AM6442: Is there a Linux (A53) driver for the EQEP peripheral in AM64X

Hello Jim,

Paula is focused more on the MCU+ side of things, I'll continue handling the Linux EQEP questions for now.

The developer told me since this driver is so new, they are still working on setting up the EQEP test infrastructure for future SDK releases. I would expect that in the future, you will be able to see test outputs for the CICD snapshots here:
https://software-dl.ti.com/cicd-report/linux/index.html?section=platform&platform=am64xx 

They also finished merging in the EQEP driver documentation for the Linux SDK docs last night. I'll attach the updated source code here, since I'm not sure if the AM64x SDK 9.2 docs will be regenerated again in the near term, or if this update will show up for the first time in SDK 10.0 documentation:

EQEP
----

.. rubric:: Introduction
   :name: introduction-linux-eqep

Linux has support for EQEP (Enhanced Quadature Encoder Pulse).
EQEP channels can be used to interface with rotary encoder hardware.

.. ifconfig:: CONFIG_part_variant in ('AM62X', 'AM62AX')

	- AM62x SK: EQEP0: k3-am62x-sk-eqep.dtso

.. ifconfig:: CONFIG_part_variant in ('AM64X')

	- AM64x SK: EQEP0: k3-am642-sk.dts

.. ifconfig:: CONFIG_part_variant in ('AM62PX')

	- AM62p SK: EQEP0: k3-am62p5-sk-eqep.dtso

.. rubric:: Kernel Configuration
   :name: driver-configuration-eqep

Using menuconfig, enable the following driver:

.. code-block:: console

   Device Drivers --->
       <M> Counter support  --->
         <M> TI eQEP counter driver

.. note::

   EQEP driver should be enabled by default as a kernel module in TI Linux kernel.

.. rubric:: Features supported in TI SDK
   :name: features-supported-eqep

- position
- overflow/underflow events
- direction
- unit timer
- latched position
- edge capture

.. rubric:: Using eQEP with SYSFS interface
   :name: sysfs-interface-eqep

Several sysfs attributes are generated by the Generic Counter interface, and reside under
the :file:`/sys/bus/counter/devices/counterX` directory, where X is to the respective counter
device id.

Please see the counter sysfs documentation `upstream <https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-bus-counter>`__
or in TI Linux kernel :file:`/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.

The following sysfs atributes can be found in :file:`/sys/bus/counter/devices/counterX` in TI SDK:

- countY/ceiling
- countY/count
- countY/direction
- countY/enable
- countY/function
- countY/latched_count
- countY/signal0_action
- countY/signal1_action
- edge_capture_unit_enable
- edge_capture_unit_latched_period
- edge_capture_unit_max_period
- edge_capture_unit_prescaler
- latch_mode
- unit_timer_enable
- unit_timer_period
- unit_timer_time

.. rubric:: Character device nodes
   :name: character-device-nodes-eqep

Counter character device nodes are created under the /dev directory as counterX, where X is the respective counter device id.

.. code-block:: console

   $ ls /dev/counter*
   /dev/counter0

.. rubric:: Counter Events
   :name: counter-events

Please read the generic counter API `upstream <https://docs.kernel.org/driver-api/generic-counter.html>`__ or in
TI Linux kernel :file:`/Documentation/driver-api/generic-counter.rst` documentation to implement events:

.. rubric:: Test eQEP on AM62x SK
   :name: test-eqep-am62x

+------------------------------------------+---------------------------------------------------------+
| .. Image:: /images/am62x_sk_eqep.jpg     | .. Image:: /images/am62x_sk_rotary_encoder_breakout.jpg |
|       :width: 380px                      |       :width: 380px                                     |
|       :align: center                     |       :align: center                                    |
+------------------------------------------+---------------------------------------------------------+
| AM62x SK & rotary encoder breakout board | Example Diligent rotary encoder breakout board          |
+------------------------------------------+---------------------------------------------------------+

The example rotary encoder breakout board used in this example can be found `here <https://www.digikey.com/en/products/detail/digilent,-inc./410-117/4090075?>`__.

The following is an example for testing eQEP using sysfs interface:

.. code-block:: console

	root@am62xx-evm:~# echo 0xff > /sys/bus/counter/devices/counter0/count0/ceiling 
	root@am62xx-evm:~# cat /sys/bus/counter/devices/counter0/count0/ceiling
	255
	root@am62xx-evm:~# echo 1 > /sys/bus/counter/devices/counter0/count0/enable 
	root@am62xx-evm:~# cat /sys/bus/counter/devices/counter0/count0/count
	0
	root@am62xx-evm:~# cat /sys/bus/counter/devices/counter0/count0/count
	92
	root@am62xx-evm:~# cat /sys/bus/counter/devices/counter0/count0/count
	148
	root@am62xx-evm:~# cat /sys/bus/counter/devices/counter0/count0/count
	188
	root@am62xx-evm:~# cat /sys/bus/counter/devices/counter0/count0/count
	252
	root@am62xx-evm:~# cat /sys/bus/counter/devices/counter0/count0/count
	12

Notice in this example that turning the rotary shaft encoder counterclockwise increases the count
until reaching the ceiling, then wraps around to zero and starts again. Also, turning the rotary
shaft encoder clockwise decreases the count.

Regards,

Nick