Messages in this thread | | | Subject | Re: [PATCH v2] hwmon: ina2xx: allow for actual measurement bandwidth above 160 Hz | From | Guenter Roeck <> | Date | Tue, 20 Oct 2015 06:30:03 -0700 |
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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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