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Date:   Sun, 8 Oct 2017 14:19:29 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     benjamin.gaignard@...aro.org, knaack.h@....de, lars@...afoo.de,
        pmeerw@...erw.net, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/6] docs: Add IIO Generic Counter Interface
 documentation

On Thu,  5 Oct 2017 14:14:24 -0400
William Breathitt Gray <vilhelm.gray@...il.com> wrote:

> This patch adds high-level documentation about the IIO Generic Counter
> Interface.
> 
> Signed-off-by: William Breathitt Gray <vilhelm.gray@...il.com>

Put it into the relevant index.rst to get and make it an rst file so it
gets built as part of the sphinx docs.

> ---
>  Documentation/driver-api/iio/generic-counter.txt | 526 +++++++++++++++++++++++
>  1 file changed, 526 insertions(+)
>  create mode 100644 Documentation/driver-api/iio/generic-counter.txt
> 
> diff --git a/Documentation/driver-api/iio/generic-counter.txt b/Documentation/driver-api/iio/generic-counter.txt
> new file mode 100644
> index 000000000000..0ce43b71e1c7
> --- /dev/null
> +++ b/Documentation/driver-api/iio/generic-counter.txt
> @@ -0,0 +1,526 @@
> +=========================
> +Generic Counter Interface
> +=========================
> +
> +Introduction
> +============
> +
> +Counter devices are prevalent within a diverse spectrum of industries.
> +The ubiquitous presence of these devices necessitates a common interface
> +and standard of interaction and exposure. This driver API attempts to
> +resolve the issue of duplicate code found among existing counter device
> +drivers by introducing a generic counter interface for consumption. The
> +generic counter interface enables drivers to support and expose a common
> +set of components and functionality present in counter devices.
> +
> +Theory
> +======
> +
> +Counter devices can vary greatly in design, but regardless of whether
> +some devices are quadrature encoder counters or pedometers, all counter
> +devices consist of a core set of components. This core set of
> +components, shared by all counter devices, is what forms the essence of
> +the generic counter interface.

Hmm. Whether it makes sense to move over pedometers is a bit of an open 
question... They tend to be fairly opaque wrt to where their data is
coming from.  We'll see, but I'd kind of prefer we keep it out of the
docs until that case has been fully analysed!

> +
> +There are three core components to a counter:
> +
> +        VALUE
> +        -----
> +        A Value represents the count data for a set of Signals. A Value
> +        has a count function mode (e.g. "increment" or "quadrature x4")
> +        which respresents the update behavior for the count data. A
> +        Value also has a set of one or more associated Signals.

The whole set of valid function modes needs to be documented somewhere.
I don't think it currently is (but I may have missed it and haven't
read all the patches yet!)

Interesting terminology - this Value is what most people would think
of as a counter.  Can we just call it that?  The issue is that we
are thinking from the hardware point of view that says a 'counter block'
can have multiple counters so value is just one of them. I'm not sure
we want to present that terminology to userspace.


> +
> +        SIGNAL
> +        ------
> +        A Signal represents a count input line. A Signal may be
> +	associated to one or more Values.
> +
> +        TRIGGER
> +	-------
> +        A Trigger represents a Value's count function trigger condition
> +        mode (e.g. "rising edge" or "double pulse") for an associated
> +        Signal.	If a Signal is associated with a Value, a respective
> +        Trigger	instance for that association exists -- albeit perhaps
> +        with a trigger condition mode of "none."

Again, we need to enumerate the full acceptable list.  Enforcing it can
be by review, but we need a list that people can refer to or we will get
multiple drivers doing subtly different variants.

> +
> +A counter is defined as a set of input signals associated to count data
> +that are generated by the evaluation of the state of the associated
> +input signals as defined by the respective count functions. Within the
> +context of the generic counter interface, a counter consists of Values
> +each associated to a set of Signals, whose respective Trigger instances
> +represent the count function update conditions for the associated
> +Values.

Could we call this a 'counter group' or 'counter device' or something to
free the term counter up for a single instance of what is currently
called value.

> +
> +Paradigm
> +========
> +
> +The most basic counter device may be expressed as a single Value
> +associated with a single Signal via a single Trigger. Take for example
> +a hypothetical counter device which simply accumulates a count of rising
> +edges on a source input line.

Drop hypothetical - this is a perfectly real example. Plenty of hardware
out there does this!

> +
> +        Value                Trigger        Signal
> +        -----                -------        ------
> ++---------------------+
> +| Data: Count         |    Rising Edge     ________
> +| Function: Increment |  <-------------   / Source \
> +|                     |                  ____________
> ++---------------------+
> +
> +In this example, the Signal is a source input line with a pulsing
> +voltage, while the Value is a persistent count which increments. The
> +Signal is associated with the Value via a respective Trigger. The
> +increment function is triggered by the condition specified by the
> +Triggered -- in this case a rising edge condition. In summary, the
> +counter device existence and behavior is aptly represented by respective
> +Value, Signal, and Trigger components: a rising edge condition triggers
> +an incrementation function on an accumulating count datum.
> +
> +A counter device is not limited to a single Signal; in fact, in theory
> +an unlimited number of Signals may be associated with a Value.
Change that that to many signals.  Otherwise someone pedantic will say
it's limited by the implementation to 2^32 or similar..
> For
> +example, a quadrature encoder counter device can keep track of position
> +based on the states of two input lines.
> +
> +           Value                 Trigger      Signal
> +           -----                 -------      ------
> ++-------------------------+
> +| Data: Position          |    Both Edges      ___
> +| Function: Quadrature x4 |  <-------------   / A \
> +|                         |                  _______
> +|                         |
> +|                         |    Both Edges      ___
> +|                         |  <-------------   / B \
> +|                         |                  _______
> ++-------------------------+
> +
> +In this example, two Signals (quadrature encoder lines A and B) are
> +associated to a single Value: a rising or falling edge on either A or B
> +triggers the "Quadrature x4" function which determines the direction of
> +movement and updates the respective position data. The "Quadrature x4"
> +function is likely implemented in the hardware of the quadrature encoder
> +counter device; the Value, Triggers, and Signals simply represent this
> +hardware behavior and functionality.

Interesting that there is a relationship between valid trigger settings
and valid Value:Function settings.

> +
> +Signal associated to the same Value can have differing trigger
> +conditions. For example, a quadrature encoder counter device operating
> +in a non-quadrature mode could have one input line dedicated for
> +movement and a second input line dedicated for direction.
> +
> +           Value                  Trigger      Signal
> +           -----                  -------      ------
> ++------------------------- +
> +| Data: Position           |    Rising Edge     ___
> +| Function: Non-quadrature |  <-------------   / A \ (Movement)
> +|                          |                  _______
> +|                          |
> +|                          |       None         ___
> +|                          |  <-------------   / B \ (Direction)
> +|                          |                  _______
> ++--------------------------+

Non-quadrature is a pretty odd way of defining it!  I'd suggest
something more positive - pulse-direction would be the standard
terminology.

> +
> +Only Signal A triggers the "Non-quadrature" update function, but the
> +state of Signal B is still required in order to know the direction in
> +order to properly update the position data. So in the end, both Signals
> +are associated to the same Value via two respective Triggers, but only
> +one Trigger has an active trigger condition while the other is left in a
> +"None" condition mode to indicate its respective Signal's availability
> +for state evaluation despite its non-triggering mode.

Interesting - there sill be some confusion around the quadrature cases
though as the 'value' will only change on say rising edge A, but the
internal state machine is tracking the previous changes of edge B.  I'd
suggest we add a note about that somewhere here.

> +
> +Although the examples thus far have been representations of physical
> +devices, this is not a necessity. A counter simply represent the
> +evaluation (Value) of input data (Signals) triggered by specific
> +conditions (Triggers). A counter can be the representation of more
> +abstract components.
> +
> +For example, suppose a counter representation is desired for a DNA
> +sequence analysizer which detects possible genetic diseases.
> +
> +        Value                     Trigger           Signal
> +        -----                     -------           ------
> ++---------------------+
> +| Data: Diseases      |    Gene Transcript (EST)     _____
> +| Function: Cancers   |  <-----------------------   / DNA \ (GAAGTGC...)
> +|                     |                            _________
> ++---------------------+
> +
> +In this scenario, the Signal is a stream of DNA nucleotide bases (As,
> +Ts, Cs, and Gs), the Trigger is expressed sequence tags (ESTs), and the
> +Value is a list of diseases discovered (in this case the function is
> +evaluating for possible cancers). Note how the Signal in this example
> +does not represent a physical voltage line, nor does the Trigger
> +represent a physical voltage line state change, nor does the Value
> +represent a strictly decimal data value.
> +
> +The DNA sequence analysizer example is contrived to illustrate the
> +flexibility of the generic counter paradigm by demonstrating its
> +capability of representing abstract concepts; however, physical devices
> +are likely to be more fitting for such a representation.
> +
> +The key concept is that the Signal, Trigger, and Value are abstract
> +representations which do not need to be closely married to their
> +respective physical sources. This allows the user of a counter to
> +divorce themselves from the nuances of physical components (such as
> +whether an input line is differential or single-ended) and focus on the
> +core idea of what the data and process represent (an accumulated count
> +of rising edges).
> +
> +Userspace Interface
> +===================
> +
> +Several sysfs attributes are generated by the generic counter interface,
> +and reside under the /sys/bus/iio/devices/iio:deviceX directory.
> +
> +Each counter has a respective set of countX-Y and signalX-Y prefixed
> +attributes, where X is the id set in the counter structure, and Y is the
> +id of the respective Value or Signal.
> +
> +The generic counter interface sysfs attributes are as follows:
> +
> +        countX-Y_function: count function mode
> +        countX-Y_function_available: available count function modes
> +        countX-Y_name: Value name
> +        countX-Y_raw: Value data
> +        countX-Y_triggers: Value's associated Signals
> +        countX-Y_trigger_signalX-Z: Value Y trigger mode for Signal Z
> +        countX-Y_trigger_signalX-Z_available: available Value Y trigger
> +                                              modes for Signal Z
> +        signalX-Y_name: Signal name
I mentioned this in one of the other patches - This naming stuff feels
like, if we want to do it we should do it throughout IIO.

Possibly it maps to the data_sheet_name.. Higher level information beyond
this device would to my mind require explicit representation of devices
beyond this - i.e. what is attached to the wires that is generating
things to count.

> +        signalX-Y_raw: Signal data
> +
> +Through these sysfs attributes, programs and scripts may interact with
> +the generic counter paradigm Values, Triggers, and Signals of respective
> +counter devices.
> +
> +Driver API
> +==========
> +
> +Driver authors may utilize the generic counter interface in their code
> +by including the include/linux/iio/counter.h header file. This header
> +file provides several core data structures and function prototypes for
> +defining a generic counter.
> +
> +struct iio_counter_signal
> +-------------------------
> +
> +This structure defines a generic counter paradigm Signal component;
> +typically this will correlate with an input channel on a physical
> +counter device. This structure is the simplest to define with only two
> +structure members which require explicit configuration:
> +
> +        id:     Unique ID used to identify the Signal
> +
> +        name:   Device-specific Signal name (typically the device input
> +                channel name)
> +
> +struct iio_counter_trigger
> +--------------------------
> +
> +This structure defines a generic counter paradigm Trigger component. To
> +properly utilize this structure, trigger modes and an associated Signal
> +must be defined:
> +
> +        mode:                   Index of the current trigger mode state
> +
> +        trigger_modes:          Array of trigger modes each represented
> +	                        by a character string

As mentioned before these have to be explicitly listed somewhere.  They
form userspace ABI.  We must not leave this empty.  Defining these
cleanly and generically may be one of the hardest aspects to agree
on.

> +
> +        num_trigger_modes:      Number of trigger modes provided in
> +	                        trigger_modes array
> +
> +        signal:                 Pointer to associated Signal
> +
> +struct iio_counter_value
> +------------------------
> +
> +This structure defines a generic counter paradigm Value component;
> +typically this will correlate with the read data (the "count" value)
> +provided by a physical counter device. This structure requires the
> +explicit configuration of an ID, name, function modes (the function
> +triggered on a Trigger condition), and optionally a set of initial
> +associated Triggers:
> +
> +        id:                     Unique ID used to identify the Signal
> +
> +        name:                   Device-specific Value name (typically
> +	                        the device read channel name)
> +
> +        mode:                   Index of the current function mode state
> +
> +        function_modes:         Array of function modes each represented
> +	                        by a character string
> +
> +        num_function_modes:     Number of function modes provided in
> +	                        function_modes array
> +
> +        triggers:               Array of associated Triggers
> +
> +        num_triggers:           Number of Triggers provided in triggers
> +	                        array

It might also be convenient to have a flag in here to say if the triggers
are fixed or not.

> +
> +struct iio_counter_ops
> +----------------------

I'd suggest you push these details down in to kernel-doc in the code
and pull it back in here using the relevant bits of markup.
That stuff is a bit clunky but any docs in here of actual elements
etc will probably rot unedited if stuff changes.

> +
> +This structure defines callbacks to interact with the Value, Trigger,
> +and Signal components:
> +
> +        signal_read:            Function to request a signal value from
> +	                        the device. Return value will specify
> +				the type of value returned by the
> +				device. val and val2 will contain the
> +				elements making up the returned value.

Mention IIO_VAL_INT type return values as well.

> +
> +	signal_write:           Function to write a signal value to the
> +	                        device. Parameters are interpreted the
> +				same as signal_read.
> +
> +        trigger_mode_set:       Function to set the trigger mode. mode
> +	                        is the index of the requested mode from
> +				the value trigger_modes array.
> +
> +        trigger_mode_get:       Function to get the current trigger
> +	                        mode. Return value will specify the
> +				index of the current mode from the value
> +				trigger_modes array.
> +
> +        value_read:             Function to request a value value from
> +	                        the device. Return value will specify
> +				the type of value returned by the
> +				device. val and val2 will contain the
> +				elements making up the returned value.

This is the counter I think?  Value is rather vague, counter_read
will be less so.

> +
> +        value_write:            Function to write a value value to the
> +	                        device. Parameters are interpreted the
> +				same as value_read.
> +
> +        value_function_set:     Function to set the value function mode.
> +	                        mode is the index of the requested mode
> +				from the value function_modes array.
> +
> +        value_function_get:     Function to get the current value
> +	                        function mode. Return value will specify
> +				the index of the current mode from the
> +				value function_modes array.
> +
> +struct iio_counter
> +------------------
> +
> +This is the main data structure for a counter device; access to all
> +respective Values, Triggers, and Signals is possible from this
> +structure. This structure allows the configuration of an ID, name,
> +function callbacks, initial Signals and initial Values, auxiliary IIO
> +core channels and callbacks, and driver-specific data:
> +
> +        id:                     Unique ID used to identify the counter
> +
> +        name:                   Name of the counter device
> +
> +        dev:                    Device structure, which should be
> +				assigned a parent and owner
> +
> +        ops:                    Function callbacks for counter
> +	                        components (Signal, Trigger, Value)
> +
> +        signals:                Array of Signals
> +
> +        num_signals:            Number of Signals specified in signals
> +	                        array
> +
> +        values:                 Array of Values
> +
> +        num_values:             Number of Values specified in values
> +	                        array
> +
> +        channels:               Optional IIO core channels specification
> +	                        structure table
> +
> +        num_channels:           Number of channels specified in channels
> +
> +        info:                   IIO core function callbacks and constant
> +	                        info from driver
> +
> +        driver_data:            Driver-specific data
> +
> +Registration functions
> +----------------------
> +
> +Counters may be registered to the system via the iio_counter_register
> +function and subsequently unregistered via the iio_counter_unregister
> +function. The devm_iio_counter_register and devm_iio_counter_unregister
> +functions serve as device memory-managed versions of the
> +iio_counter_register and iio_counter_unregister functions.
> +
> +An initialized iio_counter structure, which defines the Counter, is
> +required to be passed in for registration; the Signals, Values, and
> +Triggers for the Counter passed in via the signals and values arrays as
> +part of the iio_counter structure. If auxiliary IIO core channels and
> +functionality are required, IIO core channels and callbacks may be
> +passed in via the channels and info members of the passed-in iio_counter
> +structure.
> +
> +Implementation
> +==============
> +
> +The IIO generic counter interface piggybacks off of the IIO core. This
> +is primarily used to leverage the existing sysfs setup: the IIO_COUNT
> +channel attributes represent the Counter Value, while the IIO_SIGNAL
> +channel attributes represent the Counter Signal; auxilary IIO_COUNT
> +attributes represent the Counter Signal connections (Triggers) and their
> +respective state change configurations which trigger an associated
> +"counter function" evaluation.

If it makes more sense I wouldn't mind changing the form of that setup
code to be more generic so you can use it without jumping through so many
hoops.

> +
> +The iio_counter_ops structure serves as a container for driver callbacks
> +to communicate with the device; function callbacks are provided to read
> +and write various Signals and Values, and set and get the "trigger mode"
> +and "function mode" for various Triggers and Values respectively.
> +
> +To support a counter device, a driver must first allocate the available
> +Counter Signals via iio_counter_signal structures. These Signals should
> +be stored as an array and set to the signals array member of an
> +allocated iio_counter structure before the Counter is registered to the
> +system.
> +
> +Counter Values may be allocated via iio_counter_value structures, and
> +respective Counter Signal associations (Triggers) made via
> +iio_counter_trigger structures. Associated iio_counter_trigger
> +structures are stored as an array and set to the the triggers array
> +member of the respective iio_counter_value structure. These
> +iio_counter_value structures are set to the values array member of an
> +allocated iio_counter structure before the Counter is registered to the
> +system.
> +
> +A counter device is registered to the system by passing the respective
> +initialized iio_counter structure to the iio_counter_register function;
> +similarly, the iio_counter_unregister function unregisters the
> +respective counter.
> +
> +Architecture
> +============
> +
> +Although the IIO Generic Counter Interface utilizes IIO core under the
> +hood, driver authors are not necessarily required to interact with IIO
> +core data structures and functions directly -- in theory, such details
> +of the system are abstracted away. Driver authors only need to concern
> +themselves with the Generic Counter specific data structures and
> +functions found in the include/linux/iio/counter.h header file.
> +
> +In other words, the driver API is intended to expose itself sufficiently
> +upon the principles and concepts of the generic counter paradigm (i.e.
> +Values, Triggers, Signals, etc.) such that it may be indepedent from its
> +underlying implementation; theoretically, the IIO core code in the
> +implementation could be replaced away in its entirely by an alternative
> +implementation all without the need to update existing drivers utilizing
> +the Generic Counter Interface driver API.

Good.  I hope it's not just in theory ;)  May well make sense in the future
as in particular IIO may get based on some new core infrastructure itself.

> +
> +This paradigm separation however does result in some mapping concerns
> +between Generic Counter functions to IIO core functions; in particular,
> +parameters for the IIO core functions expect IIO core data structures
> +(e.g. iio_dev and iio_chan_spec) which are not provided directly by the
> +parameters for the respective Generic Counter functions. This results in
> +a somewhat opaque pathway from a iio_counter structure to its associated
> +iio_dev in order to support the required IIO core calls.
> +
> +The following call graphs should help illustrate some of the main IIO
> +core dependencies:
> +
> ++----------------------+
> +| iio_counter_register |
> ++----------------------+
> +  |  |  |
> +  |  |  +-----------------------------+
> +  |  +------------------+             |
> +  |                     |             |
> +  V                     V             V
> ++------------------+  +----------+  +---------------------+
> +| iio_device_alloc |  | iio_priv |  | iio_device_register |
> ++------------------+  +----------+  +---------------------+
> +
> +The iio_counter_register function allocates and initializes a new
> +iio_dev structure which will serve as the respective Counter's gateway
> +to IIO core support. The address of the parent iio_counter structure is
> +stored with the iio_dev structure via iio_priv in order to allow access
> +back to the Counter from within the IIO core functions. Finally, the
> +iio_dev structure is registered via iio_device_register.
> +
> ++-----------------------+   +----------------------+
> +| iio_read_channel_info |-->| iio_counter_read_raw |
> ++-----------------------+   +----------------------+
> +                              |  |  |
> +  +---------------------------+  |  |
> +  |                +-------------+  |
> +  |                |               ++
> +  |                |               |
> +  V                V               V
> +  IIO_SIGNAL       IIO_COUNT       IIO_*
> ++-------------+  +------------+  +----------+
> +| signal_read |  | value_read |  | read_raw |
> ++-------------+  +------------+  +----------+
> +
> ++------------------------+   +-----------------------+
> +| iio_write_channel_info |-->| iio_counter_write_raw |
> ++------------------------+   +-----------------------+
> +                               |  |  |
> +  +----------------------------+  |  |
> +  |                 +-------------+  |
> +  |                 |                |
> +  |                 |                |
> +  V                 V                V
> +  IIO_SIGNAL        IIO_COUNT        IIO_*
> ++--------------+  +-------------+  +-----------+
> +| signal_write |  | value_write |  | write_raw |
> ++--------------+  +-------------+  +-----------+
> +
> +Normally, the IIO core iio_read_channel_info and iio_write_channel_info
> +functions respectiveluy call the driver-supplied read_raw and write_raw
> +callbacks directly. Since the generic counter interface serves as an
> +abstraction above IIO core, drive authors do not generally directly
> +configure a read_raw/write_raw callback.
> +
> +Instead, the IIO Generic Counter Interface hooks on to the
> +iio_read_channel_info and iio_write_channel_info expected read_raw and
> +write_raw callbacks respectively via iio_counter_read_raw and
> +iio_counter_write_raw. The iio_counter_read_raw and
> +iio_counter_write_raw functions then call the respective driver-supplied
> +signal_read/value_read and signal_write/value_write callbacks
> +respectively for the appropriate IIO_SIGNAL OR IIO_COUNT. If an IIO core
> +channel that was not part of the generic counter paradigm was supplied
> +via the channels member of the iio_counter structure, then the
> +respective driver-supplied (via the iio_counter structure info member)
> +read_raw and write_raw are called.
> +
> ++---------------------------+        +----------------------------+
> +| iio_read_channel_ext_info |        | iio_write_channel_ext_info |
> ++---------------------------+        +----------------------------+
> +  |                                    |
> +  V                                    V
> ++-------------------------------+    +--------------------------------+
> +| iio_counter_trigger_mode_read |    | iio_counter_trigger_mode_write |
> ++-------------------------------+    +--------------------------------+
> +  |                                    |
> +  V                                    V
> ++------------------+                 +------------------+
> +| trigger_mode_get |                 | trigger_mode_set |
> ++------------------+                 +------------------+
> +
> ++---------------+                     +----------------+
> +| iio_enum_read |                     | iio_enum_write |
> ++---------------+                     +----------------+
> +  |                                     |
> +  V                                     V
> ++--------------------------------+    +--------------------------------+
> +| iio_counter_value_function_get |    | iio_counter_value_function_set |
> ++--------------------------------+    +--------------------------------+
> +  |                                     |
> +  V                                     V
> ++--------------------+                +--------------------+
> +| value_function_get |                | value_function_set |
> ++--------------------+                +--------------------+
> +
> +The driver-supplied trigger_mode_get and trigger_mode_set callbacks hook
> +on to the iio_read_channel_ext_info and iio_write_channel_ext_info
> +functions respectively via the iio_counter_trigger_mode_read and
> +iio_counter_trigger_mode_write functions. Similarly, the driver-supplied
> +value_function_get and value_function set callbacks hook on to the
> +iio_enum_read and iio_enum_write functions respectively via the
> +iio_counter_value_function_get and iio_counter_value_function set
> +functions.

I'm inclined to agree with you when you suggest that separating things
out more may well make sense.

This all seems way to involved just to share a relatively small amount
of code.

The question is whether we loose anything particularly useful if
we drop most of the complexity.  If it makes sense for a counter
to sit on the IIO bus in sysfs that's easily done (triggers in IIO
for example do that already).

The main advantage then becomes one of grouping 'somewhat' related
subsystems together...  Can also share some utility code given
the overall structures are somewhat similar of course.

Not sure...

Jonathan

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