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How to write applications which use our drivers
-----------------------------------------------
You have several choices in accessing sensor devices using the
drivers in our package. This document will briefly
describe these methods, their advantages and disadvantages,
and provide examples.
From lowest-level to the highest-level, the access methods are:
1) Character device access to the i2c bus driver
via /dev/i2c-x
2) I2C bus access functions as defined in kernel/include/i2c-dev.h
3A) /proc access to the chip device driver
3B) sysfs access to the chip device driver
4) libsensors library
5) sensors program
Details:
1. Direct /dev access using ioctls
----------------------------------
Character device access to the i2c bus driver via ioctls on a /dev
device. This device could be i2cx, i2c-x, or i2c/x, where x is an
integer. The ioctls are defined in doc/dev-interface in the
i2c package or Documentation/i2c/dev-interface in the linux kernel
sources.
This method does not use a chip device driver at all.
However it does require the i2c-dev module.
The driver must set an individual chip address on the bus via
an ioctl, so it must use locking if multiple devices on the
bus are being accessed. No access is provided for non-i2c
busses such as ISA.
For good examples, see prog/detect/i2cdetect.c and
prog/detect/sensors-detect.
2. Direct /dev access using inline functions
--------------------------------------------
I2C bus access inline functions as defined in kernel/include/i2c-dev.h,
and in doc/dev-interface in the i2c package or
Documentation/i2c/dev-interface in the linux kernel sources.
Note that these used to be defined in <linux/i2c-dev.h> in the kernel
source tree. However, userspace applications are not supposed to
include kernel headers, so the inline functions were removed from
the kernel file in recent kernels. Use the header file in this package
instead.
This method does not use a chip device driver at all.
However it does require the i2c-dev module.
The driver must set an individual chip address on the bus via
an ioctl, so it must use locking if multiple devices on the
bus are being accessed. No access is provided for non-i2c
busses such as ISA.
For good examples, see prog/detect/i2cdetect.c,
prog/dump/i2cdump.c, and prog/dump/i2cset.c.
3A. /proc access (2.4 kernels)
-----------------------------
Chip drivers using the i2c-proc module create subdirectories in
/proc/sys/dev/sensors which can be accessed directly by applications.
Naming and content standards for the entries in these subdirectories
is documented in the file doc/developers/proc. Note that these
standards may not be strictly followed.
If a new driver adheres to these standards then an application may
be able to support new devices on-the-fly.
/proc access provides a method to read and write sensor values
for any driver using i2c-proc, including ISA chip drivers.
This method also works well for shell and perl scripts
written to access a specific device. Note that i2c-proc is
standard in the kernel as of kernel 2.4.13.
Note that most drivers provide only raw sensor readings via /proc;
many readings must be scaled or adjusted, and these adjustments
must often be changed by the user. An application using /proc must
generally provide adjustment facilities and the requirements
of the adjustments can be quite complex. If you need adjustment
facilities, consider the libsensors library, below.
For examples of programs using /proc accesses, see
prog/eeprom/decode-dimms.pl, prog/matorb/displayit.pl,
prog/maxilife/writelcd.sh, prog/rrd/sens_update_rrd,
prog/pwm/fancontrol, and prog/pwm/pwmconfig.
Also search freshmeat for sensors applications.
3B. sysfs access (2.6 kernels)
------------------------------
Chip drivers using the i2c-sensor module create subdirectories in
the sysfs filesystem (usually /sys) which can be accessed
directly by applications.
Naming and content standards for the entries in these subdirectories
is documented in the file Documentation/i2c/sysfs-interface in the
2.6 kernel source tree. Note that these standards may not be
strictly followed.
If a new driver adheres to these standards then an application may
be able to support new devices on-the-fly.
sysfs access provides a method to read and write sensor values
for any driver using i2c-sensor, including ISA chip drivers.
This method may also works well for shell and perl scripts
written to access a specific device. Note that sysfs is
standard in 2.6 kernels.
Note that most drivers provide only raw sensor readings via /sys;
many readings must be scaled or adjusted, and these adjustments
must often be changed by the user. An application using /proc must
generally provide adjustment facilities and the requirements
of the adjustments can be quite complex. If you need adjustment
facilities, consider the libsensors library, below.
For an examples of a program using /sys accesses, see gkrellm.
See also lib/proc.c and prog/dump/i2cbusses.c for examples.
The sysfsutils package may also be helpful.
Also search freshmeat for sensors and sysfs applications.
4. libsensors library
---------------------
The libsensors library provides standardized access to all chip drivers.
It also provides a translation layer with settings in /etc/sensors.conf
so that your application sees adjusted (scaled) values using settings
provided by the user. Other facilities are sensor renaming, limit setting,
and ignoring individual sensors.
The libsensors library supports both 2.4 and 2.6 kernels.
Unfortunately there is little documentation for libsensors. See the
'sensors' application in prog/sensors for an example. The source
for libsensors is in the lib/ directory. Another example
is in prog/sensord. Also search freshmeat for sensors applications.
One other limitation of libsensors is that it is relatively
cumbersome to add support for new devices.
Note that libsensors falls under the GPL, not the LGPL.
In more human language, that means it is FORBIDDEN to link any application
to the library, even to the shared version, if the application itself
does not fall under the GPL. This may or may not be changed in the future.
Contact us if you wish to discuss your application.
For an examples of a program using libsensors accesses, see
prog/sensors/sensors. Also search freshmeat for sensors applications.
5. sensors program
------------------
The 'sensors' program is a text-based application that uses libsensors.
The output is fairly standardized; therefore this output could be
used by other applications.
One simple method is 'sensors|grep ALARM'.
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