esp8266: Implement vfs.rom_ioctl with support for external flash.

Not enabled by default on any board.  For a board to enable ROMFS it must:

- Add `#define MICROPY_VFS_ROM (1)` to its `mpconfigboard.h` file.

- Add a FLASH_ROMFS partition to the linker script and expose the partition
  with:

    _micropy_hw_romfs_start = ORIGIN(FLASH_ROMFS);
    _micropy_hw_romfs_size = LENGTH(FLASH_ROMFS);

Signed-off-by: Damien George <damien@micropython.org>

ports/esp8266/Makefile

@@ -134,6 +134,7 @@ SRC_C = \
 	fatfs_port.c \
 	posix_helpers.c \
 	hspi.c \
+	vfs_rom_ioctl.c \
 	$(wildcard $(BOARD_DIR)/*.c) \
 
 ifeq ($(MICROPY_PY_ESPNOW),1)

ports/esp8266/boards/esp8266_common.ld

@@ -170,6 +170,7 @@ SECTIONS
         *modonewire.o(.literal* .text*)
         *network_wlan.o(.literal* .text*)
         *esp_mphal.o(.literal* .text*)
+        *vfs_rom_ioctl.o(.literal* .text*)
 
         /* we put as much rodata as possible in this section */
         /* note that only rodata accessed as a machine word is allowed here */

ports/esp8266/modesp.c

@@ -164,9 +164,16 @@ static MP_DEFINE_CONST_FUN_OBJ_0(esp_flash_size_obj, esp_flash_size);
 #define IS_OTA_FIRMWARE() ((*(uint32_t *)0x40200000 & 0xff00) == 0x100)
 
 extern byte _firmware_size[];
+#if MICROPY_VFS_ROM_IOCTL
+extern uint8_t _micropy_hw_romfs_size;
+#endif
 
 static mp_obj_t esp_flash_user_start(void) {
-    return MP_OBJ_NEW_SMALL_INT((uint32_t)_firmware_size);
+    uint32_t flash_user_start = (uint32_t)_firmware_size;
+    #if MICROPY_VFS_ROM_IOCTL
+    flash_user_start += (uint32_t)&_micropy_hw_romfs_size;
+    #endif
+    return MP_OBJ_NEW_SMALL_INT(flash_user_start);
 }
 static MP_DEFINE_CONST_FUN_OBJ_0(esp_flash_user_start_obj, esp_flash_user_start);
 

ports/esp8266/vfs_rom_ioctl.c

@@ -0,0 +1,89 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2024-2025 Damien P. George
+ *
+ * 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 "py/mperrno.h"
+#include "py/objarray.h"
+#include "extmod/vfs.h"
+#include "ets_alt_task.h"
+#include "user_interface.h"
+
+#if MICROPY_VFS_ROM_IOCTL
+
+#define FLASH_MEM_BASE (0x40200000)
+#define FLASH_PAGE_SIZE (4096)
+
+#define MICROPY_HW_ROMFS_BASE (uintptr_t)(&_micropy_hw_romfs_start)
+#define MICROPY_HW_ROMFS_BYTES (uintptr_t)(&_micropy_hw_romfs_size)
+
+#define ROMFS_SPI_FLASH_OFFSET (MICROPY_HW_ROMFS_BASE - FLASH_MEM_BASE)
+
+extern uint8_t _micropy_hw_romfs_start;
+extern uint8_t _micropy_hw_romfs_size;
+
+static const MP_DEFINE_MEMORYVIEW_OBJ(romfs_obj, 'B', 0, MICROPY_HW_ROMFS_BYTES, (void *)MICROPY_HW_ROMFS_BASE);
+
+mp_obj_t mp_vfs_rom_ioctl(size_t n_args, const mp_obj_t *args) {
+    switch (mp_obj_get_int(args[0])) {
+        case MP_VFS_ROM_IOCTL_GET_NUMBER_OF_SEGMENTS:
+            return MP_OBJ_NEW_SMALL_INT(1);
+
+        case MP_VFS_ROM_IOCTL_GET_SEGMENT:
+            return MP_OBJ_FROM_PTR(&romfs_obj);
+
+        case MP_VFS_ROM_IOCTL_WRITE_PREPARE: {
+            uint32_t dest = ROMFS_SPI_FLASH_OFFSET;
+            uint32_t dest_max = dest + mp_obj_get_int(args[2]);
+            while (dest < dest_max) {
+                ets_loop_iter(); // flash access takes time so run any pending tasks
+                SpiFlashOpResult res = spi_flash_erase_sector(dest / FLASH_PAGE_SIZE);
+                ets_loop_iter();
+                if (res != SPI_FLASH_RESULT_OK) {
+                    return MP_OBJ_NEW_SMALL_INT(res == SPI_FLASH_RESULT_TIMEOUT ? -MP_ETIMEDOUT : -MP_EIO);
+                }
+                dest += FLASH_PAGE_SIZE;
+            }
+            return MP_OBJ_NEW_SMALL_INT(4); // minimum write size
+        }
+
+        case MP_VFS_ROM_IOCTL_WRITE: {
+            mp_int_t offset = ROMFS_SPI_FLASH_OFFSET + mp_obj_get_int(args[2]);
+            mp_buffer_info_t bufinfo;
+            mp_get_buffer_raise(args[3], &bufinfo, MP_BUFFER_READ);
+            ets_loop_iter(); // flash access takes time so run any pending tasks
+            SpiFlashOpResult res = spi_flash_write(offset, bufinfo.buf, (bufinfo.len + 3) & ~3);
+            ets_loop_iter();
+            if (res == SPI_FLASH_RESULT_OK) {
+                return MP_OBJ_NEW_SMALL_INT(0);
+            }
+            return MP_OBJ_NEW_SMALL_INT(res == SPI_FLASH_RESULT_TIMEOUT ? -MP_ETIMEDOUT : -MP_EIO);
+        }
+
+        default:
+            return MP_OBJ_NEW_SMALL_INT(-MP_EINVAL);
+    }
+}
+
+#endif // MICROPY_VFS_ROM_IOCTL