fat: extract FatFs wrapper code into its own library

Makefile

@@ -31,7 +31,7 @@ LIBDRAGON_OBJS += \
              $(BUILD_DIR)/n64sys.o $(BUILD_DIR)/interrupt.o $(BUILD_DIR)/backtrace.o \
 			 $(BUILD_DIR)/fmath.o $(BUILD_DIR)/inthandler.o $(BUILD_DIR)/entrypoint.o \
 			 $(BUILD_DIR)/debug.o $(BUILD_DIR)/debugcpp.o $(BUILD_DIR)/usb.o $(BUILD_DIR)/libcart/cart.o $(BUILD_DIR)/fatfs/ff.o \
-			 $(BUILD_DIR)/fatfs/ffunicode.o $(BUILD_DIR)/rompak.o $(BUILD_DIR)/dragonfs.o \
+			 $(BUILD_DIR)/fatfs/ffunicode.o $(BUILD_DIR)/fat.o $(BUILD_DIR)/rompak.o $(BUILD_DIR)/dragonfs.o \
 			 $(BUILD_DIR)/audio.o $(BUILD_DIR)/display.o $(BUILD_DIR)/surface.o \
 			 $(BUILD_DIR)/console.o $(BUILD_DIR)/asset.o $(BUILD_DIR)/pifile.o \
 			 $(BUILD_DIR)/compress/lzh5.o $(BUILD_DIR)/compress/lz4_dec.o $(BUILD_DIR)/compress/lz4_dec_fast.o $(BUILD_DIR)/compress/ringbuf.o \
@@ -135,6 +135,7 @@ install: install-mk libdragon
 	install -Cv -m 0644 include/display.h $(INSTALLDIR)/mips64-elf/include/display.h
 	install -Cv -m 0644 include/debug.h $(INSTALLDIR)/mips64-elf/include/debug.h
 	install -Cv -m 0644 include/debugcpp.h $(INSTALLDIR)/mips64-elf/include/debugcpp.h
+	install -Cv -m 0644 include/fat.h $(INSTALLDIR)/mips64-elf/include/fat.h
 	install -Cv -m 0644 include/usb.h $(INSTALLDIR)/mips64-elf/include/usb.h
 	install -Cv -m 0644 include/console.h $(INSTALLDIR)/mips64-elf/include/console.h
 	install -Cv -m 0644 include/joybus.h $(INSTALLDIR)/mips64-elf/include/joybus.h

include/fat.h

@@ -0,0 +1,90 @@
+/**
+ * @file fat.h
+ * @brief FAT filesystem interface
+ * @ingroup lowlevel
+ * 
+ * This file allows multiple clients to access and use the FatFs library 
+ * within libdragon for different scopes.
+ * 
+ * FatFs is a generic FAT filesystem module for small embedded systems,
+ * written by ChaN. It is available at http://elm-chan.org/fsw/ff/00index_e.html.
+ * 
+ * FatFs is currently used by libdragon for a single use case: to implement
+ * access to the SD card in flashcarts. This access is currently implemented
+ * by the debug library (debug.h), initialized via #debug_init_sdfs.
+ * 
+ * The APIs exported by this file are useful only if you need to mount a FAT
+ * volume coming from some other sources (eg: a FAT image within a ROM, or
+ * a FAT volume accessible via some custom USB protocol, or whatever else).
+ * If you need this, call #fat_mount to configure a FatFs volume (eg:
+ * #FAT_VOLUME_CUSTOM), which you will then be able to access via standard
+ * C file operations.
+ */
+
+#ifndef LIBDRAGON_FAT_H
+#define LIBDRAGON_FAT_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** 
+ * @brief Definition of the various FAT volumes.
+ * 
+ * Currently, we only allocate a single volume, which is used for the SD card
+ * in flashcarts.
+ * 
+ * If you need to mount a FAT volume coming from some other sources, you
+ * can use the #FAT_VOLUME_CUSTOM volume.
+ */
+enum {
+    FAT_VOLUME_SD = 0,          ///< Volume for SD cards
+    FAT_VOLUME_CUSTOM = 1,      ///< Custom volume, free for usage
+};
+
+/**
+ * @brief Interface for disk operations required to implement a volume.
+ * 
+ * These interfaces are identical to diskio.h from FatFs. It basically just
+ * adds one indirection layer to it, to dispatch the calls to the correct
+ * volume.
+ */
+typedef struct {
+	///@cond
+	int (*disk_initialize)(void);
+	int (*disk_status)(void);
+	int (*disk_read)(uint8_t* buff, int sector, int count);
+	int (*disk_read_sdram)(uint8_t* buff, int sector, int count);
+	int (*disk_write)(const uint8_t* buff, int sector, int count);
+	int (*disk_ioctl)(uint8_t cmd, void* buff);
+	///@endcond
+} fat_disk_t;
+
+/**
+ * @brief Mount a new FAT volume through the FatFs library.
+ * 
+ * This function allows to mount a new FAT volume through the FatFs library.
+ * Access to the actual disk is done through the provided disk operations,
+ * so that the volume can be backed by any kind of storage.
+ * 
+ * After calling this function, you will be able to access the volume via
+ * standard C file operations.
+ * 
+ * @param prefix            Prefix to use for the volume in stdio calls like 
+ *                          fopen (eg: "sd:"). If this is NULL, the volume
+ *                          will not be accessible via stdio calls, but only
+ *                          via the FatFs API.
+ * @param fatfs_volume_id   ID of the volume within FatFs (eg: #FAT_VOLUME_CUSTOM)
+ * @param disk              Table of disk operations to use for this volume
+ * 
+ * @return 0 on success, -1 on failure (errno will be set)
+ */
+int fat_mount(const char *prefix, int fatfs_volume_id, const fat_disk_t* disk);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

include/libdragon.h

@@ -25,6 +25,7 @@
 #include "audio.h"
 #include "console.h"
 #include "debug.h"
+#include "fat.h"
 #include "joybus.h"
 #include "joybus_accessory.h"
 #include "pixelfx.h"