Date:	Tue, 20 Oct 2015 06:30:03 -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 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 ?

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

Thanks,
Guenter

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