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

[Marc Titinger]

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
> > registers,
> > 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
> through
> 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
> > for
> > four measurements around (1/(4*800us) = 312 Hz. Since for any readout
> > rate
> > faster than 160 Hz the interval is set by the i2c transactions
> > completion,
> > 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
> > driver
> > 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
> caching,
> 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.
Agreed.

> 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.

Cheers,
Marc.


> Guenter

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