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

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Message-ID: <20151020135213.20687.7466@quantum>
Date:	Tue, 20 Oct 2015 06:52:13 -0700
From:	Michael Turquette <mturquette@...libre.com>
To:	Guenter Roeck <linux@...ck-us.net>,
	"Marc Titinger" <mtitinger@...libre.com>, jdelvare@...e.com
Cc:	lm-sensors@...sensors.org, linux-kernel@...r.kernel.org,
	bcousson@...libre.com
Subject: Re: [PATCH v2] hwmon: ina2xx: allow for actual measurement bandwidth above
 160 Hz

Hi Guenter,

Quoting Guenter Roeck (2015-10-20 06:30:03)
> On 10/20/2015 06:17 AM, Marc Titinger wrote:
> > 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.
> >
> That is an invasive change, though, not just a simple read. Sure, it is
> a risk as well. But it is a different level of risk than someone using
> i2cget or i2cdump while the driver is running.
> 
> >>
> >>> 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.
> >
> 
> Sorry, you lost me there. How are you twisting the hwmon interface ?
> Because I am concerned about multiple accesses from multiple sources ?
> How is iio going to solve that problem ? By ignoring it ?

There is no twisting going on here :-)

We're going to investigate IIO with or without the multi-client/regmap
issues raised in this thread. The purpose is to investigate if we can
get better performance/higher sample rate for our application (the ACME
power measurement device).

> 
> Not that I mind if you want to convert the driver to iio. One less driver
> to take care of. Just asking.

Yup, it's orthogonal to the issues raised here.

Regards,
Mike

> 
> Thanks,
> Guenter
> 
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