[Hexagon] Add doc on TVM - Hexagon RPC flow

[masahi]

Rendered

This PR adds documentation on the complex interaction between TVM and Hexagon RPC.

@kparzysz-quic @mehrdadh @areusch @tmoreau89

[masahi]

@kparzysz-quic As my doc shows, LocalSession is not involved in TVM - Hexagon RPC. So we can delete

tvm/cmake/modules/Hexagon.cmake

Line 249 in cb7f773

"${TVMRT_SOURCE_DIR}/rpc/rpc_local_session.cc"

Can I go ahead and do that?

[kparzysz-quic]

You can try that, but when I did it, it caused failures. Make sure that once you delete it, it still works.

Edit: please check both, hardware and simulator.

[masahi]

You can try that, but when I did it, it caused failures. Make sure that once you delete it, it still works.

Edit: please check both, hardware and simulator.

Ok, I'll do that later. I wonder why you got the failure. Karl (don't know his github) got a call trace on the Hexagon side which showed:

* thread #2, stop reason = breakpoint 7.1
  * frame #0: 0xfdf4bc30 libhexagon_rpc_skel.so`tvm::runtime::hexagon::HexagonDeviceAPIv2::CopyDataFromTo(DLTensor*, DLTensor*, void*)
    frame #1: 0xfde9b4a4 libhexagon_rpc_skel.so`TVMDeviceCopyDataFromTo + 68
    frame #2: 0xfde78b20 libhexagon_rpc_skel.so`tvm::runtime::MinRPCServer<tvm::runtime::hexagon::HexagonIOHandler>::HandleCopyToRemote() + 272
    frame #3: 0xfde785e4 libhexagon_rpc_skel.so`tvm::runtime::MinRPCServer<tvm::runtime::hexagon::HexagonIOHandler>::ProcessOnePacket() + 212
    frame #4: 0xfde777b8 libhexagon_rpc_skel.so`hexagon_rpc_send + 52
    frame #5: 0x00180970 fastrpc_shell_unsigned_3`mod_table_invoke + 528
    frame #6: 0x00171b6c fastrpc_shell_unsigned_3`fastrpc_invoke_dispatch + 556

So there is no LocalSession involved on the Hexagon side.

[masahi]

@kparzysz-quic please review