From be18475e7028662b7944c4e818592616d3e5c154 Mon Sep 17 00:00:00 2001
From: SaiRishithaDharoor <sairishithadharoor@gmail.com>
Date: Tue, 31 Mar 2026 21:16:58 +0530
Subject: [PATCH] Add deadlock detection example using SymbiYosys

---
 docs/examples/deadlock_detection/README.md    | 104 ++++++++++++++++++
 docs/examples/deadlock_detection/deadlock.sby |  13 +++
 docs/examples/deadlock_detection/fsm.v        |  47 ++++++++
 3 files changed, 164 insertions(+)
 create mode 100644 docs/examples/deadlock_detection/README.md
 create mode 100644 docs/examples/deadlock_detection/deadlock.sby
 create mode 100644 docs/examples/deadlock_detection/fsm.v

diff --git a/docs/examples/deadlock_detection/README.md b/docs/examples/deadlock_detection/README.md
new file mode 100644
index 0000000..d94bada
--- /dev/null
+++ b/docs/examples/deadlock_detection/README.md
@@ -0,0 +1,104 @@
+\# Deadlock Detection using SymbiYosys
+
+
+
+\## 1. Introduction
+
+
+
+Deadlocks are a critical class of bugs in digital designs where the system enters a state with no possible forward progress. Detecting such issues early is important for ensuring correct system behavior.
+
+This example demonstrates how **Formal verification** can be used to identify deadlocks in RTL designs using SymbiYosys.
+
+
+
+\## 2. Design Overview
+
+
+
+The design is a simple Finite State Machine (FSM) with three states: IDLE, WAIT, GRANT
+
+State Transitions:
+
+\- From 'IDLE', the FSM moves to 'WAIT' when 'req' is asserted.
+
+\- The 'WAIT' state has no outgoing transitions.
+
+\- The 'GRANT' state is never reached.
+
+This creates a Deadlock condition, where the FSM gets stuck in the 'WAIT' state indefinitely.
+
+
+
+\## 3. Deadlock Scenario
+
+
+
+IDLE --(req)--> WAIT --(no exit)--> WAIT (forever)
+
+Once the FSM enters the 'WAIT' state, it cannot transition to any other state.
+
+
+
+\## 4. Formal Verification Approach
+
+
+
+Instead of simulating specific test cases, formal verification explores all possible behaviors of the design.
+
+Property Used: cover(grant);
+
+
+
+\## 5. Interpretation of Results
+
+If the cover condition is reached -> the GRANT state is reachable (no deadlock)
+
+If the cover condition is not reached -> the GRANT state is unreachable
+
+In this design, The GRANT state is never reached. Therefore, the FSM is deadlocked
+
+
+
+\##6. Running the Example
+
+Requirements:
+
+Yosys
+
+SymbiYosys
+
+Run Command: sby -f deadlock.sby
+
+
+
+\## 7. Expected Outcome
+
+
+
+Status: PASSED
+
+This indicates that the cover condition (grant = 1) was not reached, implying that the GRANT state is unreachable.
+
+
+
+Status: UNKNOWN
+
+This indicates that the tool could not fully determine the result within the given exploration depth.
+
+However, since the cover condition was not reached during exploration, it still suggests that the GRANT state is likely unreachable.
+
+
+
+Interpretation:
+
+
+
+In both cases, the GRANT state is not reached during formal exploration.
+
+This means there is no valid execution path that allows the FSM to progress beyond the WAIT state.
+
+
+
+Therefore, the FSM is stuck in a state with no forward progress, indicating a deadlock condition.
+
diff --git a/docs/examples/deadlock_detection/deadlock.sby b/docs/examples/deadlock_detection/deadlock.sby
new file mode 100644
index 0000000..d672d8d
--- /dev/null
+++ b/docs/examples/deadlock_detection/deadlock.sby
@@ -0,0 +1,13 @@
+[options]
+mode bmc
+depth 10
+
+[engines]
+smtbmc
+
+[script]
+read -formal fsm.v
+prep -top fsm
+
+[files]
+fsm.v
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diff --git a/docs/examples/deadlock_detection/fsm.v b/docs/examples/deadlock_detection/fsm.v
new file mode 100644
index 0000000..06aab5d
--- /dev/null
+++ b/docs/examples/deadlock_detection/fsm.v
@@ -0,0 +1,47 @@
+module fsm (
+    input clk,
+    input rst,
+    input req,
+    output reg grant
+);
+
+parameter IDLE  = 2'b00;
+parameter WAIT  = 2'b01;
+parameter GRANT = 2'b10;
+
+reg [1:0] state;
+
+always @(posedge clk) begin
+    if (rst) begin
+        state <= IDLE;
+        grant <= 0;
+    end else begin
+        case (state)
+            IDLE: begin
+                if (req)
+                    state <= WAIT;
+            end
+
+            WAIT: begin
+                // Deadlock: no exit from WAIT state
+                state <= WAIT;
+            end
+
+            GRANT: begin
+                grant <= 1;
+                state <= IDLE;
+            end
+        endcase
+    end
+end
+
+`ifdef FORMAL
+
+// Check if GRANT state is reachable
+always @(posedge clk) begin
+    cover(grant);
+end
+
+`endif
+
+endmodule
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