[PATCH v2 4/4] docs: counter: Document character device interface

[William Breathitt Gray <vilhelm.gray@gmail.com>]

This patch adds high-level documentation about the Counter subsystem
character device interface.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
---
 Documentation/driver-api/generic-counter.rst | 112 +++++++++++++------
 1 file changed, 76 insertions(+), 36 deletions(-)

diff --git a/Documentation/driver-api/generic-counter.rst b/Documentation/driver-api/generic-counter.rst
index 8f85c30dea0b..58045b33b576 100644
--- a/Documentation/driver-api/generic-counter.rst
+++ b/Documentation/driver-api/generic-counter.rst
@@ -223,19 +223,6 @@ whether an input line is differential or single-ended) and instead focus
 on the core idea of what the data and process represent (e.g. position
 as interpreted from quadrature encoding data).
 
-Userspace Interface
-===================
-
-Several sysfs attributes are generated by the Generic Counter interface,
-and reside under the /sys/bus/counter/devices/counterX directory, where
-counterX refers to the respective counter device. Please see
-Documentation/ABI/testing/sysfs-bus-counter for detailed
-information on each Generic Counter interface sysfs attribute.
-
-Through these sysfs attributes, programs and scripts may interact with
-the Generic Counter paradigm Counts, Signals, and Synapses of respective
-counter devices.
-
 Driver API
 ==========
 
@@ -377,13 +364,13 @@ driver can be exemplified by the following::
                 +----------------------------+          |
                 | Processes data from device |   -------------------
                 |----------------------------|  / driver callbacks /
-                | Type: unsigned long        |  -------------------
+                | Type: u64                  |  -------------------
                 | Value: 42                  |          |
                 +----------------------------+          |
                         |                               |
-                 ----------------                       |
-                / unsigned long /                       |
-                ----------------                        |
+                 ----------                             |
+                / u64     /                             |
+                ----------                              |
                         |                               |
                         |                               V
                         |               +----------------------+
@@ -398,25 +385,32 @@ driver can be exemplified by the following::
                         |               / driver callbacks /
                         |               -------------------
                         |                       |
-                +-------+                       |
+                +-------+---------------+       |
+                |                       |       |
+                |               +-------|-------+
+                |               |       |
+                V               |       V
+        +--------------------+  |  +---------------------+
+        | Counter sysfs      |<-+->| Counter chrdev      |
+        +--------------------+     +---------------------+
+        | Translates to the  |     | Translates to the   |
+        | standard Counter   |     | standard Counter    |
+        | sysfs output       |     | character device    |
+        |--------------------|     |---------------------+
+        | Type: const char * |     | Type: u64           |
+        | Value: "42"        |     | Value: 42           |
+        +--------------------+     +---------------------+
                 |                               |
-                |               +---------------+
-                |               |
-                V               |
-        +--------------------+  |
-        | Counter sysfs      |<-+
-        +--------------------+
-        | Translates to the  |
-        | standard Counter   |
-        | sysfs output       |
-        |--------------------|
-        | Type: const char * |
-        | Value: "42"        |
-        +--------------------+
-                |
-         ---------------
-        / const char * /
-        ---------------
+         ---------------                 ----------
+        / const char * /                / u64     /
+        ---------------                 ----------
+                |                               |
+                |                               V
+                |                       +-----------+
+                |                       | read      |
+                |                       +-----------+
+                |                       \ Count: 42 /
+                |                        -----------
                 |
                 V
         +--------------------------------------------------+
@@ -425,7 +419,7 @@ driver can be exemplified by the following::
         \ Count: "42"                                      /
          --------------------------------------------------
 
-There are three primary components involved:
+There are four primary components involved:
 
 Counter device driver
 ---------------------
@@ -445,3 +439,49 @@ 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.
+
+Counter chrdev
+--------------
+Translates counter data to the standard Counter character device; data
+is transferred via standard character device read/write calls.
+
+Sysfs Interface
+===============
+
+Several sysfs attributes are generated by the Generic Counter interface,
+and reside under the `/sys/bus/counter/devices/counterX` directory,
+where `X` is to the respective counter device id. Please see
+Documentation/ABI/testing/sysfs-bus-counter for detailed information on
+each Generic Counter interface sysfs attribute.
+
+Through these sysfs attributes, programs and scripts may interact with
+the Generic Counter paradigm Counts, Signals, and Synapses of respective
+counter devices.
+
+Counter Character Device
+========================
+
+Counter character device nodes are created under the `/dev` directory as
+`counterX`, where `X` is the respective counter device id. Defines for
+the standard Counter data types are exposed via the userspace
+`include/uapi/linux/counter-types.h` file.
+
+The first 196095 bytes of the character device serve as a control
+selection area where control exposure of desired Counter components and
+extensions may be selected. Each byte serves as a boolean selection
+indicator for a respective Counter component or extension. The format of
+this area is as follows:
+
+* For each device extension, a byte is required.
+* For each Signal, a byte is reserved for the Signal component, and a
+  byte is reserved for each Signal extension.
+* For each Count, a byte is reserved for the Count component, a byte is
+  reserved for the count function, a byte is reserved for each Synapse
+  action, and byte is reserved for each Count extension.
+
+The selected Counter components and extensions may then be interfaced
+after the first 196095 bytes via standard character device read/write
+operations. The number of bytes available for each component or
+extension is dependent on their respective data type: u8 will have 1
+byte available, u64 will have 8 bytes available, strings will have 64
+bytes available, etc.
-- 
2.26.2