sw: Fix snrt_dma_wait implementation

docs/rm/custom_instructions.md

@@ -104,7 +104,7 @@ The DMSTATI instruction can be used to implement a blocking wait for the complet
 
         dmcpyi a0, ...
     1:  dmstati t0, 0
-        bltu a0, t0, 1b
+        bltu t0, a0, 1b
 
 Similarly, waiting for the completion of *all* DMA transfers:
 

sw/snRuntime/src/dma.h

@@ -51,7 +51,7 @@ inline snrt_dma_txid_t snrt_dma_start_1d_wideptr(uint64_t dst, uint64_t src,
 
         return reg_txid;
     } else {
-        return -1;
+        return 0;
     }
 }
 
@@ -129,7 +129,7 @@ inline snrt_dma_txid_t snrt_dma_start_2d_wideptr(uint64_t dst, uint64_t src,
 
         return reg_txid;
     } else {
-        return -1;
+        return 0;
     }
 }
 
@@ -151,8 +151,7 @@ inline void snrt_dma_wait(snrt_dma_txid_t tid) {
                (    0b000 << 12) | \
                (      (5) <<  7) | \
                (0b0101011 <<  0)   \n"
-        "sub t0, t0, %0 \n"
-        "blez t0, 1b \n" ::"r"(tid)
+        "bltu t0, %0, 1b \n" ::"r"(tid)
         : "t0");
 }
 

sw/tests/dma_empty_transfer.c

@@ -0,0 +1,25 @@
+// Copyright 2020 ETH Zurich and University of Bologna.
+// Licensed under the Apache License, Version 2.0, see LICENSE for details.
+// SPDX-License-Identifier: Apache-2.0
+
+#include <snrt.h>
+
+// Allocate a buffer in the main memory which we will use to copy data around
+// with the DMA.
+uint32_t buffer[32];
+
+int main() {
+    if (snrt_global_core_idx() != 8) return 0;  // only DMA core
+
+    // Populate source buffer.
+    uint32_t buffer_src[32];
+    for (uint32_t i = 0; i < 32; i++) {
+        buffer_src[i] = i + 1;
+    }
+
+    // Start a DMA transfer of zero size.
+    snrt_dma_txid_t id = snrt_dma_start_1d(buffer, buffer_src, 0);
+    snrt_dma_wait(id);
+
+    // Test is successful if the previous transfer doesn't block.
+}

sw/tests/dma_simple.c

@@ -21,8 +21,8 @@ int main() {
     }
 
     // Copy data to main memory.
-    snrt_dma_start_1d(buffer, buffer_src, sizeof(buffer));
-    snrt_dma_wait_all();
+    snrt_dma_txid_t id = snrt_dma_start_1d(buffer, buffer_src, sizeof(buffer));
+    snrt_dma_wait(id);
 
     // Check that the main memory buffer contains the correct data.
     for (uint32_t i = 0; i < 32; i++) {

target/snitch_cluster/sw/run.yaml

@@ -9,6 +9,7 @@ runs:
     simulators: [vsim, vcs, verilator] # banshee fails with exit code 0x4
   - elf: tests/build/barrier.elf
   - elf: tests/build/data_mover.elf
+  - elf: tests/build/dma_empty_transfer.elf
   - elf: tests/build/dma_simple.elf
   - elf: tests/build/event_unit.elf
   - elf: tests/build/fence_i.elf