rp2: Implement vfs.rom_ioctl with support for external flash.

Not enabled by default on any board.  A board can enable a ROMFS partition
by defining `MICROPY_HW_ROMFS_BYTES` in its `mpconfigboard.h` file.  For
example:

    #define MICROPY_HW_ROMFS_BYTES (128 * 1024)

The ROMFS partition is placed at the end of the flash allocated for the
firmware, giving less space for the firmware.  It then lives between the
firmware and the read/write filesystem.

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

ports/rp2/mpconfigport.h

@@ -67,6 +67,16 @@
 #endif
 #endif
 
+// Number of bytes of flash to allocate to the ROMFS partition.
+#ifndef MICROPY_HW_ROMFS_BYTES
+#define MICROPY_HW_ROMFS_BYTES (0)
+#endif
+
+// Number of bytes of flash to allocate to read/write filesystem storage.
+#ifndef MICROPY_HW_FLASH_STORAGE_BYTES
+#define MICROPY_HW_FLASH_STORAGE_BYTES (1408 * 1024)
+#endif
+
 #ifndef MICROPY_CONFIG_ROM_LEVEL
 #define MICROPY_CONFIG_ROM_LEVEL                (MICROPY_CONFIG_ROM_LEVEL_EXTRA_FEATURES)
 #endif
@@ -170,6 +180,7 @@
 #define MICROPY_VFS                             (1)
 #define MICROPY_VFS_LFS2                        (1)
 #define MICROPY_VFS_FAT                         (1)
+#define MICROPY_VFS_ROM                         (MICROPY_HW_ROMFS_BYTES > 0)
 #define MICROPY_SSL_MBEDTLS                     (1)
 #define MICROPY_PY_LWIP_PPP                     (MICROPY_PY_NETWORK_PPP_LWIP)
 #define MICROPY_PY_LWIP_SOCK_RAW                (MICROPY_PY_LWIP)

ports/rp2/rp2_flash.c

@@ -28,6 +28,7 @@
 
 #include "py/mphal.h"
 #include "py/runtime.h"
+#include "py/mperrno.h"
 #include "extmod/vfs.h"
 #include "modrp2.h"
 #include "hardware/flash.h"
@@ -35,15 +36,16 @@
 
 #define BLOCK_SIZE_BYTES (FLASH_SECTOR_SIZE)
 
-#ifndef MICROPY_HW_FLASH_STORAGE_BYTES
+static_assert(MICROPY_HW_ROMFS_BYTES % 4096 == 0, "ROMFS size must be a multiple of 4K");
-#define MICROPY_HW_FLASH_STORAGE_BYTES (1408 * 1024)
-#endif
 static_assert(MICROPY_HW_FLASH_STORAGE_BYTES % 4096 == 0, "Flash storage size must be a multiple of 4K");
 
 #ifndef MICROPY_HW_FLASH_STORAGE_BASE
 #define MICROPY_HW_FLASH_STORAGE_BASE (PICO_FLASH_SIZE_BYTES - MICROPY_HW_FLASH_STORAGE_BYTES)
 #endif
 
+// Put ROMFS at the upper end of the code space.
+#define MICROPY_HW_ROMFS_BASE (MICROPY_HW_FLASH_STORAGE_BASE - MICROPY_HW_ROMFS_BYTES)
+
 static_assert(MICROPY_HW_FLASH_STORAGE_BYTES <= PICO_FLASH_SIZE_BYTES, "MICROPY_HW_FLASH_STORAGE_BYTES too big");
 static_assert(MICROPY_HW_FLASH_STORAGE_BASE + MICROPY_HW_FLASH_STORAGE_BYTES <= PICO_FLASH_SIZE_BYTES, "MICROPY_HW_FLASH_STORAGE_BYTES too big");
 
@@ -53,6 +55,14 @@ typedef struct _rp2_flash_obj_t {
     uint32_t flash_size;
 } rp2_flash_obj_t;
 
+#if MICROPY_HW_ROMFS_BYTES > 0
+static rp2_flash_obj_t rp2_flash_romfs_obj = {
+    .base = { &rp2_flash_type },
+    .flash_base = MICROPY_HW_ROMFS_BASE,
+    .flash_size = MICROPY_HW_ROMFS_BYTES,
+};
+#endif
+
 static rp2_flash_obj_t rp2_flash_obj = {
     .base = { &rp2_flash_type },
     .flash_base = MICROPY_HW_FLASH_STORAGE_BASE,
@@ -138,6 +148,19 @@ static mp_obj_t rp2_flash_make_new(const mp_obj_type_t *type, size_t n_args, siz
     return MP_OBJ_FROM_PTR(self);
 }
 
+static mp_int_t rp2_flash_get_buffer(mp_obj_t self_in, mp_buffer_info_t *bufinfo, mp_uint_t flags) {
+    rp2_flash_obj_t *self = MP_OBJ_TO_PTR(self_in);
+    if (flags == MP_BUFFER_READ) {
+        bufinfo->buf = (void *)(XIP_BASE + self->flash_base);
+        bufinfo->len = self->flash_size;
+        bufinfo->typecode = 'B';
+        return 0;
+    } else {
+        // Write unsupported.
+        return 1;
+    }
+}
+
 static mp_obj_t rp2_flash_readblocks(size_t n_args, const mp_obj_t *args) {
     rp2_flash_obj_t *self = MP_OBJ_TO_PTR(args[0]);
     uint32_t offset = mp_obj_get_int(args[1]) * BLOCK_SIZE_BYTES;
@@ -218,5 +241,21 @@ MP_DEFINE_CONST_OBJ_TYPE(
     MP_QSTR_Flash,
     MP_TYPE_FLAG_NONE,
     make_new, rp2_flash_make_new,
+    buffer, rp2_flash_get_buffer,
     locals_dict, &rp2_flash_locals_dict
     );
+
+#if MICROPY_VFS_ROM_IOCTL
+mp_obj_t mp_vfs_rom_ioctl(size_t n_args, const mp_obj_t *args) {
+    switch (mp_obj_get_int(args[0])) {
+        #if MICROPY_HW_ROMFS_BYTES > 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(&rp2_flash_romfs_obj);
+        #endif
+        default:
+            return MP_OBJ_NEW_SMALL_INT(-MP_EINVAL);
+    }
+}
+#endif