Raw HID: documentation improvements (#21596)

docs/feature_rawhid.md

@@ -1,69 +1,156 @@
-# Raw HID
+# Raw HID :id=raw-hid
 
-Raw HID allows for bidirectional communication between QMK and the host computer over an HID interface. This has many potential use cases, such as switching keymaps on the fly or changing RGB LED colors and modes.
+The Raw HID feature allows for bidirectional communication between QMK and the host computer over an HID interface. This has many potential use cases, such as switching keymaps on the fly or sending useful metrics like CPU/RAM usage.
 
-There are two main components to getting raw HID working with your keyboard.
+In order to communicate with the keyboard using this feature, you will need to write a program that runs on the host. As such, some basic programming skills are required - more if you intend to implement complex behaviour.
 
-## Keyboard firmware
+## Usage :id=usage
 
-The implementation is fairly straightforward for the firmware.
-In your `rules.mk` add:
+Add the following to your `rules.mk`:
 
 ```make
 RAW_ENABLE = yes
 ```
 
-In your `keymap.c` include `"raw_hid.h"` and implement the following:
+## Basic Configuration :id=basic-configuration
+
+By default, the HID Usage Page and Usage ID for the Raw HID interface are `0xFF60` and `0x61`. However, they can be changed if necessary by adding the following to your `config.h`:
+
+|Define          |Default |Description                            |
+|----------------|--------|---------------------------------------|
+|`RAW_USAGE_PAGE`|`0xFF60`|The usage page of the Raw HID interface|
+|`RAW_USAGE_ID`  |`0x61`  |The usage ID of the Raw HID interface  |
+
+## Sending Data to the Keyboard :id=sending-data-to-the-keyboard
+
+To send data to the keyboard, you must first find a library for communicating with HID devices in the programming language of your choice. Here are some examples:
+
+* **Node.js:** [node-hid](https://github.com/node-hid/node-hid)
+* **C/C++:** [hidapi](https://github.com/libusb/hidapi)
+* **Java:** [purejavahidapi](https://github.com/nyholku/purejavahidapi) and [hid4java](https://github.com/gary-rowe/hid4java)
+* **Python:** [pyhidapi](https://pypi.org/project/hid/)
+
+Please refer to these libraries' own documentation for instructions on usage. Remember to close the device once you are finished with it!
+
+Next, you will need to know the USB Vendor and Product IDs of the device. These can easily be found by looking at your keyboard's `info.json`, under the `usb` object (alternatively, you can also use Device Manager on Windows, System Information on macOS, or `lsusb` on Linux). For example, the Vendor ID for the Planck Rev 6 is `0x03A8`, and the Product ID is `0xA4F9`.
+
+It's also a good idea to narrow down the list of potential HID devices the library may give you by filtering on the usage page and usage ID, to avoid accidentally opening the interface on the same device for the keyboard, or mouse, or media keys, etc.
+
+Once you are able to open the HID device and send reports to it, it's time to handle them on the keyboard side. Implement the following function in your `keymap.c` and start coding:
 
 ```c
 void raw_hid_receive(uint8_t *data, uint8_t length) {
-    // Your code goes here. data is the packet received from host.
+    // Your code goes here
+    // `data` is a pointer to the buffer containing the received HID report
+    // `length` is the length of the report - always `RAW_EPSIZE`
 }
 ```
 
-The `"raw_hid.h"` header also declares `void raw_hid_send(uint8_t *data, uint8_t length);` which allows sending packets from keyboard to host. As an example, it can also be used for debugging when building your host application by returning all data back to the host.
+!> Because the HID specification does not support variable length reports, all reports in both directions must be exactly `RAW_EPSIZE` (currently 32) bytes long, regardless of actual payload length. However, variable length payloads can potentially be implemented on top of this by creating your own data structure that may span multiple reports.
+
+## Receiving Data from the Keyboard :id=receiving-data-from-the-keyboard
+
+If you need the keyboard to send data back to the host, simply call the `raw_hid_send()` function. It requires two arguments - a pointer to a 32-byte buffer containing the data you wish to send, and the length (which should always be `RAW_EPSIZE`).
+
+The received report can then be handled in whichever way your HID library provides.
+
+## Simple Example :id=simple-example
+
+The following example reads the first byte of the received report from the host, and if it is an ASCII "A", responds with "B". `memset()` is used to fill the response buffer (which could still contain the previous response) with null bytes.
 
 ```c
 void raw_hid_receive(uint8_t *data, uint8_t length) {
-    raw_hid_send(data, length);
+    uint8_t response[length];
+    memset(response, 0, length);
+    response[0] = 'B';
+
+    if(data[0] == 'A') {
+        raw_hid_send(response, length);
+    }
 }
 ```
 
-These two functions send and receive packets of length `RAW_EPSIZE` bytes to and from the host (32 on LUFA/ChibiOS/V-USB, 64 on ATSAM).
+On the host side (here we are using Python and the `pyhidapi` library), the HID device is opened by enumerating the interfaces on the USB device, then filtering on the usage page and usage ID. Then, a report containing a single ASCII "A" (hex `0x41`) is constructed and sent.
+
+For demonstration purposes, the manufacturer and product strings of the device, along with the request and response, are also printed.
+
+```python
+import sys
+import hid
+
+vendor_id     = 0x4335
+product_id    = 0x0002
+
+usage_page    = 0xFF60
+usage         = 0x61
+report_length = 32
+
+def get_raw_hid_interface():
+    device_interfaces = hid.enumerate(vendor_id, product_id)
+    raw_hid_interfaces = [i for i in device_interfaces if i['usage_page'] == usage_page and i['usage'] == usage]
+
+    if len(raw_hid_interfaces) == 0:
+        return None
 
-Make sure to flash raw enabled firmware before proceeding with working on the host side.
+    interface = hid.Device(path=raw_hid_interfaces[0]['path'])
 
-## Host (Windows/macOS/Linux)
+    print(f"Manufacturer: {interface.manufacturer}")
+    print(f"Product: {interface.product}")
 
-This is the more complicated part as it will require some digging.
+    return interface
 
-To connect your host computer to your keyboard with raw HID you need four pieces of information about your keyboard:
+def send_raw_report(data):
+    interface = get_raw_hid_interface()
 
-1. Vendor ID
-2. Product ID
-3. Usage Page
-4. Usage
+    if interface is None:
+        print("No device found")
+        sys.exit(1)
+
+    request_data = [0x00] * (report_length + 1) # First byte is Report ID
+    request_data[1:len(data) + 1] = data
+    request_report = bytes(request_data)
+
+    print("Request:")
+    print(request_report)
+
+    try:
+        interface.write(request_report)
+
+        response_report = interface.read(report_length, timeout=1000)
+
+        print("Response:")
+        print(response_report)
+    finally:
+        interface.close()
+
+if __name__ == '__main__':
+    send_raw_report([
+        0x41
+    ])
+```
 
-The first two can easily be found in your keyboard's `config.h` in the keyboard's main directory under `VENDOR_ID` and `PRODUCT_ID`.
+## API :id=api
 
-The final two can be overridden in your keyboard's `config.h` in the keyboard's main directory by redefining the values: `#define RAW_USAGE_PAGE 0xFF60` and `#define RAW_USAGE_ID 0x61`.
+### `void raw_hid_receive(uint8_t *data, uint8_t length)` :id=api-raw-hid-receive
 
-By default, **Usage Page** is `0xFF60` and **Usage** is `0x61`.
+Callback, invoked when a raw HID report has been received from the host.
 
-### Building your host
+#### Arguments :id=api-raw-hid-receive-arguments
 
-You can build your host using any language that has an available HID implementation library if you don't wish to make your own. The ones we know of for popular languages are:
+ - `uint8_t *data`  
+   A pointer to the received data. Always 32 bytes in length.
+ - `uint8_t length`  
+   The length of the buffer. Always 32.
 
-* Node: [node-hid](https://github.com/node-hid/node-hid).
-* C: [hidapi](https://github.com/libusb/hidapi).
-* Java: [purejavahidapi](https://github.com/nyholku/purejavahidapi) and [hid4java](https://github.com/gary-rowe/hid4java).
-* Python: [pyhidapi](https://pypi.org/project/hid/).
+---
 
-This is not an exhaustive cross-platform list but should get you started. There are no special requirements for using raw HID so any HID library should work.
+### `void raw_hid_send(uint8_t *data, uint8_t length)` :id=api-raw-hid-send
 
-Now that you have all four pieces of information required to open HID interface to your keyboard. All you need to do is use your library's available functions to open the device with its ID parameters.
+Send an HID report.
 
-Note that Vendor ID and Product ID are not actually required to open the device. They are used only to filter to a specific device out of the many HID devices you have plugged in. Many libraries will give you the option to open the device using Product Name or Manufacturer Name instead, `node-hid` being a prime example. This will create issues for devices with builtin USB Hub or any extra HID interfaces where you will have multiple interfaces with the same name or from the same manufacturer. The Vendor ID together with Product ID create a unique designation to a single interface and will not exhibit this problem. Therefore, even if your library doesn't require you to, it is best to use them to avoid issues.
-Unlike Vendor ID and Product ID though, Usage Page and Usage are necessary for successful communication.
+#### Arguments :id=api-raw-hid-send-arguments
 
-It should go without saying that regardless of the library you're using, you should always make sure to close the interface when finished. Depending on the operating system and your particular environment there may be issues connecting to it again afterwards with another client or another instance of the same client if it's not explicitly closed.
+ - `uint8_t *data`  
+   A pointer to the data to send. Must always be 32 bytes in length.
+ - `uint8_t length`  
+   The length of the buffer. Must always be 32.

quantum/raw_hid.h

@@ -1,5 +1,31 @@
+// Copyright 2023 QMK
+// SPDX-License-Identifier: GPL-2.0-or-later
+
 #pragma once
 
+#include <stdint.h>
+
+/**
+ * \file
+ *
+ * \defgroup raw_hid Raw HID API
+ * \{
+ */
+
+/**
+ * \brief Callback, invoked when a raw HID report has been received from the host.
+ *
+ * \param data A pointer to the received data. Always 32 bytes in length.
+ * \param length The length of the buffer. Always 32.
+ */
 void raw_hid_receive(uint8_t *data, uint8_t length);
 
+/**
+ * \brief Send an HID report.
+ *
+ * \param data A pointer to the data to send. Must always be 32 bytes in length.
+ * \param length The length of the buffer. Must always be 32.
+ */
 void raw_hid_send(uint8_t *data, uint8_t length);
+
+/** \} */