fatfs: Added lower bound to block sizes

Some block devices (for example I2C EEPROM) can be erased at the byte
level. These aren't really block devices, but fall under the block
device API. For these devices, the fat filesystem needs to use a lower
bound on the block size. In this case we used 512 bytes is used since
it is already a standard.

Author: geky

features/filesystem/fat/FATFileSystem.cpp

@@ -160,6 +160,21 @@ void ff_memfree(void *p)
 }
 
 // Implementation of diskio functions (see ChaN/diskio.h)
+static WORD disk_get_sector_size(BYTE pdrv)
+{
+    WORD ssize = _ffs[pdrv]->get_erase_size();
+    if (ssize < 512) {
+        ssize = 512;
+    }
+
+    return ssize;
+}
+
+static DWORD disk_get_sector_count(BYTE pdrv)
+{
+    return _ffs[pdrv]->size() / disk_get_sector_size(pdrv);
+}
+
 DSTATUS disk_status(BYTE pdrv)
 {
     debug_if(FFS_DBG, "disk_status on pdrv [%d]\n", pdrv);
@@ -175,15 +190,15 @@ DSTATUS disk_initialize(BYTE pdrv)
 DRESULT disk_read(BYTE pdrv, BYTE *buff, DWORD sector, UINT count)
 {
     debug_if(FFS_DBG, "disk_read(sector %d, count %d) on pdrv [%d]\n", sector, count, pdrv);
-    bd_size_t ssize = _ffs[pdrv]->get_erase_size();
+    DWORD ssize = disk_get_sector_size(pdrv);
     int err = _ffs[pdrv]->read(buff, sector*ssize, count*ssize);
     return err ? RES_PARERR : RES_OK;
 }
 
 DRESULT disk_write(BYTE pdrv, const BYTE *buff, DWORD sector, UINT count)
 {
     debug_if(FFS_DBG, "disk_write(sector %d, count %d) on pdrv [%d]\n", sector, count, pdrv);
-    bd_size_t ssize = _ffs[pdrv]->get_erase_size();
+    DWORD ssize = disk_get_sector_size(pdrv);
     int err = _ffs[pdrv]->erase(sector*ssize, count*ssize);
     if (err) {
         return RES_PARERR;
@@ -211,16 +226,14 @@ DRESULT disk_ioctl(BYTE pdrv, BYTE cmd, void *buff)
             if (_ffs[pdrv] == NULL) {
                 return RES_NOTRDY;
             } else {
-                DWORD count = _ffs[pdrv]->size() / _ffs[pdrv]->get_erase_size();
-                *((DWORD*)buff) = count;
+                *((DWORD*)buff) = disk_get_sector_count(pdrv);
                 return RES_OK;
             }
         case GET_SECTOR_SIZE:
             if (_ffs[pdrv] == NULL) {
                 return RES_NOTRDY;
             } else {
-                WORD size = _ffs[pdrv]->get_erase_size();
-                *((WORD*)buff) = size;
+                *((WORD*)buff) = disk_get_sector_size(pdrv);
                 return RES_OK;
             }
         case GET_BLOCK_SIZE: