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

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Message-ID: <56263973.300@roeck-us.net>
Date:	Tue, 20 Oct 2015 05:54:11 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Marc Titinger <mtitinger@...libre.com>, jdelvare@...e.com
Cc:	lm-sensors@...sensors.org, linux-kernel@...r.kernel.org,
	bcousson@...libre.com, mturquette@...libre.com
Subject: Re: [PATCH v2] hwmon: ina2xx: allow for actual measurement bandwidth
 above 160 Hz

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.

> 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.

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

Guenter

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