// SPDX-License-Identifier: GPL-2.0-only

  /*
   * Copyright (C) 2004, OGAWA Hirofumi

   */

  #include <linux/blkdev.h>

  #include <linux/sched/signal.h>

  #include "fat.h"

  
  struct fatent_operations {
  	void (*ent_blocknr)(struct super_block *, int, int *, sector_t *);
  	void (*ent_set_ptr)(struct fat_entry *, int);
  	int (*ent_bread)(struct super_block *, struct fat_entry *,
  			 int, sector_t);
  	int (*ent_get)(struct fat_entry *);
  	void (*ent_put)(struct fat_entry *, int);
  	int (*ent_next)(struct fat_entry *);
  };

  static DEFINE_SPINLOCK(fat12_entry_lock);

  static void fat12_ent_blocknr(struct super_block *sb, int entry,
  			      int *offset, sector_t *blocknr)
  {
  	struct msdos_sb_info *sbi = MSDOS_SB(sb);
  	int bytes = entry + (entry >> 1);

  	WARN_ON(!fat_valid_entry(sbi, entry));

  	*offset = bytes & (sb->s_blocksize - 1);
  	*blocknr = sbi->fat_start + (bytes >> sb->s_blocksize_bits);
  }
  
  static void fat_ent_blocknr(struct super_block *sb, int entry,
  			    int *offset, sector_t *blocknr)
  {
  	struct msdos_sb_info *sbi = MSDOS_SB(sb);
  	int bytes = (entry << sbi->fatent_shift);

  	WARN_ON(!fat_valid_entry(sbi, entry));

  	*offset = bytes & (sb->s_blocksize - 1);
  	*blocknr = sbi->fat_start + (bytes >> sb->s_blocksize_bits);
  }
  
  static void fat12_ent_set_ptr(struct fat_entry *fatent, int offset)
  {
  	struct buffer_head **bhs = fatent->bhs;
  	if (fatent->nr_bhs == 1) {
  		WARN_ON(offset >= (bhs[0]->b_size - 1));
  		fatent->u.ent12_p[0] = bhs[0]->b_data + offset;
  		fatent->u.ent12_p[1] = bhs[0]->b_data + (offset + 1);
  	} else {
  		WARN_ON(offset != (bhs[0]->b_size - 1));
  		fatent->u.ent12_p[0] = bhs[0]->b_data + offset;
  		fatent->u.ent12_p[1] = bhs[1]->b_data;
  	}
  }
  
  static void fat16_ent_set_ptr(struct fat_entry *fatent, int offset)
  {
  	WARN_ON(offset & (2 - 1));
  	fatent->u.ent16_p = (__le16 *)(fatent->bhs[0]->b_data + offset);
  }
  
  static void fat32_ent_set_ptr(struct fat_entry *fatent, int offset)
  {
  	WARN_ON(offset & (4 - 1));
  	fatent->u.ent32_p = (__le32 *)(fatent->bhs[0]->b_data + offset);
  }
  
  static int fat12_ent_bread(struct super_block *sb, struct fat_entry *fatent,
  			   int offset, sector_t blocknr)
  {
  	struct buffer_head **bhs = fatent->bhs;
  
  	WARN_ON(blocknr < MSDOS_SB(sb)->fat_start);

  	fatent->fat_inode = MSDOS_SB(sb)->fat_inode;

  	bhs[0] = sb_bread(sb, blocknr);
  	if (!bhs[0])
  		goto err;
  
  	if ((offset + 1) < sb->s_blocksize)
  		fatent->nr_bhs = 1;
  	else {
  		/* This entry is block boundary, it needs the next block */
  		blocknr++;
  		bhs[1] = sb_bread(sb, blocknr);
  		if (!bhs[1])
  			goto err_brelse;
  		fatent->nr_bhs = 2;
  	}
  	fat12_ent_set_ptr(fatent, offset);
  	return 0;
  
  err_brelse:
  	brelse(bhs[0]);
  err:

  	fat_msg(sb, KERN_ERR, "FAT read failed (blocknr %llu)", (llu)blocknr);

  	return -EIO;
  }
  
  static int fat_ent_bread(struct super_block *sb, struct fat_entry *fatent,
  			 int offset, sector_t blocknr)
  {

  	const struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;

  
  	WARN_ON(blocknr < MSDOS_SB(sb)->fat_start);

  	fatent->fat_inode = MSDOS_SB(sb)->fat_inode;

  	fatent->bhs[0] = sb_bread(sb, blocknr);
  	if (!fatent->bhs[0]) {

  		fat_msg(sb, KERN_ERR, "FAT read failed (blocknr %llu)",

  		       (llu)blocknr);

  		return -EIO;
  	}
  	fatent->nr_bhs = 1;
  	ops->ent_set_ptr(fatent, offset);
  	return 0;
  }
  
  static int fat12_ent_get(struct fat_entry *fatent)
  {
  	u8 **ent12_p = fatent->u.ent12_p;
  	int next;

  	spin_lock(&fat12_entry_lock);

  	if (fatent->entry & 1)
  		next = (*ent12_p[0] >> 4) | (*ent12_p[1] << 4);
  	else
  		next = (*ent12_p[1] << 8) | *ent12_p[0];

  	spin_unlock(&fat12_entry_lock);

  	next &= 0x0fff;
  	if (next >= BAD_FAT12)
  		next = FAT_ENT_EOF;
  	return next;
  }
  
  static int fat16_ent_get(struct fat_entry *fatent)
  {
  	int next = le16_to_cpu(*fatent->u.ent16_p);
  	WARN_ON((unsigned long)fatent->u.ent16_p & (2 - 1));
  	if (next >= BAD_FAT16)
  		next = FAT_ENT_EOF;
  	return next;
  }
  
  static int fat32_ent_get(struct fat_entry *fatent)
  {
  	int next = le32_to_cpu(*fatent->u.ent32_p) & 0x0fffffff;
  	WARN_ON((unsigned long)fatent->u.ent32_p & (4 - 1));
  	if (next >= BAD_FAT32)
  		next = FAT_ENT_EOF;
  	return next;
  }
  
  static void fat12_ent_put(struct fat_entry *fatent, int new)
  {
  	u8 **ent12_p = fatent->u.ent12_p;
  
  	if (new == FAT_ENT_EOF)
  		new = EOF_FAT12;

  	spin_lock(&fat12_entry_lock);

  	if (fatent->entry & 1) {
  		*ent12_p[0] = (new << 4) | (*ent12_p[0] & 0x0f);
  		*ent12_p[1] = new >> 4;
  	} else {
  		*ent12_p[0] = new & 0xff;
  		*ent12_p[1] = (*ent12_p[1] & 0xf0) | (new >> 8);
  	}

  	spin_unlock(&fat12_entry_lock);

  	mark_buffer_dirty_inode(fatent->bhs[0], fatent->fat_inode);

  	if (fatent->nr_bhs == 2)

  		mark_buffer_dirty_inode(fatent->bhs[1], fatent->fat_inode);

  }
  
  static void fat16_ent_put(struct fat_entry *fatent, int new)
  {
  	if (new == FAT_ENT_EOF)
  		new = EOF_FAT16;
  
  	*fatent->u.ent16_p = cpu_to_le16(new);

  	mark_buffer_dirty_inode(fatent->bhs[0], fatent->fat_inode);

  }
  
  static void fat32_ent_put(struct fat_entry *fatent, int new)
  {

  	WARN_ON(new & 0xf0000000);
  	new |= le32_to_cpu(*fatent->u.ent32_p) & ~0x0fffffff;
  	*fatent->u.ent32_p = cpu_to_le32(new);

  	mark_buffer_dirty_inode(fatent->bhs[0], fatent->fat_inode);

  }
  
  static int fat12_ent_next(struct fat_entry *fatent)
  {
  	u8 **ent12_p = fatent->u.ent12_p;
  	struct buffer_head **bhs = fatent->bhs;
  	u8 *nextp = ent12_p[1] + 1 + (fatent->entry & 1);
  
  	fatent->entry++;
  	if (fatent->nr_bhs == 1) {

  		WARN_ON(ent12_p[0] > (u8 *)(bhs[0]->b_data +
  							(bhs[0]->b_size - 2)));
  		WARN_ON(ent12_p[1] > (u8 *)(bhs[0]->b_data +
  							(bhs[0]->b_size - 1)));

  		if (nextp < (u8 *)(bhs[0]->b_data + (bhs[0]->b_size - 1))) {
  			ent12_p[0] = nextp - 1;
  			ent12_p[1] = nextp;
  			return 1;
  		}
  	} else {

  		WARN_ON(ent12_p[0] != (u8 *)(bhs[0]->b_data +
  							(bhs[0]->b_size - 1)));

  		WARN_ON(ent12_p[1] != (u8 *)bhs[1]->b_data);
  		ent12_p[0] = nextp - 1;
  		ent12_p[1] = nextp;
  		brelse(bhs[0]);
  		bhs[0] = bhs[1];
  		fatent->nr_bhs = 1;
  		return 1;
  	}
  	ent12_p[0] = NULL;
  	ent12_p[1] = NULL;
  	return 0;
  }
  
  static int fat16_ent_next(struct fat_entry *fatent)
  {
  	const struct buffer_head *bh = fatent->bhs[0];
  	fatent->entry++;
  	if (fatent->u.ent16_p < (__le16 *)(bh->b_data + (bh->b_size - 2))) {
  		fatent->u.ent16_p++;
  		return 1;
  	}
  	fatent->u.ent16_p = NULL;
  	return 0;
  }
  
  static int fat32_ent_next(struct fat_entry *fatent)
  {
  	const struct buffer_head *bh = fatent->bhs[0];
  	fatent->entry++;
  	if (fatent->u.ent32_p < (__le32 *)(bh->b_data + (bh->b_size - 4))) {
  		fatent->u.ent32_p++;
  		return 1;
  	}
  	fatent->u.ent32_p = NULL;
  	return 0;
  }

  static const struct fatent_operations fat12_ops = {

  	.ent_blocknr	= fat12_ent_blocknr,
  	.ent_set_ptr	= fat12_ent_set_ptr,
  	.ent_bread	= fat12_ent_bread,
  	.ent_get	= fat12_ent_get,
  	.ent_put	= fat12_ent_put,
  	.ent_next	= fat12_ent_next,
  };

  static const struct fatent_operations fat16_ops = {

  	.ent_blocknr	= fat_ent_blocknr,
  	.ent_set_ptr	= fat16_ent_set_ptr,
  	.ent_bread	= fat_ent_bread,
  	.ent_get	= fat16_ent_get,
  	.ent_put	= fat16_ent_put,
  	.ent_next	= fat16_ent_next,
  };

  static const struct fatent_operations fat32_ops = {

  	.ent_blocknr	= fat_ent_blocknr,
  	.ent_set_ptr	= fat32_ent_set_ptr,
  	.ent_bread	= fat_ent_bread,
  	.ent_get	= fat32_ent_get,
  	.ent_put	= fat32_ent_put,
  	.ent_next	= fat32_ent_next,
  };
  
  static inline void lock_fat(struct msdos_sb_info *sbi)
  {

  	mutex_lock(&sbi->fat_lock);

  }
  
  static inline void unlock_fat(struct msdos_sb_info *sbi)
  {

  	mutex_unlock(&sbi->fat_lock);

  }
  
  void fat_ent_access_init(struct super_block *sb)
  {
  	struct msdos_sb_info *sbi = MSDOS_SB(sb);

  	mutex_init(&sbi->fat_lock);

  	if (is_fat32(sbi)) {

  		sbi->fatent_shift = 2;
  		sbi->fatent_ops = &fat32_ops;

  	} else if (is_fat16(sbi)) {

  		sbi->fatent_shift = 1;
  		sbi->fatent_ops = &fat16_ops;

  	} else if (is_fat12(sbi)) {

  		sbi->fatent_shift = -1;
  		sbi->fatent_ops = &fat12_ops;

  	} else {
  		fat_fs_error(sb, "invalid FAT variant, %u bits", sbi->fat_bits);

  	}
  }

  static void mark_fsinfo_dirty(struct super_block *sb)
  {

  	struct msdos_sb_info *sbi = MSDOS_SB(sb);

  	if (sb_rdonly(sb) || !is_fat32(sbi))

  		return;
  
  	__mark_inode_dirty(sbi->fsinfo_inode, I_DIRTY_SYNC);

  }

  static inline int fat_ent_update_ptr(struct super_block *sb,
  				     struct fat_entry *fatent,
  				     int offset, sector_t blocknr)
  {
  	struct msdos_sb_info *sbi = MSDOS_SB(sb);

  	const struct fatent_operations *ops = sbi->fatent_ops;

  	struct buffer_head **bhs = fatent->bhs;
  
  	/* Is this fatent's blocks including this entry? */
  	if (!fatent->nr_bhs || bhs[0]->b_blocknr != blocknr)
  		return 0;

  	if (is_fat12(sbi)) {

  		if ((offset + 1) < sb->s_blocksize) {
  			/* This entry is on bhs[0]. */
  			if (fatent->nr_bhs == 2) {
  				brelse(bhs[1]);
  				fatent->nr_bhs = 1;
  			}
  		} else {
  			/* This entry needs the next block. */
  			if (fatent->nr_bhs != 2)
  				return 0;
  			if (bhs[1]->b_blocknr != (blocknr + 1))
  				return 0;
  		}

  	}
  	ops->ent_set_ptr(fatent, offset);
  	return 1;
  }
  
  int fat_ent_read(struct inode *inode, struct fat_entry *fatent, int entry)
  {
  	struct super_block *sb = inode->i_sb;
  	struct msdos_sb_info *sbi = MSDOS_SB(inode->i_sb);

  	const struct fatent_operations *ops = sbi->fatent_ops;

  	int err, offset;
  	sector_t blocknr;

  	if (!fat_valid_entry(sbi, entry)) {

  		fatent_brelse(fatent);

  		fat_fs_error(sb, "invalid access to FAT (entry 0x%08x)", entry);

  		return -EIO;
  	}
  
  	fatent_set_entry(fatent, entry);
  	ops->ent_blocknr(sb, entry, &offset, &blocknr);
  
  	if (!fat_ent_update_ptr(sb, fatent, offset, blocknr)) {
  		fatent_brelse(fatent);
  		err = ops->ent_bread(sb, fatent, offset, blocknr);
  		if (err)
  			return err;
  	}
  	return ops->ent_get(fatent);
  }
  
  /* FIXME: We can write the blocks as more big chunk. */
  static int fat_mirror_bhs(struct super_block *sb, struct buffer_head **bhs,
  			  int nr_bhs)
  {
  	struct msdos_sb_info *sbi = MSDOS_SB(sb);
  	struct buffer_head *c_bh;
  	int err, n, copy;
  
  	err = 0;
  	for (copy = 1; copy < sbi->fats; copy++) {
  		sector_t backup_fat = sbi->fat_length * copy;
  
  		for (n = 0; n < nr_bhs; n++) {
  			c_bh = sb_getblk(sb, backup_fat + bhs[n]->b_blocknr);
  			if (!c_bh) {
  				err = -ENOMEM;
  				goto error;
  			}

  			/* Avoid race with userspace read via bdev */
  			lock_buffer(c_bh);

  			memcpy(c_bh->b_data, bhs[n]->b_data, sb->s_blocksize);
  			set_buffer_uptodate(c_bh);

  			unlock_buffer(c_bh);

  			mark_buffer_dirty_inode(c_bh, sbi->fat_inode);

  			if (sb->s_flags & SB_SYNCHRONOUS)

  				err = sync_dirty_buffer(c_bh);
  			brelse(c_bh);
  			if (err)
  				goto error;
  		}
  	}
  error:
  	return err;
  }
  
  int fat_ent_write(struct inode *inode, struct fat_entry *fatent,
  		  int new, int wait)
  {
  	struct super_block *sb = inode->i_sb;

  	const struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;

  	int err;
  
  	ops->ent_put(fatent, new);
  	if (wait) {
  		err = fat_sync_bhs(fatent->bhs, fatent->nr_bhs);
  		if (err)
  			return err;
  	}
  	return fat_mirror_bhs(sb, fatent->bhs, fatent->nr_bhs);
  }
  
  static inline int fat_ent_next(struct msdos_sb_info *sbi,
  			       struct fat_entry *fatent)
  {
  	if (sbi->fatent_ops->ent_next(fatent)) {
  		if (fatent->entry < sbi->max_cluster)
  			return 1;
  	}
  	return 0;
  }
  
  static inline int fat_ent_read_block(struct super_block *sb,
  				     struct fat_entry *fatent)
  {

  	const struct fatent_operations *ops = MSDOS_SB(sb)->fatent_ops;

  	sector_t blocknr;
  	int offset;
  
  	fatent_brelse(fatent);
  	ops->ent_blocknr(sb, fatent->entry, &offset, &blocknr);
  	return ops->ent_bread(sb, fatent, offset, blocknr);
  }
  
  static void fat_collect_bhs(struct buffer_head **bhs, int *nr_bhs,
  			    struct fat_entry *fatent)
  {
  	int n, i;
  
  	for (n = 0; n < fatent->nr_bhs; n++) {
  		for (i = 0; i < *nr_bhs; i++) {
  			if (fatent->bhs[n] == bhs[i])
  				break;
  		}
  		if (i == *nr_bhs) {
  			get_bh(fatent->bhs[n]);
  			bhs[i] = fatent->bhs[n];
  			(*nr_bhs)++;
  		}
  	}
  }
  
  int fat_alloc_clusters(struct inode *inode, int *cluster, int nr_cluster)
  {
  	struct super_block *sb = inode->i_sb;
  	struct msdos_sb_info *sbi = MSDOS_SB(sb);

  	const struct fatent_operations *ops = sbi->fatent_ops;

  	struct fat_entry fatent, prev_ent;
  	struct buffer_head *bhs[MAX_BUF_PER_PAGE];
  	int i, count, err, nr_bhs, idx_clus;
  
  	BUG_ON(nr_cluster > (MAX_BUF_PER_PAGE / 2));	/* fixed limit */
  
  	lock_fat(sbi);

  	if (sbi->free_clusters != -1 && sbi->free_clus_valid &&
  	    sbi->free_clusters < nr_cluster) {

  		unlock_fat(sbi);
  		return -ENOSPC;
  	}
  
  	err = nr_bhs = idx_clus = 0;
  	count = FAT_START_ENT;
  	fatent_init(&prev_ent);
  	fatent_init(&fatent);
  	fatent_set_entry(&fatent, sbi->prev_free + 1);
  	while (count < sbi->max_cluster) {
  		if (fatent.entry >= sbi->max_cluster)
  			fatent.entry = FAT_START_ENT;
  		fatent_set_entry(&fatent, fatent.entry);
  		err = fat_ent_read_block(sb, &fatent);
  		if (err)
  			goto out;
  
  		/* Find the free entries in a block */
  		do {
  			if (ops->ent_get(&fatent) == FAT_ENT_FREE) {
  				int entry = fatent.entry;
  
  				/* make the cluster chain */
  				ops->ent_put(&fatent, FAT_ENT_EOF);
  				if (prev_ent.nr_bhs)
  					ops->ent_put(&prev_ent, entry);
  
  				fat_collect_bhs(bhs, &nr_bhs, &fatent);
  
  				sbi->prev_free = entry;
  				if (sbi->free_clusters != -1)
  					sbi->free_clusters--;
  
  				cluster[idx_clus] = entry;
  				idx_clus++;
  				if (idx_clus == nr_cluster)
  					goto out;
  
  				/*
  				 * fat_collect_bhs() gets ref-count of bhs,
  				 * so we can still use the prev_ent.
  				 */
  				prev_ent = fatent;
  			}
  			count++;
  			if (count == sbi->max_cluster)
  				break;
  		} while (fat_ent_next(sbi, &fatent));
  	}
  
  	/* Couldn't allocate the free entries */
  	sbi->free_clusters = 0;

  	sbi->free_clus_valid = 1;

  	err = -ENOSPC;
  
  out:
  	unlock_fat(sbi);

  	mark_fsinfo_dirty(sb);

  	fatent_brelse(&fatent);
  	if (!err) {
  		if (inode_needs_sync(inode))
  			err = fat_sync_bhs(bhs, nr_bhs);
  		if (!err)
  			err = fat_mirror_bhs(sb, bhs, nr_bhs);
  	}
  	for (i = 0; i < nr_bhs; i++)
  		brelse(bhs[i]);

  
  	if (err && idx_clus)
  		fat_free_clusters(inode, cluster[0]);
  
  	return err;
  }
  
  int fat_free_clusters(struct inode *inode, int cluster)
  {
  	struct super_block *sb = inode->i_sb;
  	struct msdos_sb_info *sbi = MSDOS_SB(sb);

  	const struct fatent_operations *ops = sbi->fatent_ops;

  	struct fat_entry fatent;
  	struct buffer_head *bhs[MAX_BUF_PER_PAGE];
  	int i, err, nr_bhs;

  	int first_cl = cluster, dirty_fsinfo = 0;

  
  	nr_bhs = 0;
  	fatent_init(&fatent);
  	lock_fat(sbi);
  	do {
  		cluster = fat_ent_read(inode, &fatent, cluster);
  		if (cluster < 0) {
  			err = cluster;
  			goto error;
  		} else if (cluster == FAT_ENT_FREE) {

  			fat_fs_error(sb, "%s: deleting FAT entry beyond EOF",

  				     __func__);

  			err = -EIO;
  			goto error;
  		}

  		if (sbi->options.discard) {
  			/*
  			 * Issue discard for the sectors we no longer
  			 * care about, batching contiguous clusters
  			 * into one request
  			 */
  			if (cluster != fatent.entry + 1) {
  				int nr_clus = fatent.entry - first_cl + 1;
  
  				sb_issue_discard(sb,
  					fat_clus_to_blknr(sbi, first_cl),

  					nr_clus * sbi->sec_per_clus,

  					GFP_NOFS, 0);

  
  				first_cl = cluster;
  			}

  		}

  		ops->ent_put(&fatent, FAT_ENT_FREE);

  		if (sbi->free_clusters != -1) {

  			sbi->free_clusters++;

  			dirty_fsinfo = 1;

  		}

  
  		if (nr_bhs + fatent.nr_bhs > MAX_BUF_PER_PAGE) {

  			if (sb->s_flags & SB_SYNCHRONOUS) {

  				err = fat_sync_bhs(bhs, nr_bhs);
  				if (err)
  					goto error;
  			}
  			err = fat_mirror_bhs(sb, bhs, nr_bhs);
  			if (err)
  				goto error;
  			for (i = 0; i < nr_bhs; i++)
  				brelse(bhs[i]);
  			nr_bhs = 0;
  		}
  		fat_collect_bhs(bhs, &nr_bhs, &fatent);
  	} while (cluster != FAT_ENT_EOF);

  	if (sb->s_flags & SB_SYNCHRONOUS) {

  		err = fat_sync_bhs(bhs, nr_bhs);
  		if (err)
  			goto error;
  	}
  	err = fat_mirror_bhs(sb, bhs, nr_bhs);
  error:
  	fatent_brelse(&fatent);
  	for (i = 0; i < nr_bhs; i++)
  		brelse(bhs[i]);
  	unlock_fat(sbi);

  	if (dirty_fsinfo)
  		mark_fsinfo_dirty(sb);

  	return err;
  }

  EXPORT_SYMBOL_GPL(fat_free_clusters);

  struct fatent_ra {
  	sector_t cur;
  	sector_t limit;
  
  	unsigned int ra_blocks;
  	sector_t ra_advance;
  	sector_t ra_next;
  	sector_t ra_limit;
  };

  static void fat_ra_init(struct super_block *sb, struct fatent_ra *ra,
  			struct fat_entry *fatent, int ent_limit)

  {

  	struct msdos_sb_info *sbi = MSDOS_SB(sb);
  	const struct fatent_operations *ops = sbi->fatent_ops;
  	sector_t blocknr, block_end;
  	int offset;
  	/*
  	 * This is the sequential read, so ra_pages * 2 (but try to
  	 * align the optimal hardware IO size).
  	 * [BTW, 128kb covers the whole sectors for FAT12 and FAT16]
  	 */
  	unsigned long ra_pages = sb->s_bdi->ra_pages;
  	unsigned int reada_blocks;

  	if (fatent->entry >= ent_limit)
  		return;

  	if (ra_pages > sb->s_bdi->io_pages)
  		ra_pages = rounddown(ra_pages, sb->s_bdi->io_pages);
  	reada_blocks = ra_pages << (PAGE_SHIFT - sb->s_blocksize_bits + 1);
  
  	/* Initialize the range for sequential read */

  	ops->ent_blocknr(sb, fatent->entry, &offset, &blocknr);

  	ops->ent_blocknr(sb, ent_limit - 1, &offset, &block_end);
  	ra->cur = 0;
  	ra->limit = (block_end + 1) - blocknr;

  	/* Advancing the window at half size */
  	ra->ra_blocks = reada_blocks >> 1;
  	ra->ra_advance = ra->cur;
  	ra->ra_next = ra->cur;
  	ra->ra_limit = ra->cur + min_t(sector_t, reada_blocks, ra->limit);
  }
  
  /* Assuming to be called before reading a new block (increments ->cur). */
  static void fat_ent_reada(struct super_block *sb, struct fatent_ra *ra,
  			  struct fat_entry *fatent)
  {
  	if (ra->ra_next >= ra->ra_limit)
  		return;
  
  	if (ra->cur >= ra->ra_advance) {
  		struct msdos_sb_info *sbi = MSDOS_SB(sb);
  		const struct fatent_operations *ops = sbi->fatent_ops;
  		struct blk_plug plug;
  		sector_t blocknr, diff;
  		int offset;
  
  		ops->ent_blocknr(sb, fatent->entry, &offset, &blocknr);
  
  		diff = blocknr - ra->cur;
  		blk_start_plug(&plug);
  		/*
  		 * FIXME: we would want to directly use the bio with
  		 * pages to reduce the number of segments.
  		 */
  		for (; ra->ra_next < ra->ra_limit; ra->ra_next++)
  			sb_breadahead(sb, ra->ra_next + diff);
  		blk_finish_plug(&plug);
  
  		/* Advance the readahead window */
  		ra->ra_advance += ra->ra_blocks;
  		ra->ra_limit += min_t(sector_t,
  				      ra->ra_blocks, ra->limit - ra->ra_limit);
  	}
  	ra->cur++;

  }

  int fat_count_free_clusters(struct super_block *sb)
  {
  	struct msdos_sb_info *sbi = MSDOS_SB(sb);

  	const struct fatent_operations *ops = sbi->fatent_ops;

  	struct fat_entry fatent;

  	struct fatent_ra fatent_ra;

  	int err = 0, free;
  
  	lock_fat(sbi);

  	if (sbi->free_clusters != -1 && sbi->free_clus_valid)

  		goto out;
  
  	free = 0;
  	fatent_init(&fatent);
  	fatent_set_entry(&fatent, FAT_START_ENT);

  	fat_ra_init(sb, &fatent_ra, &fatent, sbi->max_cluster);

  	while (fatent.entry < sbi->max_cluster) {

  		/* readahead of fat blocks */

  		fat_ent_reada(sb, &fatent_ra, &fatent);

  		err = fat_ent_read_block(sb, &fatent);
  		if (err)
  			goto out;
  
  		do {
  			if (ops->ent_get(&fatent) == FAT_ENT_FREE)
  				free++;
  		} while (fat_ent_next(sbi, &fatent));

  		cond_resched();

  	}
  	sbi->free_clusters = free;

  	sbi->free_clus_valid = 1;

  	mark_fsinfo_dirty(sb);

  	fatent_brelse(&fatent);
  out:
  	unlock_fat(sbi);
  	return err;
  }

  
  static int fat_trim_clusters(struct super_block *sb, u32 clus, u32 nr_clus)
  {
  	struct msdos_sb_info *sbi = MSDOS_SB(sb);
  	return sb_issue_discard(sb, fat_clus_to_blknr(sbi, clus),
  				nr_clus * sbi->sec_per_clus, GFP_NOFS, 0);
  }
  
  int fat_trim_fs(struct inode *inode, struct fstrim_range *range)
  {
  	struct super_block *sb = inode->i_sb;
  	struct msdos_sb_info *sbi = MSDOS_SB(sb);
  	const struct fatent_operations *ops = sbi->fatent_ops;
  	struct fat_entry fatent;

  	struct fatent_ra fatent_ra;

  	u64 ent_start, ent_end, minlen, trimmed = 0;
  	u32 free = 0;

  	int err = 0;
  
  	/*
  	 * FAT data is organized as clusters, trim at the granulary of cluster.
  	 *
  	 * fstrim_range is in byte, convert vaules to cluster index.
  	 * Treat sectors before data region as all used, not to trim them.
  	 */
  	ent_start = max_t(u64, range->start>>sbi->cluster_bits, FAT_START_ENT);
  	ent_end = ent_start + (range->len >> sbi->cluster_bits) - 1;
  	minlen = range->minlen >> sbi->cluster_bits;
  
  	if (ent_start >= sbi->max_cluster || range->len < sbi->cluster_size)
  		return -EINVAL;
  	if (ent_end >= sbi->max_cluster)
  		ent_end = sbi->max_cluster - 1;

  	fatent_init(&fatent);
  	lock_fat(sbi);
  	fatent_set_entry(&fatent, ent_start);

  	fat_ra_init(sb, &fatent_ra, &fatent, ent_end + 1);

  	while (fatent.entry <= ent_end) {
  		/* readahead of fat blocks */

  		fat_ent_reada(sb, &fatent_ra, &fatent);

  
  		err = fat_ent_read_block(sb, &fatent);
  		if (err)
  			goto error;
  		do {
  			if (ops->ent_get(&fatent) == FAT_ENT_FREE) {
  				free++;
  			} else if (free) {
  				if (free >= minlen) {
  					u32 clus = fatent.entry - free;
  
  					err = fat_trim_clusters(sb, clus, free);
  					if (err && err != -EOPNOTSUPP)
  						goto error;
  					if (!err)
  						trimmed += free;
  					err = 0;
  				}
  				free = 0;
  			}
  		} while (fat_ent_next(sbi, &fatent) && fatent.entry <= ent_end);
  
  		if (fatal_signal_pending(current)) {
  			err = -ERESTARTSYS;
  			goto error;
  		}
  
  		if (need_resched()) {
  			fatent_brelse(&fatent);
  			unlock_fat(sbi);
  			cond_resched();
  			lock_fat(sbi);
  		}
  	}
  	/* handle scenario when tail entries are all free */
  	if (free && free >= minlen) {
  		u32 clus = fatent.entry - free;
  
  		err = fat_trim_clusters(sb, clus, free);
  		if (err && err != -EOPNOTSUPP)
  			goto error;
  		if (!err)
  			trimmed += free;
  		err = 0;
  	}
  
  error:
  	fatent_brelse(&fatent);
  	unlock_fat(sbi);
  
  	range->len = trimmed << sbi->cluster_bits;
  
  	return err;
  }