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Subject[PATCH v2 2/4] docs: counter: Update to reflect sysfs internalization
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 <>
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

+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
+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":

-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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