rfnoc: Implement overrun handling using action API

In order to enable overrun handling through the action API, a few new
features are implemented:
- The RX streamer can now accept stream command actions. The streamer
  will interpret stream command actions as a request to send stream
  commands upstream to all producers.
- A new action type is defined ('restart request') which is understood
  by the radio and streamer, and is a handshake between producers and
  consumers. In this case, it will ask the radio to send a stream
  command itself.

When an RX streamer receives an overrun, it will now run the following
algorithm:
1. Stop all upstream producers (this was already in the code before this
   commit).
2. If no restart is required, Wait for the radios to have space in the
   downstream blocks.

The radio, if it was in continuous streaming mode before the overrun,
includes a flag in its initial action whether or not to restart the
streaming. Also, it will wait for the stop stream command from the
streamer. When it receives that, it will initiate a restart request
handshake.

3. The streamer submits a restart request action upstream. This action
   will be received by the radio.
   The radio will then check the current time, and send a stream command
   action back downstream.
4. The RX streamer receives the stream command action, and uses it to
   send another stream command to all upstream producers. This way, all
   upstream producers receive a start command for the same time.

1 files changed, 1 insertions, 0 deletions

diff --git a/host/include/uhd/rfnoc/defaults.hpp b/host/include/uhd/rfnoc/defaults.hpp
index 696d31f30..e1046ada2 100644
--- a/host/include/uhd/rfnoc/defaults.hpp
+++ b/host/include/uhd/rfnoc/defaults.hpp
@@ -29,6 +29,7 @@ static const io_type_t IO_TYPE_SC16 = "sc16";
 
 static const std::string ACTION_KEY_STREAM_CMD("stream_cmd");
 static const std::string ACTION_KEY_RX_EVENT("rx_event");
+static const std::string ACTION_KEY_RX_RESTART_REQ("restart_request");
 
 //! If the block name can't be automatically detected, this name is used
 static const std::string DEFAULT_BLOCK_NAME = "Block";