pifo-hardware: update README

Author: depctg

pifo-hardware/README.md

@@ -1,8 +1,80 @@
 # Runtime reconfigurable pifo hardware
 
 ```bash
-sbt "runMain rio.sim.BasicPifoSim"
+sbt "runMain rio.sim.PifoMeshSim"
 ```
 
+This contains a PifoMesh Implementation. Current implementation assumes the insert input to be a `PacketToken = engineId ## flowId`. The `PacketToken` will be processed in PifoEngine in the following process:
+
+### Enqueue Path
+
+```
+    ┌─────────────────────┐
+    │  PacketToken        │-───────────────────┐
+    │  engineId ## flowId │                    │
+    └──────────┬──────────┘                    │
+               │                               │
+               ▼                               │
+    ┌─────────────────────────────┐            │
+    │  EnqueueMapper              │-───────────┤
+    │  flowId → vPifoId           │            │
+    └──────────┬──────────────────┘            │
+               │ vPifoId                       │
+               ▼                               │
+    ┌─────────────────────────────┐            │
+    │  Brain                      │            │
+    │  {engineId, vPifo, Flow}    │            │
+    │  → Rank                     │            │
+    └──────────┬──────────────────┘            │
+               │ Rank                          │
+               ▼                               ▼
+    ┌──────────────────────────────────────────────┐
+    │  PIFO.insert                                 │
+    │  { vPifo=vPifoId, rank=Rank, data=Token }    │
+    └──────────────────────────────────────────────┘
+```
+
+### Dequeue Path
+
+```
+    ┌─────────────────────┐
+    │  DequeueRequest     │
+    │  engineId ## vPifoId│
+    └──────────┬──────────┘
+               │
+               ▼
+    ┌─────────────────────┐
+    │  PIFO.dequeue       │
+    │  vPifoId →          │
+    │{PacketToken, exist?}│-- if not exist? --> Drop
+    └──────────┬──────────┘
+               │ if exist?
+               ▼
+    ┌──────────────────────────────┐
+    │  DequeueMapper               │
+    │  data (PacketToken)          │
+    │  → PacketToken               │
+    └──────────┬───────────────────┘
+               │ PacketToken
+               ▼
+    ┌──────────────────────────────┐
+    │  Crossbar (xbar)             │
+    |  case engineId               |
+    │    Port 0 → Output           │
+    │    Other  → Other Engines    │
+    └──────────────────────────────┘
+```
+
+**Note**: For simplicity, `flowId` and `vPifoId` have the same width and use the name `vPifoId` in the code.
+
+## TODO List
+
+- [ ] Support per-PIFO copy, make non-exist pifo pop return an invalid message
+- [ ] Support packet meta data and packet identifier in mesh message and brain
+- [ ] Support configurable brain policy
+    - [ ] support configurable rank in WFQ
+- [ ] Hardware controller for insertion and pop
+- [ ] Transactional update of 
+