3/7/21 8:37 PM, Chris Packham wrote:
[ ... ]

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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