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Date: Tue, 4 Aug 2015 12:04:53 +0300
From: Daniel Baluta <daniel.baluta@...el.com>
To: Daniel Baluta <daniel.baluta@...el.com>
Cc: Peter Meerwald <pmeerw@...erw.net>,
Jonathan Cameron <jic23@...nel.org>,
Jonathan Corbet <corbet@....net>,
Hartmut Knaack <knaack.h@....de>,
Lars-Peter Clausen <lars@...afoo.de>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
linux-doc@...r.kernel.org
Subject: Re: [PATCH v4] DocBook: Add initial documentation for IIO
Sorry, hit send by mistake :)
>>> + An IIO buffer has an associated attributes directory under <filename>
>>> + /sys/bus/iio/iio:deviceX/buffer/</filename>. Here are the existing
>>> + attributes:
>>> + <itemizedlist>
>>> + <listitem>
>>> + <emphasis>length</emphasis>, number of data samples contained by the
>>> + buffer.
>>
>> wording is not very clean; length is is the buffer length/capacity, not
>> the number of samples currently in the buffer
>
> Oh, I see. The same wording is in the ABI file:
http://lxr.free-electrons.com/source/Documentation/ABI/testing/sysfs-bus-iio#L975
<snip>
>>> + </listitem>
>>> + <listitem><emphasis>bits</emphasis> is the number of bits of data
>>
>> number of data bits
Ok.
>>
>>> + </listitem>
>>> + <listitem><emphasis>storagebits</emphasis> is the space (after padding)
>>> + that it occupies in the buffer.
>>
>> is the number of bits (after padding) that...
Ok.
>>
>>> + </listitem>
>>> + <listitem>
>>> + <emphasis>shift</emphasis> if specified, is the shift that needs
>>> + to be a applied prior to masking out unused bits
>>
>> that needs to be applied -- delete a
Ok.
>>
>>> + </listitem>
>>> + <listitem>
>>> + <emphasis>repeat</emphasis>, specifies the number of real/storage bits
>>
>> what is real? -- undefined at this point;
>> "real/storage" doesn't make it clear if the padded or unpadded data bits
>> are repeated -- I think the later
>> maybe: "specifies the number of unpadded data repetitions"
Will fix.
>>
>>> + repetitions. When the repeat element is 0 or 1, then the repeat
>>> + value is omitted.
>>> + </listitem>
>>> + </itemizedlist>
>>> + </listitem>
>>> + </itemizedlist>
>>> + For example, a driver for a 3-axis accelerometer with 12 bit
>>> + resolution where data is stored in two 8-bits registers as
>>> + follows:
>>> + <programlisting>
>>> + 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)
>>> + +---+---+---+---+---+---+---+---+
>>> + </programlisting>
>>> +
>>> + will have the following scan element type for each axis:
>>> + <programlisting>
>>> + $ cat /sys/bus/iio/devices/iio:device0/scan_elements/in_accel_y_type
>>> + le:s12/16>>4
>>> + </programlisting>
>>> + The userspace applications will interpret data samples read from buffer
>>
>> user space -- inconsistent
>> maybe "A user space application will..."
>> the buffer -- the
>>
>>> + as two byte little endian signed data, that needs a 4 bits right
>>> + shift before masking out the only 12 valid bits of real data.
>>
>> masking out the 12 valid bits of data
Agree.
>>
>>> + </para>
>>> + <para>
>>> + For implementing buffer support a driver should initialize the following
>>> + fields in <type>iio_chan_spec</type> definition:
>>> + <programlisting>
>>> + 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;
>>> + };
>>> + </programlisting>
>>> + The driver implementing the accelerometer described above will
>>> + have the following channel definition:
>>> + <programlisting>
>>> + struct 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,
>>> + .storgebits = 16,
>>> + .shift = 4,
>>> + .endianness = IIO_LE,
>>> + },
>>> + }
>>> + /* similar for Y and Z axis */
>>
>> two spaces before Z
>> maybe: "similar for Y (with channel2 = IIO_MOD_Y, scan_index = 1) and Z
>> (with channel2 = IIO_MOD_Z, scan_index=2) axis"
Ok.
>>
>>> + }
>>> + </programlisting>
>>> + </para>
>>> + <para>
>>> + Here <emphasis> scan_index </emphasis> defines the relative order in which
>>
>> why relative?
>> it is simply the order
>>
>>> + 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.
>>> + </para>
>>> + <para>
>>> + It is important to realize that the scan_index does not define the
>>> + absolute position in the buffer. E.g. a channel with the scan_index = 3
>>> + will not be at offset 3 bytes or 3 words, but rather will be placed in the
>>> + buffer after any channel with a scan_index lower than 3 and before
>>> + any channel with a scan_index larger than 3.
>>
>> I'd drop the paragraph above, this is just confusing; better mention that
>> there are padding rules (e.g. for the timestamp channel) and it follows
>> that the scan_index is not a byte offset into the buffer
Will try to add a better wording for this, although it looks clear enough to me.
>>
>>> + Furthermore the scan indices do not have to be consecutive. E.g. A
>>> + channel spec array that defines 3 channels with the indices 1, 2 and 3 is
>>> + just as valid as a channel spec that uses the indices 100, 200, 300. The
>>> + relative order of the channels will be the same.
>>> + </para>
>>> + <para>
>>> + 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.
>>> + </para>
>>> + </sect2>
>>> + </sect1>
>>> +
>>> + <sect1 id="iiotrigger"> <title> Industrial I/O triggers </title>
>>> +!Finclude/linux/iio/trigger.h iio_trigger
>>> +!Edrivers/iio/industrialio-trigger.c
>>> + <para>
>>> + In many situations it is useful for a driver to be able to
>>> + capture data based on some external event (trigger) as opposed
>>> + to periodically polling for data. An IIO trigger can be provided
>>> + by a device driver that also has an IIO device based on hardware
>>> + generated events (e.g. data ready or threshold exceeded) or
>>> + provided by a separate driver from an independent interrupt
>>> + source (e.g. GPIO line connected to some external system, timer
>>> + interrupt or user space reading a specific file in sysfs). A
>>
>> writing a specific file
Ok.
>>
>>> + trigger may initialize data capture for a number of sensors and
>>
>> initiate -- not initialize
:)
>>
>>> + also it may be completely unrelated to the sensor itself.
>>> + </para>
>>> +
>>> + <sect2 id="iiotrigsysfs"> <title> IIO trigger sysfs interface </title>
>>> + There are two locations in sysfs related to triggers:
>>> + <itemizedlist>
>>> + <listitem><filename>/sys/bus/iio/devices/triggerY</filename>,
>>> + this file is created once an IIO triggered is registered with
>>
>> an IIO trigger
Ok.
>>
>>> + the IIO core and corresponds to trigger with index Y. Because
>>> + triggers can be very different depending on type there are few
>>> + standard attributes that we can describe here:
>>> + <itemizedlist>
>>> + <listitem>
>>> + <emphasis>name</emphasis>, trigger name that can be later
>>> + used to for association with a device.
>>
>> used for association
:), ok.
>>
>>> + </listitem>
>>> + <listitem>
>>> + <emphasis>sampling_frequency</emphasis>, some timer based
>>> + triggers use this attribute to specify the frequency for
>>> + trigger calls.
>>> + </listitem>
>>> + </itemizedlist>
>>> + </listitem>
>>> + <listitem>
>>> + <filename>/sys/bus/iio/devices/iio:deviceX/trigger/</filename>, this
>>> + directory is created once the device supports a triggered
>>> + buffer. We can associate a trigger with our device by writing
>>> + trigger's name in the<filename>current_trigger</filename> file.
>>
>> the trigger's name -- the
>> the <filename> -- add space before tag
>>
>>> + </listitem>
>>> + </itemizedlist>
>>> + </sect2>
>>> +
>>> + <sect2 id="iiotrigattr"> <title> IIO trigger setup</title>
>>> +
>>> + <para>
>>> + Let's see a simple example of how to setup a trigger to be used
>>> + by a driver.
>>> +
>>> + <programlisting>
>>> + struct iio_trigger_ops trigger_ops = {
>>> + .set_trigger_state = sample_trigger_state,
>>> + .validate_device = sample_validate_device,
>>> + }
>>> +
>>> + struct iio_trigger *trig;
>>> +
>>> + /* first, allocate memory for our trigger */
>>> + trig = iio_trigger_alloc(dev, "trig-%s-%d", name, idx);
>>> +
>>> + /* setup trigger operations field */
>>> + trig->ops = &trigger_ops;
>>> +
>>> + /* now register the trigger with the IIO core */
>>> + iio_trigger_register(trig);
>>> + </programlisting>
>>> + </para>
>>> + </sect2>
>>> +
>>> + <sect2 id="iiotrigsetup"> <title> IIO trigger ops</title>
>>> +!Finclude/linux/iio/trigger.h iio_trigger_ops
>>> + <para>
>>> + Notice that a trigger has a set of operations attached:
>>> + <itemizedlist>
>>> + <listitem>
>>> + <function>set_trigger_state</function>, switch the trigger on/off
>>> + on demand.
>>> + </listitem>
>>> + <listitem>
>>> + <function>validate_device</function>, function to validate the
>>> + device when the current trigger gets changed.
>>> + </listitem>
>>> + </itemizedlist>
>>> + </para>
>>> + </sect2>
>>> + </sect1>
>>> + <sect1 id="iiotriggered_buffer">
>>> + <title> Industrial I/O triggered buffers </title>
>>> + <para>
>>> + Now that we know what buffers and triggers are let's see how they
>>> + work together.
>>> + </para>
>>> + <sect2 id="iiotrigbufsetup"> <title> IIO triggered buffer setup</title>
>>> +!Edrivers/iio/industrialio-triggered-buffer.c
>>> +!Finclude/linux/iio/iio.h iio_buffer_setup_ops
>>> +
>>> +
>>> + <para>
>>> + A typical triggered buffer setup looks like this:
>>> + <programlisting>
>>> + const struct iio_buffer_setup_ops sensor_buffer_setup_ops = {
>>> + .preenable = sensor_buffer_preenable,
>>> + .postenable = sensor_buffer_postenable,
>>> + .postdisable = sensor_buffer_postdisable,
>>> + .predisable = sensor_buffer_predisable,
>>> + };
>>> +
>>> + irqreturn_t sensor_iio_pollfunc(int irq, void *p)
>>> + {
>>> + pf->timestamp = iio_get_time_ns();
>>> + return IRQ_WAKE_THREAD;
>>> + }
>>> +
>>> + irqreturn_t sensor_trigger_handler(int irq, void *p)
>>> + {
>>> + u16 buf[8];
>>
>> int i = 0;
>>
>>> +
>>> + /* read data for each active channel */
>>> + for_each_set_bit(bit, active_scan_mask, masklength)
>>> + buf[i++] = sensor_get_data(bit)
>>> +
>>> + iio_push_to_buffers_with_timestamp(indio_dev, buffer, timestamp);
>>
>> buf -- not buffer
ok.
>>
>>> +
>>> + iio_trigger_notify_done(trigger);
>>
>> return IRQ_HANDLED;
ok.
>>
>>> + }
>>> +
>>> + /* setup triggered buffer, usually in probe function */
>>> + iio_triggered_buffer_setup(indio_dev, sensor_iio_polfunc,
>>> + sensor_trigger_handler,
>>> + sensor_buffer_setup_ops);
>>> + </programlisting>
>>> + </para>
>>> + The important things to notice here are:
>>> + <itemizedlist>
>>> + <listitem><function> iio_buffer_setup_ops</function>, the buffer setup
>>> + functions to be called at predefined points in buffer configuration
>>
>> the buffer configuration -- the
ok.
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