[PATCH v4 06/11] exfat: add data_start_bytes and exfat_cluster_to_phys_bytes() helper

[Namjae Jeon <linkinjeon@kernel.org>]

This caches the data area start offset in bytes (data_start_bytes)
and introduces a helper function exfat_cluster_to_phys_bytes() to compute
the physical byte position of a given cluster.

Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
---
 fs/exfat/exfat_fs.h | 8 ++++++++
 fs/exfat/super.c    | 1 +
 2 files changed, 9 insertions(+)

diff --git a/fs/exfat/exfat_fs.h b/fs/exfat/exfat_fs.h
index 6e7fd6822b01..cd38e71d3e8a 100644
--- a/fs/exfat/exfat_fs.h
+++ b/fs/exfat/exfat_fs.h
@@ -227,6 +227,7 @@ struct exfat_sb_info {
 	unsigned long long FAT1_start_sector; /* FAT1 start sector */
 	unsigned long long FAT2_start_sector; /* FAT2 start sector */
 	unsigned long long data_start_sector; /* data area start sector */
+	unsigned long long data_start_bytes;
 	unsigned int num_FAT_sectors; /* num of FAT sectors */
 	unsigned int root_dir; /* root dir cluster */
 	unsigned int dentries_per_clu; /* num of dentries per cluster */
@@ -400,6 +401,13 @@ static inline loff_t exfat_ondisk_size(const struct inode *inode)
 	return ((loff_t)inode->i_blocks) << 9;
 }
 
+static inline loff_t exfat_cluster_to_phys_bytes(struct exfat_sb_info *sbi,
+		unsigned int clus)
+{
+	return ((loff_t)(clus - EXFAT_RESERVED_CLUSTERS) << sbi->cluster_size_bits) +
+		sbi->data_start_bytes;
+}
+
 /*
  * helpers for cluster size to byte conversion.
  */
diff --git a/fs/exfat/super.c b/fs/exfat/super.c
index cb2f8eefff99..388db271c6bf 100644
--- a/fs/exfat/super.c
+++ b/fs/exfat/super.c
@@ -499,6 +499,7 @@ static int exfat_read_boot_sector(struct super_block *sb)
 	if (p_boot->num_fats == 2)
 		sbi->FAT2_start_sector += sbi->num_FAT_sectors;
 	sbi->data_start_sector = le32_to_cpu(p_boot->clu_offset);
+	sbi->data_start_bytes = sbi->data_start_sector << p_boot->sect_size_bits;
 	sbi->num_sectors = le64_to_cpu(p_boot->vol_length);
 	/* because the cluster index starts with 2 */
 	sbi->num_clusters = le32_to_cpu(p_boot->clu_count) +
-- 
2.25.1