sw: Add possibility to get ocd putchar on gvsoc (#126)

Co-authored-by: Germain Haugou <[email protected]>

sw/snRuntime/src/openocd.h

@@ -0,0 +1,34 @@
+// Copyright 2023 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 <stdint.h>
+
+/* riscv semihosting standard:
+ * IN: a0 holds syscall number
+ * IN: a1 holds pointer to arg struct
+ * OUT: a0 holds return value (if exists)
+ */
+static inline long __ocd_semihost(long n, long _a1) {
+    register long a0 asm("a0") = n;
+    register long a1 asm("a1") = _a1;
+
+    // riscv magic values for semihosting
+    asm volatile(
+        ".option norvc;\t\n"
+        "slli    zero,zero,0x1f\t\n"
+        "ebreak\t\n"
+        "srai    zero,zero,0x7\t\n"
+        ".option rvc;\t\n"
+        : "+r"(a0)
+        : "r"(a1));
+
+    return a0;
+}
+
+static inline int __ocd_semihost_write(int fd, uint8_t *buffer, int len) {
+    uint32_t args[3] = {(long)fd, (long)buffer, (long)len};
+    __asm__ __volatile__("" : : : "memory");
+    return __ocd_semihost(0x05, (long)args);
+}

sw/snRuntime/src/start.c

@@ -99,8 +99,12 @@ static inline void snrt_init_libs() { snrt_alloc_init(); }
 #ifdef SNRT_CRT0_EXIT
 static inline void snrt_exit_default(int exit_code) {
     exit_code = snrt_global_all_to_all_reduction(exit_code);
+#ifdef OPENOCD_SEMIHOSTING
+    if (snrt_global_core_idx() == 0) __ocd_semihost(0x18, (long)exit_code);
+#else
     if (snrt_global_core_idx() == 0)
         *(snrt_exit_code_destination()) = (exit_code << 1) | 1;
+#endif
 }
 #ifndef SNRT_CRT0_ALTERNATE_EXIT
 static inline void snrt_exit(int exit_code) { snrt_exit_default(exit_code); }

target/snitch_cluster/README.md

@@ -60,6 +60,9 @@ make DEBUG=ON sw
 
 # for Banshee simulation (requires slightly different runtime)
 make SELECT_RUNTIME=banshee DEBUG=ON sw
+
+# to use OpenOCD semi-hosting for putchar and termination
+make DEBUG=ON OPENOCD_SEMIHOSTING=ON sw
 ```
 
 The `sw` target first generates some C header files which depend on the hardware configuration. Hence, the need to generate the software for the same configuration as your hardware. Afterwards, it recursively invokes the `make` target in the `sw` subdirectory to build the apps/kernels which have been developed in that directory.
@@ -70,6 +73,8 @@ The `SELECT_RUNTIME` flag is set by default to `rtl`. To build the software with
 
 ___Note:__ the RTL is not the only source which is generated from the configuration file. The software stack also depends on the configuration file. Make sure you always build the software with the same configuration of the hardware you are going to run it on._
 
+___Note:__ on GVSOC, it is better to use OpenOCD semi-hosting to prevent putchar from disturbing the DRAMSys timing model._
+
 ### Running a simulation
 
 Create the `logs` directory to host the simulation traces:

target/snitch_cluster/sw/runtime/rtl/src/putchar.c

@@ -2,6 +2,26 @@
 // Licensed under the Apache License, Version 2.0, see LICENSE for details.
 // SPDX-License-Identifier: Apache-2.0
 
+#ifdef OPENOCD_SEMIHOSTING
+
+#include <openocd.h>
+
+#define PUTC_BUFFER_LEN 1024
+static int putchar_index[SNRT_CLUSTER_NUM * SNRT_CLUSTER_CORE_NUM] = {0};
+static char putchar_data[SNRT_CLUSTER_NUM * SNRT_CLUSTER_CORE_NUM]
+                        [PUTC_BUFFER_LEN];
+
+// Provide an implementation for putchar.
+void _putchar(char character) {
+    volatile char *buf = putchar_data[snrt_hartid()];
+    buf[putchar_index[snrt_hartid()]++] = character;
+    if (putchar_index[snrt_hartid()] == PUTC_BUFFER_LEN || character == '\n') {
+        __ocd_semihost_write(1, (uint8_t *)buf, putchar_index[snrt_hartid()]);
+        putchar_index[snrt_hartid()] = 0;
+    }
+}
+
+#else
 extern uintptr_t volatile tohost, fromhost;
 
 // Rudimentary string buffer for putc calls.
@@ -34,3 +54,5 @@ void _putchar(char character) {
         buf->hdr.size = 0;
     }
 }
+
+#endif

target/snitch_cluster/sw/runtime/rtl/src/snitch_cluster_start.c

@@ -11,8 +11,10 @@
 #define SNRT_CRT0_POST_BARRIER
 #define SNRT_CRT0_EXIT
 
+#ifndef OPENOCD_SEMIHOSTING
 static inline volatile uint32_t* snrt_exit_code_destination() {
     return (volatile uint32_t*)&tohost;
 }
+#endif
 
 #include "start.c"

target/snitch_cluster/sw/toolchain.mk

@@ -42,6 +42,12 @@ RISCV_CFLAGS += -O3
 ifeq ($(DEBUG), ON)
 RISCV_CFLAGS += -g
 endif
+
+ifeq ($(OPENOCD_SEMIHOSTING), ON)
+RISCV_CFLAGS += -DOPENOCD_SEMIHOSTING
+endif
+
+
 # Required by math library to avoid conflict with stdint definition
 RISCV_CFLAGS += -D__DEFINED_uint64_t