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