Re: [PATCH v1 0/7] iio: adc: ina2xx: Rework CNVR alignment, fix busy loops

[Stefan Brüns]

On Sunday, December 10, 2017 6:36:54 PM CET Jonathan Cameron wrote:
> On Fri, 8 Dec 2017 18:41:45 +0100
> 
> Stefan Brüns <stefan.bruens@xxxxxxxxxxxxxx> wrote:
> > Currently, the INA2xx driver may end up causing 100% load on a single core
> > and fully loading the I2C bus, which is caused by two different issues:
> > 
> > The code uses a udelay to bridge the gab between two subsequent samples.
> > As the sampling interval may be up to 16 seconds, the CPU is busy
> > waiting most of the time.
> > 
> > The second issue manifests when using the (default) "synchronous" mode.
> > The code polls for a set conversion ready flag, but fails to align the
> > sampling interval to the raising flag. The polling interval is
> > (rightfully) slighly shorter than the sampling interval, so after some
> > samples the sampling thread is continously polling.
> 
> I'm confused.  Would you mind doing an asci art example perhaps?

Lets assume the conversion interval is set to 2 ms. If the polling is done at 
the sampling frequency, it might be slightly to long due to differences 
between the host and device clocks, so the polling has to run somewhat faster. 
Somewhat faster means 200 us, this is kept unchanged.

In case the CNVR flag is not set, the status register is read again until the 
flag raises. The time instant the flag raises is the reference for later 
reads.

The following shows the timing for the fixed code. Each character corresponds 
to 200us, first row: real time (ms), second row: conversion finished by 
device, third row: register read
s: status, CNVR not set
S: status, CNVR set
S: value register, e.g. shunt voltage
_: bus busy (each reads needs 400 us @ 100 kBit/s)

__0____1____2____3____4____5____6____7____8____9
...C.........C.........C.........C.........C....
..s_S_V_.....S_V_......S_V_....s_S_V_......S_V_.

At 0 ms, the conversion has not yet finished, but a 0.4 it has. Further reads 
are done at 0.4 + n * (2 - 0.2) ms. The next read happens at 2.2 ms, the third 
should at 4.0, but happens slightly late at 4.2. The read at 5.8 gets an unset 
CNVR flag, so the sampling is readjusted to happen at 6.2 + n' (2 - 0.2) ms.

The old code does the following:
__0____1____2____3____4____5____6____7____8____9
...C.........C.........C.........C.........C....
..s_S_V_...s_S_V_...s_s_S_V_.s_s_S_V_.s_s_s_S_V_

The first read happens at 0 ms, it measures the time for the reading (1.2 ms), 
sleeps for the remainder (0.6 ms) and reads again. The third read takes 1.6, 
so sleep for 0.2 ms.

The old code does not differentiate between time spent in the status poll and 
time spent for reading. Time spent in the status poll should not be subtracted 
from the delay until the next read (well, halfway, only the time spent with 
the poll returning !CNVR).

Kind regards,

Stefan

-- 
Stefan Brüns  /  Bergstraße 21  /  52062 Aachen
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