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Date:   Sun, 24 May 2020 17:12:21 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     William Breathitt Gray <vilhelm.gray@...il.com>
Cc:     kamel.bouhara@...tlin.com, gwendal@...omium.org,
        alexandre.belloni@...tlin.com, david@...hnology.com,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, syednwaris@...il.com,
        patrick.havelange@...ensium.com, fabrice.gasnier@...com,
        mcoquelin.stm32@...il.com, alexandre.torgue@...com
Subject: Re: [PATCH v2 2/4] docs: counter: Update to reflect sysfs
 internalization

On Sat, 16 May 2020 15:20:00 -0400
William Breathitt Gray <vilhelm.gray@...il.com> wrote:

> The Counter subsystem architecture and driver implementations have
> changed in order to handle Counter sysfs interactions in a more
> consistent way. This patch updates the Generic Counter interface
> documentation to reflect the changes.
> 
> Signed-off-by: William Breathitt Gray <vilhelm.gray@...il.com>
Looks good to me.

Jonathan

> ---
>  Documentation/driver-api/generic-counter.rst | 215 +++++++++++++------
>  1 file changed, 149 insertions(+), 66 deletions(-)
> 
> diff --git a/Documentation/driver-api/generic-counter.rst b/Documentation/driver-api/generic-counter.rst
> index e622f8f6e56a..8f85c30dea0b 100644
> --- a/Documentation/driver-api/generic-counter.rst
> +++ b/Documentation/driver-api/generic-counter.rst
> @@ -250,8 +250,8 @@ for defining a counter device.
>  .. kernel-doc:: drivers/counter/counter.c
>     :export:
>  
> -Implementation
> -==============
> +Driver Implementation
> +=====================
>  
>  To support a counter device, a driver must first allocate the available
>  Counter Signals via counter_signal structures. These Signals should
> @@ -267,25 +267,58 @@ respective counter_count structure. These counter_count structures are
>  set to the counts array member of an allocated counter_device structure
>  before the Counter is registered to the system.
>  
> -Driver callbacks should be provided to the counter_device structure via
> -a constant counter_ops structure in order to communicate with the
> -device: to read and write various Signals and Counts, and to set and get
> -the "action mode" and "function mode" for various Synapses and Counts
> -respectively.
> +Driver callbacks must be provided to the counter_device structure in
> +order to communicate with the device: to read and write various Signals
> +and Counts, and to set and get the "action mode" and "function mode" for
> +various Synapses and Counts respectively.
>  
>  A defined counter_device structure may be registered to the system by
>  passing it to the counter_register function, and unregistered by passing
>  it to the counter_unregister function. Similarly, the
> -devm_counter_register and devm_counter_unregister functions may be used
> -if device memory-managed registration is desired.
> -
> -Extension sysfs attributes can be created for auxiliary functionality
> -and data by passing in defined counter_device_ext, counter_count_ext,
> -and counter_signal_ext structures. In these cases, the
> -counter_device_ext structure is used for global/miscellaneous exposure
> -and configuration of the respective Counter device, while the
> -counter_count_ext and counter_signal_ext structures allow for auxiliary
> -exposure and configuration of a specific Count or Signal respectively.
> +devm_counter_register function may be used if device memory-managed
> +registration is desired.
> +
> +The struct counter_data structure is used to define counter extensions
> +for Signals, Synapses, and Counts.
> +
> +The "type" member specifies the type of data (e.g. unsigned long,
> +boolean, etc.) handled by this extension. The "read" and "write" members
> +can then be set by the counter device driver with callbacks to handle
> +that data.
> +
> +Convenience macros such as `COUNTER_DATA_COUNT_U64` are provided for use
> +by driver authors. In particular, driver authors are expected to use
> +the provided macros for standard Counter subsystem attributes in order
> +to maintain a consistent interface for userspace. For example, a counter
> +device driver may define several standard attributes like so::
> +
> +	struct counter_data count_ext[] = {
> +		COUNTER_DATA_DIRECTION(count_direction_read),
> +		COUNTER_DATA_ENABLE(count_enable_read, count_enable_write),
> +		COUNTER_DATA_CEILING(count_ceiling_read, count_ceiling_write),
> +	};
> +
> +This makes it intuitive to see, add, and modify the attributes that are
> +supported by this driver ("direction", "enable", and "ceiling") and to
> +maintain this code without getting lost in a web of struct braces.
> +
> +Callbacks must match the function type expected for the respective
> +component or extension. These function types are defined in the struct
> +counter_data structure as the "`*_read`" and "`*_write`" union members.
> +
> +The corresponding callback prototypes for the extensions mentioned in
> +the previous example above would be::
> +
> +	int count_direction_read(struct counter_device *counter,
> +				 struct counter_count *count, u8 *direction);
> +	int count_enable_read(struct counter_device *counter,
> +			      struct counter_count *count, u8 *enable);
> +	int count_enable_write(struct counter_device *counter,
> +			       struct counter_count *count, u8 enable);
> +	int count_ceiling_read(struct counter_device *counter,
> +			       struct counter_count *count, u64 *ceiling);
> +	int count_ceiling_write(struct counter_device *counter,
> +				struct counter_count *count, u64 ceiling);
>  
>  Determining the type of extension to create is a matter of scope.
>  
> @@ -313,52 +346,102 @@ Determining the type of extension to create is a matter of scope.
>    chip overheated via a device extension called "error_overtemp":
>    /sys/bus/counter/devices/counterX/error_overtemp
>  
> -Architecture
> -============
> -
> -When the Generic Counter interface counter module is loaded, the
> -counter_init function is called which registers a bus_type named
> -"counter" to the system. Subsequently, when the module is unloaded, the
> -counter_exit function is called which unregisters the bus_type named
> -"counter" from the system.
> -
> -Counter devices are registered to the system via the counter_register
> -function, and later removed via the counter_unregister function. The
> -counter_register function establishes a unique ID for the Counter
> -device and creates a respective sysfs directory, where X is the
> -mentioned unique ID:
> -
> -    /sys/bus/counter/devices/counterX
> -
> -Sysfs attributes are created within the counterX directory to expose
> -functionality, configurations, and data relating to the Counts, Signals,
> -and Synapses of the Counter device, as well as options and information
> -for the Counter device itself.
> -
> -Each Signal has a directory created to house its relevant sysfs
> -attributes, where Y is the unique ID of the respective Signal:
> -
> -    /sys/bus/counter/devices/counterX/signalY
> -
> -Similarly, each Count has a directory created to house its relevant
> -sysfs attributes, where Y is the unique ID of the respective Count:
> -
> -    /sys/bus/counter/devices/counterX/countY
> -
> -For a more detailed breakdown of the available Generic Counter interface
> -sysfs attributes, please refer to the
> -Documentation/ABI/testing/sysfs-bus-counter file.
> -
> -The Signals and Counts associated with the Counter device are registered
> -to the system as well by the counter_register function. The
> -signal_read/signal_write driver callbacks are associated with their
> -respective Signal attributes, while the count_read/count_write and
> -function_get/function_set driver callbacks are associated with their
> -respective Count attributes; similarly, the same is true for the
> -action_get/action_set driver callbacks and their respective Synapse
> -attributes. If a driver callback is left undefined, then the respective
> -read/write permission is left disabled for the relevant attributes.
> -
> -Similarly, extension sysfs attributes are created for the defined
> -counter_device_ext, counter_count_ext, and counter_signal_ext
> -structures that are passed in.
> +Subsystem Architecture
> +======================
> +
> +Counter drivers pass and take data natively (i.e. `u8`, `u64`, `char *`,
> +etc.) and the shared counter module handles the translation between the
> +sysfs interface. This gurantees a standard userspace interface for all
> +counter drivers, and helps generalize the Generic Counter driver ABI in
> +order to support the Generic Counter chrdev interface without
> +significant changes to the existing counter drivers.
> +
> +A high-level view of how a count value is passed down from a counter
> +driver can be exemplified by the following::
> +
> +       Count data request:
> +       ~~~~~~~~~~~~~~~~~~~
> +                 ----------------------
> +                / Counter device       \
> +                +----------------------+
> +                | Count register: 0x28 |
> +                +----------------------+
> +                        |
> +                 -----------------
> +                / raw count data /
> +                -----------------
> +                        |
> +                        V
> +                +----------------------------+
> +                | Counter device driver      |----------+
> +                +----------------------------+          |
> +                | Processes data from device |   -------------------
> +                |----------------------------|  / driver callbacks /
> +                | Type: unsigned long        |  -------------------
> +                | Value: 42                  |          |
> +                +----------------------------+          |
> +                        |                               |
> +                 ----------------                       |
> +                / unsigned long /                       |
> +                ----------------                        |
> +                        |                               |
> +                        |                               V
> +                        |               +----------------------+
> +                        |               | Counter core         |
> +                        |               +----------------------+
> +                        |               | Routes device driver |
> +                        |               | callbacks to the     |
> +                        |               | userspace interfaces |
> +                        |               +----------------------+
> +                        |                       |
> +                        |                -------------------
> +                        |               / driver callbacks /
> +                        |               -------------------
> +                        |                       |
> +                +-------+                       |
> +                |                               |
> +                |               +---------------+
> +                |               |
> +                V               |
> +        +--------------------+  |
> +        | Counter sysfs      |<-+
> +        +--------------------+
> +        | Translates to the  |
> +        | standard Counter   |
> +        | sysfs output       |
> +        |--------------------|
> +        | Type: const char * |
> +        | Value: "42"        |
> +        +--------------------+
> +                |
> +         ---------------
> +        / const char * /
> +        ---------------
> +                |
> +                V
> +        +--------------------------------------------------+
> +        | `/sys/bus/counter/devices/counterX/countY/count` |
> +        +--------------------------------------------------+
> +        \ Count: "42"                                      /
> +         --------------------------------------------------
> +
> +There are three primary components involved:
> +
> +Counter device driver
> +---------------------
> +Communicates with the hardware device to read/write data; e.g. counter
> +drivers for quadrature encoders, timers, etc.
> +
> +Counter core
> +------------
> +Registers the counter device driver to the system so that the respective
> +callbacks are called during userspace interaction.
> +
> +Counter sysfs
> +-------------
> +Translates counter data to the standard Counter sysfs interface format
> +and vice versa.
> +
> +Please refer to the `Documentation/ABI/testing/sysfs-bus-counter` file
> +for a detailed breakdown of the available Generic Counter interface
> +sysfs attributes.

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