linux-kernel - [IIO] Proposal for sysfs attributes

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Message-ID: <4C84A93B.2050507@iis.fraunhofer.de>
Date:	Mon, 06 Sep 2010 10:41:31 +0200
From:	Manuel Stahl <manuel.stahl@....fraunhofer.de>
To:	LKML <linux-kernel@...r.kernel.org>
CC:	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>
Subject: [IIO] Proposal for sysfs attributes

Hi all,

there were some sysfs userspace questions on linux-iio where we would 
like to get some more comments on.

First thing, there are some attributes describing the layout of a ring 
buffer that can be read out via a char device. The buffer contains 
several entries with each containing so called scan elements. All 
entries have the same layout. It is described in a directory 
scan_elements (see below) that contains three attributes per scan element.

> OK, no packed buffers for now, but we should implement variable
 > sample sizes for standard types. Indeed we already have this for
 > the timestamp, which is always 64 bit.
>
> To be compatible with future extensions we could have:
>    |- /sys/bus/iio/device0/buffer0/scan_elements/
>       |- accel_x_en    (0 or 1)
>       |- accel_x_type  (i.e. s14/16, see *)
>       |- accel_x_index (position inside the buffer entry)
>
> * s14/16 means signed 14 bits, stored in 16 bits, right aligned.
 >   If it's left aligned we can just modify the scale attribute and
 >   give the 16 bit interpretation in <channel>_raw.

Is the 's14/16' self explaining or should we use some other format?
Is 'type' a good postfix for the attribute?

Next question: how strictly we want to resamble the hwmon ABI?

To sum up the discussion:
Hwmon has files with the postfix _input to read values scaled to
reasonable units for fixed point representation. So the units are
sometimes scaled down to 'millis' e.g. millivolt, millidegree Celsius.
We agreed that we want to keep this, whenever we use the _input postfix.
For IIO there is also the postfix triple _raw, _scale and _offset,
where the final value is calculated by (_raw + _offset) * _scale.
Floating point values are allowed for any of these files.

The question in place was:
Do we want to resamble the 'milli' units or should we stick to
standard SI units (radians, kelvin, etc.) as floating point math is
necessary anyway.

Regards,
-- 
Dipl.-Inf. Manuel Stahl
Fraunhofer-Institut für Integrierte Schaltungen IIS
- Leistungsoptimierte Systeme -
Nordostpark 93                Telefon  +49 (0)911/58061-6419
90411 Nürnberg                Fax      +49 (0)911/58061-6398
http://www.iis.fraunhofer.de  manuel.stahl@....fraunhofer.de

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