RRP Protocol

RRP protocol is 'high level' protocol like HTTP.

Indicators

Value	Name	Meaning
01010101(0x55)	START	Request/Response starts
00000000(0x00)	END	This request/response flow is finished
11111111(0xFF)	CONTINUE	This request/response continues

Request

Normal

Step	Operation	Data size
1	Send `START` indicator	1byte
2	Send request id	1byte
3	Send endpoint id	1byte
4	Send `CONTINUE` indicator	1byte
5	Send data size	4byte
6	Send data	Size specified in step 5
7	Send `END` indicator	1byte

Stream

Step	Operation	Data size
1	Send `START` indicator	1byte
2	Send request id	1byte
3	Send endpoint id	1byte
4	If stream continues: Send `CONTINUE` If stream finished: Send `END`	1byte
5	Send data size	4byte
6	Send data	Size specified in step 5
7	Goto step 4

Response

Normal

Step	Operation	Data size
1	Send `START` indicator	1byte
2	Send request id tied to this response	1byte
3	Send status code	1byte
4	Send `CONTINUE` indicator	1byte
5	Send data size	4byte
6	Send data	Size specified in step 5
7	Send `END` indicator	1byte

Stream

Step	Operation	Data size
1	Send `START` indicator	1byte
2	Send request id tied to this response	1byte
3	Send status code	1byte
4	If stream continues: Send `CONTINUE` If stream finished: Send `END`	1byte
5	Send data size	4byte
6	Send data	Size specified in step 5
7	Goto step 4
If status code is other than 0, event stream endpoints respond with normal response.

About streaming

Although it is called streaming, this is not primarily intended for streaming, but rather to save memory when passing large vec.

If a request/response is streaming, its size should be greater than 0 so it can be distinguished from an End indicator.

RRP-hid protocol

RRP-hid protocol is low level protocol like udp to transfer RRP through hid.

Use Output Report for request
- Output report is [u8; 32]
Use Input Report for response
- Input report is [u8; u32]
  (This report descriptor's structure is same as via's one.)
Usage page id: 0xFF70, Usage id: 0x71

One report is considered as a packet. A packet contains the RRP data and the size of the data included in the packet. So the data structure looks like this.
1: Data size (1byte, 0-31)
2: Data (arbitrary size specified in Data Size)
Report size is always 32 bytes, and remained bytes are undefined (usually filled with 0).

This protocol doesn't have flow control. While one rrp request or response is ongoing, other requests or responses muse be blocked.

Descriptor

Changed usage id of rmk's via code

#[gen_hid_descriptor(
    (collection = APPLICATION, usage_page = 0xFF70, usage = 0x71) = {
        (usage = 0x72, logical_min = 0x0) = {
            #[item_settings data,variable,absolute] input_data=input;
        };
        (usage = 0x73, logical_min = 0x0) = {
            #[item_settings data,variable,absolute] output_data=output;
        };
    }
)]
pub(crate) struct ViaReport {
    pub(crate) input_data: [u8; 32],
    pub(crate) output_data: [u8; 32],
}

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