fatfs: add FAT_IOCTL_GET_VOLUME_ID

This patch, originally from Android kernel, adds vfat ioctl command
FAT_IOCTL_GET_VOLUME_ID, with this command we can get the vfat volume ID
using following code:

	ioctl(fd, FAT_IOCTL_GET_VOLUME_ID, &volume_ID)

This patch is a modified version of the patch by Mike Lockwood, with
changes from Dmitry Pervushin, who noticed the original patch makes some
volume IDs abiguous with error returns: for example, if volume id is
0xFFFFFDAD, that matches -ENOIOCTLCMD, we get "FFFFFFFF" from the user
space.

So add a parameter to ioctl to get the correct volume ID.

Android uses vfat volume ID to identify different sd card, when a new sd
card is inserted to device, android can scan the media on it and pop up
new contents.

Signed-off-by: Bintian Wang <bintian.wang@linaro.org>
Cc: dmitry pervushin <dpervushin@gmail.com>
Cc: Mike Lockwood <lockwood@android.com>
Cc: Colin Cross <ccross@android.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Sean McNeil <sean@mcneil.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index 5d4513cb1b3c..11b51bb55b42 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -1415,6 +1415,18 @@ int fat_fill_super(struct super_block *sb, void *data, int silent, int isvfat,
                 brelse(fsinfo_bh);
         }
 
+        /* interpret volume ID as a little endian 32 bit integer */
+        if (sbi->fat_bits == 32)
+                sbi->vol_id = (((u32)b->fat32.vol_id[0]) |
+                                        ((u32)b->fat32.vol_id[1] << 8) |
+                                        ((u32)b->fat32.vol_id[2] << 16) |
+                                        ((u32)b->fat32.vol_id[3] << 24));
+        else /* fat 16 or 12 */
+                sbi->vol_id = (((u32)b->fat16.vol_id[0]) |
+                                        ((u32)b->fat16.vol_id[1] << 8) |
+                                        ((u32)b->fat16.vol_id[2] << 16) |
+                                        ((u32)b->fat16.vol_id[3] << 24));
+
         sbi->dir_per_block = sb->s_blocksize / sizeof(struct msdos_dir_entry);
         sbi->dir_per_block_bits = ffs(sbi->dir_per_block) - 1;