Re: [PATCH v2] hwmon: ina2xx: allow for actual measurement bandwidth above 160 Hz

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On 20/10/2015 14:54, Guenter Roeck wrote:
On 10/20/2015 01:20 AM, Marc Titinger wrote:
With the current implementation, the driver will prevent a readout at a
pace faster than the default conversion time (2ms) times the averaging
setting, min AVG being 1:1.

Any sysfs "show" read access from the client app faster than 500 Hz
will be
'cached' by the driver, but actually since do_update reads all 8
the best achievable measurement rate is roughly 8*800 us (for the time
spent in i2c-core) i.e. <= 156Hz with Beagle Bone Black.

This change set uses a register mask to allow for the readout of a single
i2c register at a time. Furthermore, performing subsequent reads on the
same register will make use of the ability of the i2c chip to retain the
last reg offset, hence use a shorter i2c message (roughly 400us
instead of
800us spent in i2c-core.c).

That doesn't work. There could be accesses from other sources (such as
i2c-dev, or in multi-master systems) between two reads.

Re-setting the register address with each read transaction will not prevent another master to change the configuration in your back, in this case. That sounds like a general issue of concurrent clients for one device, this is beyond just reading one register IMO.

The best readout rate for a single measurement is now around 2kHz. And
four measurements around (1/(4*800us) = 312 Hz. Since for any readout
faster than 160 Hz the interval is set by the i2c transactions
the 'last-update' anti-flooding code will not have a limiting effect in
practice. Hence I also remove the elapsed time checking in the hwmon
for ina2xx.

To summarize, the patch provides a max bandwidth improvement with hwmon
client apps from ~160 Hz to ~320 Hz, and better in single-channel
polling mode.

Overall your patch pretty much re-implements regmap. Since you drop
it is also unnecessary to read all registers at a time, so you can just use
a function to read _one_ register and returns its value (with retries).
Or use regmap. Either case, do_update() and ina2xx_update_device() are no
longer needed.

If you want to convert the driver to regmap, just look for 'regmap' in
drivers/hwmon for examples.

Fair enough, but based on your comments, I may look into an iio driver instead for this device, given our application, rather than 'twisting' the hwmon interface.



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