Re: [PATCH v2] hwmon: ina2xx: allow for actual measurement bandwidth above 160 Hz
From: Guenter Roeck
Date: Tue Oct 20 2015 - 08:54:19 EST
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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/