=======
Buffers
=======
* struct iio_buffer — general buffer structure
* :c:func:`iio_validate_scan_mask_onehot` — Validates that exactly one channel
is selected
* :c:func:`iio_buffer_get` — Grab a reference to the buffer
* :c:func:`iio_buffer_put` — Release the reference to the buffer
The Industrial I/O core offers a way for continuous data capture based on a
trigger source. Multiple data channels can be read at once from
:file:`/dev/iio:device{X}` character device node, thus reducing the CPU load.
IIO buffer sysfs interface
==========================
An IIO buffer has an associated attributes directory under
:file:`/sys/bus/iio/devices/iio:device{X}/buffer/*`. Here are some of the
existing attributes:
* :file:`length`, the total number of data samples (capacity) that can be
stored by the buffer.
* :file:`enable`, activate buffer capture.
IIO buffer setup
================
The meta information associated with a channel reading placed in a buffer is
called a scan element. The important bits configuring scan elements are
exposed to userspace applications via the
:file:`/sys/bus/iio/devices/iio:device{X}/scan_elements/` directory. This
directory contains attributes of the following form:
* :file:`enable`, used for enabling a channel. If and only if its attribute
is non *zero*, then a triggered capture will contain data samples for this
channel.
* :file:`index`, the scan_index of the channel.
* :file:`type`, description of the scan element data storage within the buffer
and hence the form in which it is read from user space.
Format is [be|le]:[s|u]bits/storagebits[Xrepeat][>>shift] .
* *be* or *le*, specifies big or little endian.
* *s* or *u*, specifies if signed (2's complement) or unsigned.
* *bits*, is the number of valid data bits.
* *storagebits*, is the number of bits (after padding) that it occupies in the
buffer.
* *repeat*, specifies the number of bits/storagebits repetitions. When the
repeat element is 0 or 1, then the repeat value is omitted.
* *shift*, if specified, is the shift that needs to be applied prior to
masking out unused bits.
For example, a driver for a 3-axis accelerometer with 12 bit resolution where
data is stored in two 8-bits registers as follows::
7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+
|D3 |D2 |D1 |D0 | X | X | X | X | (LOW byte, address 0x06)
+---+---+---+---+---+---+---+---+
7 6 5 4 3 2 1 0
+---+---+---+---+---+---+---+---+
|D11|D10|D9 |D8 |D7 |D6 |D5 |D4 | (HIGH byte, address 0x07)
+---+---+---+---+---+---+---+---+
will have the following scan element type for each axis::
$ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type
le:s12/16>>4
A user space application will interpret data samples read from the buffer as
two byte little endian signed data, that needs a 4 bits right shift before
masking out the 12 valid bits of data.
For implementing buffer support a driver should initialize the following
fields in iio_chan_spec definition::
struct iio_chan_spec {
/* other members */
int scan_index
struct {
char sign;
u8 realbits;
u8 storagebits;
u8 shift;
u8 repeat;
enum iio_endian endianness;
} scan_type;
};
The driver implementing the accelerometer described above will have the
following channel definition::
struct iio_chan_spec accel_channels[] = {
{
.type = IIO_ACCEL,
.modified = 1,
.channel2 = IIO_MOD_X,
/* other stuff here */
.scan_index = 0,
.scan_type = {
.sign = 's',
.realbits = 12,
.storagebits = 16,
.shift = 4,
.endianness = IIO_LE,
},
}
/* similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1)
* and Z (with channel2 = IIO_MOD_Z, scan_index = 2) axis
*/
}
Here **scan_index** defines the order in which the enabled channels are placed
inside the buffer. Channels with a lower **scan_index** will be placed before
channels with a higher index. Each channel needs to have a unique
**scan_index**.
Setting **scan_index** to -1 can be used to indicate that the specific channel
does not support buffered capture. In this case no entries will be created for
the channel in the scan_elements directory.
More details
============
.. kernel-doc:: include/linux/iio/buffer.h
.. kernel-doc:: drivers/iio/industrialio-buffer.c
:export:
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