lib/utils: Add ARM semihosting utility functions.

This can be a replacement for a UART in custom ports.

lib/utils/semihosting.c

@@ -0,0 +1,132 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2018 Ayke van Laethem
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "semihosting.h"
+
+// Resources:
+// http://embed.rs/articles/2016/semi-hosting-rust/
+// https://wiki.dlang.org/Minimal_semihosted_ARM_Cortex-M_%22Hello_World%22
+// https://github.com/arduino/OpenOCD/blob/master/src/target/arm_semihosting.c
+
+#define SYS_OPEN   0x01
+#define SYS_WRITEC 0x03
+#define SYS_WRITE  0x05
+#define SYS_READC  0x07
+
+// Constants:
+#define OPEN_MODE_READ  (0) // mode "r"
+#define OPEN_MODE_WRITE (4) // mode "w"
+
+#ifndef __thumb__
+#error Semihosting is only implemented for ARM microcontrollers.
+#endif
+
+static int mp_semihosting_stdout;
+
+static uint32_t mp_semihosting_call(uint32_t num, const void *arg) {
+    // A semihosting call works as follows, similar to a SVCall:
+    //   * the call is invoked by a special breakpoint: 0xAB
+    //   * the command is placed in r0
+    //   * a pointer to the arguments is placed in r1
+    //   * the return value is placed in r0
+    // Note that because it uses the breakpoint instruction, applications
+    // will hang if they're not connected to a debugger. And they'll be
+    // stuck in a breakpoint if semihosting is not specifically enabled in
+    // the debugger.
+    // Also note that semihosting is extremely slow (sometimes >100ms per
+    // call).
+    register uint32_t num_reg __asm__ ("r0") = num;
+    register const void *args_reg __asm__ ("r1") = arg;
+    __asm__ __volatile__ (
+        "bkpt 0xAB\n"         // invoke semihosting call
+        : "+r" (num_reg)      // call number and result
+        : "r" (args_reg)      // arguments
+        : "memory");          // make sure args aren't optimized away
+    return num_reg; // r0, which became the result
+}
+
+static int mp_semihosting_open_console(uint32_t mode) {
+    struct {
+        char *name;
+        uint32_t mode;
+        uint32_t name_len;
+    } args = {
+        .name = ":tt",     // magic path to console
+        .mode = mode,      // e.g. "r", "w" (see OPEN_MODE_* constants)
+        .name_len = 3,     // strlen(":tt")
+    };
+    return mp_semihosting_call(SYS_OPEN, &args);
+}
+
+void mp_semihosting_init() {
+    mp_semihosting_stdout = mp_semihosting_open_console(OPEN_MODE_WRITE);
+}
+
+int mp_semihosting_rx_char() {
+    return mp_semihosting_call(SYS_READC, NULL);
+}
+
+static void mp_semihosting_tx_char(char c) {
+    mp_semihosting_call(SYS_WRITEC, &c);
+}
+
+uint32_t mp_semihosting_tx_strn(const char *str, size_t len) {
+    if (len == 0) {
+        return 0; // nothing to do
+    }
+    if (len == 1) {
+        mp_semihosting_tx_char(*str); // maybe faster?
+        return 0;
+    }
+
+    struct {
+        uint32_t fd;
+        const char *str;
+        uint32_t len;
+    } args = {
+        .fd = mp_semihosting_stdout,
+        .str = str,
+        .len = len,
+    };
+    return mp_semihosting_call(SYS_WRITE, &args);
+}
+
+uint32_t mp_semihosting_tx_strn_cooked(const char *str, size_t len) {
+    // Write chunks of data until (excluding) the first '\n' character,
+    // insert a '\r' character, and then continue with the next chunk
+    // (starting with '\n').
+    // Doing byte-by-byte writes would be easier to implement but is far
+    // too slow.
+    size_t start = 0;
+    for (size_t i = 0; i < len; i++) {
+        if (str[i] == '\n') {
+            mp_semihosting_tx_strn(str + start, i - start);
+            mp_semihosting_tx_char('\r');
+            start = i;
+        }
+    }
+    return mp_semihosting_tx_strn(str + start, len - start);
+}

lib/utils/semihosting.h

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+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2018 Ayke van Laethem
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+#ifndef MICROPY_INCLUDED_LIB_UTILS_SEMIHOSTING_H
+#define MICROPY_INCLUDED_LIB_UTILS_SEMIHOSTING_H
+
+/*
+
+To use semi-hosting for a replacement UART:
+- Add lib/semihosting/semihosting.c to the Makefile sources.
+- Call mp_semihosting_init() in main(), around the time UART is initialized.
+- Replace mp_hal_stdin_rx_chr and similar in mphalport.c with the semihosting equivalent.
+- Include lib/semihosting/semihosting.h in the relevant files.
+
+Then make sure the debugger is attached and enables semihosting.  In OpenOCD this is
+done with ARM semihosting enable followed by reset.  The terminal will need further
+configuration to work with MicroPython (bash: stty raw -echo).
+
+*/
+
+#include <stddef.h>
+#include <stdint.h>
+
+void mp_semihosting_init();
+int mp_semihosting_rx_char();
+uint32_t mp_semihosting_tx_strn(const char *str, size_t len);
+uint32_t mp_semihosting_tx_strn_cooked(const char *str, size_t len);
+
+#endif // MICROPY_INCLUDED_LIB_UTILS_SEMIHOSTING_H