[PATCH] fat: Provide option for setting timezone offset

[Jan Kara <jack@suse.cz>]

So far FAT either offsets time stamps by sys_tz.minuteswest or leaves them
as they are (when tz=UTC mount option is used). However in some cases it
is useful if one can specify time stamp offset on his own (e.g. when time
zone of the camera connected is different from time zone of the computer,
or when HW clock is in UTC and thus sys_tz.minuteswest == 0).

So provide a mount option tz= which allows user to specify offset in minutes
that should be applied to time stamps on the filesystem.

Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/fat/fat.h   |    3 ++-
 fs/fat/inode.c |   20 +++++++++++++++-----
 fs/fat/misc.c  |    9 ++++++---
 3 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/fs/fat/fat.h b/fs/fat/fat.h
index 2deeeb8..5f19eac 100644
--- a/fs/fat/fat.h
+++ b/fs/fat/fat.h
@@ -28,6 +28,7 @@ struct fat_mount_options {
 	unsigned short fs_fmask;
 	unsigned short fs_dmask;
 	unsigned short codepage;  /* Codepage for shortname conversions */
+	short tz_offset;	  /* Offset of timestamps from UTC (in minutes) */
 	char *iocharset;          /* Charset used for filename input/display */
 	unsigned short shortname; /* flags for shortname display/create rule */
 	unsigned char name_check; /* r = relaxed, n = normal, s = strict */
@@ -44,7 +45,7 @@ struct fat_mount_options {
 		 flush:1,	  /* write things quickly */
 		 nocase:1,	  /* Does this need case conversion? 0=need case conversion*/
 		 usefree:1,	  /* Use free_clusters for FAT32 */
-		 tz_utc:1,	  /* Filesystem timestamps are in UTC */
+		 tz_set:1,	  /* Filesystem timestamps' offset set */
 		 rodir:1,	  /* allow ATTR_RO for directory */
 		 discard:1;	  /* Issue discard requests on deletions */
 };
diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index 05e897f..23c29a2 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -850,8 +850,8 @@ static int fat_show_options(struct seq_file *m, struct dentry *root)
 	}
 	if (opts->flush)
 		seq_puts(m, ",flush");
-	if (opts->tz_utc)
-		seq_puts(m, ",tz=UTC");
+	if (opts->tz_set)
+		seq_printf(m, ",tz=%d", opts->tz_offset);
 	if (opts->errors == FAT_ERRORS_CONT)
 		seq_puts(m, ",errors=continue");
 	else if (opts->errors == FAT_ERRORS_PANIC)
@@ -872,7 +872,7 @@ enum {
 	Opt_charset, Opt_shortname_lower, Opt_shortname_win95,
 	Opt_shortname_winnt, Opt_shortname_mixed, Opt_utf8_no, Opt_utf8_yes,
 	Opt_uni_xl_no, Opt_uni_xl_yes, Opt_nonumtail_no, Opt_nonumtail_yes,
-	Opt_obsolete, Opt_flush, Opt_tz_utc, Opt_rodir, Opt_err_cont,
+	Opt_obsolete, Opt_flush, Opt_tz_utc, Opt_tz, Opt_rodir, Opt_err_cont,
 	Opt_err_panic, Opt_err_ro, Opt_discard, Opt_err,
 };
 
@@ -898,6 +898,7 @@ static const match_table_t fat_tokens = {
 	{Opt_immutable, "sys_immutable"},
 	{Opt_flush, "flush"},
 	{Opt_tz_utc, "tz=UTC"},
+	{Opt_tz, "tz=%d"},
 	{Opt_err_cont, "errors=continue"},
 	{Opt_err_panic, "errors=panic"},
 	{Opt_err_ro, "errors=remount-ro"},
@@ -981,7 +982,7 @@ static int parse_options(struct super_block *sb, char *options, int is_vfat,
 	opts->utf8 = opts->unicode_xlate = 0;
 	opts->numtail = 1;
 	opts->usefree = opts->nocase = 0;
-	opts->tz_utc = 0;
+	opts->tz_set = 0;
 	opts->errors = FAT_ERRORS_RO;
 	*debug = 0;
 
@@ -1072,8 +1073,17 @@ static int parse_options(struct super_block *sb, char *options, int is_vfat,
 		case Opt_flush:
 			opts->flush = 1;
 			break;
+		case Opt_tz:
+			if (match_int(&args[0], &option))
+				return 0;
+			if (option < -12 * 60 || option > 12 * 60)
+				return 0;
+			opts->tz_set = 1;
+			opts->tz_offset = option;
+			break;
 		case Opt_tz_utc:
-			opts->tz_utc = 1;
+			opts->tz_set = 1;
+			opts->tz_offset = 0;
 			break;
 		case Opt_err_cont:
 			opts->errors = FAT_ERRORS_CONT;
diff --git a/fs/fat/misc.c b/fs/fat/misc.c
index 6d93360..cc20a78 100644
--- a/fs/fat/misc.c
+++ b/fs/fat/misc.c
@@ -212,8 +212,10 @@ void fat_time_fat2unix(struct msdos_sb_info *sbi, struct timespec *ts,
 		   + days_in_year[month] + day
 		   + DAYS_DELTA) * SECS_PER_DAY;
 
-	if (!sbi->options.tz_utc)
+	if (!sbi->options.tz_set)
 		second += sys_tz.tz_minuteswest * SECS_PER_MIN;
+	else
+		second -= sbi->options.tz_offset * SECS_PER_MIN;
 
 	if (time_cs) {
 		ts->tv_sec = second + (time_cs / 100);
@@ -229,8 +231,9 @@ void fat_time_unix2fat(struct msdos_sb_info *sbi, struct timespec *ts,
 		       __le16 *time, __le16 *date, u8 *time_cs)
 {
 	struct tm tm;
-	time_to_tm(ts->tv_sec, sbi->options.tz_utc ? 0 :
-		   -sys_tz.tz_minuteswest * 60, &tm);
+	time_to_tm(ts->tv_sec,
+		   (sbi->options.tz_set ? sbi->options.tz_offset :
+		   -sys_tz.tz_minuteswest) * SECS_PER_MIN, &tm);
 
 	/*  FAT can only support year between 1980 to 2107 */
 	if (tm.tm_year < 1980 - 1900) {
-- 
1.7.1