qmk_firmware/docs/xap_0.0.1.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
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.
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
Handler	A piece of code that is executed when a specific route is received.
ID	A single octet / 8-bit byte.
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.
Response Flags	An u8 containing the status of the request.
Route	A sequence of IDs describing the route to invoke a handler.
Subsystem	A high-level area of functionality within XAP.
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 repsonse 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.

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
-	-	-	-	-	-	-	Success

• Bit 0: 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