qmk_firmware/docs/xap_0.1.0.md

QMK Firmware XAP Specs

This document describes the requirements of the QMK XAP ("extensible application protocol") API.

Types

All integral types are little-endian.

Name	Definition
struct{}	A structure of data, packing different objects together. Data is "compacted" -- there are no padding bytes between fields. Equivalent to a packed C-style struct. The order in which they're defined matches the order of the data in the response packet.
type[n]	An array of type, with array extent of N -- e.g. u8[2] signifies two consecutive octets.
u16	An unsigned 16-bit integral, commonly seen as uint16_t from stdint.h.
u32	An unsigned 32-bit integral, commonly seen as uint32_t from stdint.h.
u64	An unsigned 64-bit integral, commonly seen as uint64_t from stdint.h.
u8	An unsigned 8-bit integral (octet, or byte), commonly seen as uint8_t from stdint.h.

Definitions

This list defines the terms used across the entire set of XAP protocol documentation.

Name	Definition
Capability	A way to determine if certain functionality is enabled in the firmware. Any subsystem that provides build-time restriction of functionality must provide a route for a capabilities query.
Handler	A piece of code that is executed when a specific route is received.
Payload	Any received data appended to the route, which gets delivered to the handler when received.
Response	The data sent back to the host during execution of a handler.
Route	A sequence of IDs describing the route to invoke a handler.
Secure Route	A route which has potentially destructive consequences, necessitating prior approval by the user before executing.
Subsystem	A high-level area of functionality within XAP.
Unlock sequence	A physical sequence initiated by the user to enable execution of secure routes.
Broadcast Header	Packet format for broadcast messages. Takes the format: token - token u8 - type u8 - length
ID	A single octet / 8-bit byte, representing Subsystem or Route index.
Request Header	Packet format for inbound data. Takes the format: token - token u8 - length
Response Flags	An u8 containing the status of the request.
Response Header	Packet format for outbound data. Takes the format: token - token response_flags - flags u8 - length
Token	A u16 associated with a specific request as well as its corresponding response.

Requests and Responses

Communication generally follows a request/response pattern.

Each request needs to include a token -- this u16 value prefixes each outbound request from the host application and its corresponding response, allowing response messages to be correlated with their request, even if multiple host applications are communicating with the firmware simultaneously. Host applications should randomly generate a token ID for every outbound request, unless using a reserved token defined below.

This token is followed by a u8 signifying the length of data in the request.

Two token values are reserved: 0x0000 and 0xFFFF:

• 0x0000: A message sent by a host application may use this token if no response is to be sent -- a "fire and forget" message.
• 0xFFFF: Signifies a "broadcast" message sent by the firmware without prompting from the host application. Broadcast messages are defined later in this document.

Any request will generate at least one corresponding response, with the exception of messages using reserved tokens. Maximum total message length is 128 bytes due to RAM constraints.

Response messages will always be prefixed by the originating request token, directly followed by that request's response flags, then the response payload length:

Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
UNLOCKED	UNLOCK_IN_PROGRESS	-	-	-	-	SECURE_FAILURE	SUCCESS

• Bit 7 (UNLOCKED): When this bit is set, an unlock sequence has completed, and secure routes may be invoked.
• Bit 6 (UNLOCK_IN_PROGRESS): When this bit is set, an unlock sequence is in progress.
• Bit 1 (SECURE_FAILURE): When this bit is set, the requested route was marked secure but an unlock sequence has not completed.
• Bit 0 (SUCCESS): When this bit is set, the request was successfully handled. If not set, all payload data should be disregarded, and the request retried if appropriate (with a new token).

Example "conversation":

Request -- version query:

Byte	0	1	2	3	4
Purpose	Token	Token	Payload Length	Route	Route
Value	0x43	0x2B	0x02	0x00	0x00

Response -- matching token, successful flag, payload of 0x03170192 = 3.17.192:

Byte	0	1	2	3	4	5	6	7
Purpose	Token	Token	Response Flags	Payload Length	Payload	Payload	Payload	Payload
Value	0x43	0x2B	0x01	0x04	0x92	0x01	0x17	0x03

Routes

Subsystem validity should be queried through the “Enabled-in-firmware subsystem query” under the QMK subsystem (route=0x00,0x01). This is the primary method for determining if a subsystem has been enabled in the running firmware.

XAP - 0x00

This subsystem is always present, and provides the ability to query information about the XAP protocol of the connected device.

Name	Route	Definition
Version Query	0x00 0x00	XAP protocol version query.
Capabilities Query	0x00 0x01	XAP subsystem capabilities query. Each bit should be considered as a "usable" route within this subsystem.
Enabled subsystem query	0x00 0x02	XAP protocol subsystem query. Each bit should be considered as a "usable" subsystem. For example, checking (value & (1 << XAP_ROUTE_QMK) != 0) means the QMK subsystem is enabled and available for querying.
Secure Status	0x00 0x03	Query secure route status
Secure Unlock	0x00 0x04	Initiate secure route unlock sequence
Secure Lock	0x00 0x05	Disable secure routes

QMK - 0x01

This subsystem is always present, and provides the ability to address QMK-specific functionality.

Name	Route	Definition
Version Query	0x01 0x00	QMK protocol version query.
Capabilities Query	0x01 0x01	QMK subsystem capabilities query. Each bit should be considered as a "usable" route within this subsystem.
Board identifiers	0x01 0x02	Retrieves the set of identifying information for the board.
Board Manufacturer	0x01 0x03	Retrieves the name of the manufacturer
Product Name	0x01 0x04	Retrieves the product name
info.json length	0x01 0x05	Retrieves the length of info.json
info.json	0x01 0x06	Retrieves a chunk of info.json
Jump to bootloader	0x01 0x07	Jump to bootloader
info.json	0x01 0x08	Retrieves a unique identifier for the board.

Keyboard - 0x02

This subsystem is always present, and reserved for user-specific functionality. No routes are defined by XAP.

User - 0x03

This subsystem is always present, and reserved for user-specific functionality. No routes are defined by XAP.

Broadcast messages

Broadcast messages may be sent by the firmware to the host, without a corresponding inbound request. Each broadcast message uses the token 0xFFFF, and does not expect a response from the host. Tokens are followed by an ID signifying the type of broadcast, with corresponding payload.

Log message - 0x00

Replicates and replaces the same functionality as if using the standard QMK CONSOLE_ENABLE = yes in rules.mk. Normal prints within the firmware will manifest as log messages broadcast to the host. hid_listen will not be functional with XAP enabled.

Log message payloads include a u8 signifying the length of the text, followed by the u8[Length] containing the text itself.

Example Log Broadcast -- log message "Hello QMK!"

Byte	0	1	2	3	4	5	6	7	8	9	10	11	12	13
Purpose	Token	Token	Broadcast Type	Length	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload	Payload
Value	0xFF	0xFF	0x00	0x0A(10)	0x48(H)	0x65(e)	0x6C(l)	0x6C(l)	0x6F(o)	0x20( )	0x51(Q)	0x4D(M)	0x4B(K)	0x21(!)

Secure Status - 0x01

Secure status has changed.