Add versioning to IPC

[mrquincle]

• This needs to be properly documented as well.
• There is a version required for the IPC structs. When those structs, e.g. between bootloader and bluenet firmware, the corresponding functions need to be able to implement backwards compatibility. The implement version is a major/minor combination.
• Bluenet and a microapp also communicates over IPC. In this struct a callback can be written by bluenet to be called by the microapp. If this struct changes, a major/minor update make sense.
• The messages between bluenet and microapps are communicated back and forth by a separate buffer (maintained by the microapp so it doesn't incur penalties at the bluenet side if no microapp is present). This is according to a versioned protocol. This protocol version is also present in the IPC struct [1].
• The major/minor with respect to the SDK is not clearly separated from the protocol version that is used for uploading microapps [2].

[1] It is perhaps sufficient to have just a minor update for a protocol update? [2] Naming must be so that it is very clear what is done where. The reason to have a protocol version for uploading microapps is to allow for different upload mechanisms without having to bump the general BLE protocol version. Updating the general BLE protocol version is very invasive.

[vliedel]

So the question is how to deal with IPC major version updates. The problem is that the version can impact 2 different things: the payload data (bootloader or microapp specific data), or the IPC header itself.

So a solution could be: add yet another version: the IPC header version. In that case, we sort of go back to having the data like:

struct ipc_header_t {
uint8_t version;
uint8_t index;
uint8_t dataSize;
uint16_t checksum;
};

struct ipc_bootloader_t {
uint8_t major;
uint8_t minor;
uint16_t dfuVersion;
...
};

struct ipc_microapp_t {
// Version of this struct.
uint8_t ipcMajor;
uint8_t ipcMinor;
microappCallbackFunc microappCallback;
...
};

Another solution would be to increase all majors to the same value on a breaking change of the IPC header.

I wouldn't let the caller make another call to getIpcData(), but instead let getIpcData() deal with the different IPC headers.

[mrquincle]

So you propose the thing that is implemented in this PR?

[mrquincle]

All concerns are addressed.

[vliedel]

Thanks :)