USB Improvements

* Introduce shell module for basic serial shell with argument parsing
* Introduce shell_cmd_list module for basic compile-time command
  registration
* Harden USB handling to hang less and drop fewer inputs
  - Service tud_task() with periodic TC0 timer interrupt
  - Service cdc_task() with periodic TC1 timer interrupt
  - Handle shell servicing in main app loop
  - Add a circular buffering layer for reads/writes
* Change newline prints to also send carriage return
* Refactor filesystem commands for shell subsystem
* Introduce new shell commands:
  - 'help' command
  - 'flash' command to reset into bootloader
  - 'stress' command to stress CDC writes

Testing:
* Shell validated on Sensor Watch Blue w/ Linux host
* Shell validated in emscripten emulator
* Tuned by spamming inputs during `stress` cmd until stack didn't crash

watch-library/hardware/watch/watch_private_cdc.c

@@ -0,0 +1,160 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2020 Joey Castillo
+ * Copyright (c) 2023 Edward Shin
+ *
+ * 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 "watch_private_cdc.h"
+
+#include <stddef.h>
+
+#include "watch_utility.h"
+#include "tusb.h"
+
+/*
+ * Implement a circular buffer for the USB CDC Serial read buffer.
+ * The size of the buffer must be a power of two for this circular buffer
+ * implementation to work.
+ */
+
+// Size of the circular buffer. Must be a power of two.
+#define CDC_WRITE_BUF_SZ  (1024)
+// Macro function to perform modular arithmetic on an index.
+// eg. (63 + 2) & (64 - 1) -> 1
+#define CDC_WRITE_BUF_IDX(x)  ((x) & (CDC_WRITE_BUF_SZ - 1))
+static char s_write_buf[CDC_WRITE_BUF_SZ] = {0};
+static size_t s_write_buf_pos = 0;
+static size_t s_write_buf_len = 0;
+
+#define CDC_READ_BUF_SZ  (256)
+#define CDC_READ_BUF_IDX(x)  ((x) & (CDC_READ_BUF_SZ - 1))
+static char s_read_buf[CDC_READ_BUF_SZ] = {0};
+static size_t s_read_buf_pos = 0;
+static size_t s_read_buf_len = 0;
+
+// Mask TC1 interrupts, preventing calls to cdc_task()
+static inline void prv_critical_section_enter(void) {
+    NVIC_DisableIRQ(TC1_IRQn);
+}
+
+// Unmask TC1 interrupts, allowing calls to cdc_task()
+static inline void prv_critical_section_exit(void) {
+    NVIC_EnableIRQ(TC1_IRQn);
+}
+
+int _write(int file, char *ptr, int len) {
+    (void) file;
+
+    if (ptr == NULL || len <= 0) {
+        return -1;
+    }
+
+    int bytes_written = 0;
+
+    prv_critical_section_enter();
+
+    for (int i = 0; i < len; i++) {
+        s_write_buf[s_write_buf_pos] = ptr[i];
+        s_write_buf_pos = CDC_WRITE_BUF_IDX(s_write_buf_pos + 1);
+        if (s_write_buf_len < CDC_WRITE_BUF_SZ) {
+            s_write_buf_len++;
+        }
+        bytes_written++;
+    }
+
+    prv_critical_section_exit();
+
+    return bytes_written;
+}
+
+int _read(int file, char *ptr, int len) {
+    (void) file;
+
+    prv_critical_section_enter();
+
+    if (ptr == NULL || len <= 0 || s_read_buf_len == 0) {
+        prv_critical_section_exit();
+        return -1;
+    }
+
+    // Clamp to the length of the read buffer
+    if ((size_t) len > s_read_buf_len) {
+        len = s_read_buf_len;
+    }
+
+    // Calculate the start of the circular buffer, and iterate from there
+    const size_t start_pos = CDC_READ_BUF_IDX(s_read_buf_pos - len);
+    for (size_t i = 0; i < (size_t) len; i++) {
+        const size_t idx = CDC_READ_BUF_IDX(start_pos + i);
+        ptr[i] = s_read_buf[idx];
+        s_read_buf[idx] = 0;
+    }
+
+    // Update circular buffer position and length
+    s_read_buf_len -= len;
+    s_read_buf_pos = CDC_READ_BUF_IDX(s_read_buf_pos - len);
+
+    prv_critical_section_exit();
+
+    return len;
+}
+
+static void prv_handle_reads(void) {
+    while (tud_cdc_available()) {
+        int c = tud_cdc_read_char();
+        if (c < 0) {
+            continue;
+        }
+        s_read_buf[s_read_buf_pos] = c;
+        s_read_buf_pos = CDC_READ_BUF_IDX(s_read_buf_pos + 1);
+        if (s_read_buf_len < CDC_READ_BUF_SZ) {
+            s_read_buf_len++;
+        }
+    }
+}
+
+static void prv_handle_writes(void) {
+    if (s_write_buf_len > 0) {
+        const size_t start_pos =
+            CDC_WRITE_BUF_IDX(s_write_buf_pos - s_write_buf_len);
+        for (size_t i = 0; i < (size_t) s_write_buf_len; i++) {
+            const size_t idx = CDC_WRITE_BUF_IDX(start_pos + i);
+            if (tud_cdc_available() > 0) {
+                // If we receive data while doing a large write, we need to
+                // fully service it before continuing to write, or the
+                // stack will crash.
+                prv_handle_reads();
+            }
+            if (tud_cdc_write_available()) {
+                tud_cdc_write(&s_write_buf[idx], 1);
+            }
+            s_write_buf[idx] = 0;
+            s_write_buf_len--;
+        }
+        tud_cdc_write_flush();
+    }
+}
+
+void cdc_task(void) {
+    prv_handle_reads();
+    prv_handle_writes();
+}