fat: provide option for setting timezone offset

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 time_offset= which allows user to specify offset
in minutes that should be applied to time stamps on the filesystem.

akpm: this code would work incorrectly when used via `mount -o remount',
because cached inodes would not be updated.  But fatfs's fat_remount() is
basically a no-op anyway.

Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/Documentation/filesystems/vfat.txt b/Documentation/filesystems/vfat.txt
index de1e6c4dccff..d230dd9c99b0 100644
--- a/Documentation/filesystems/vfat.txt
+++ b/Documentation/filesystems/vfat.txt
@@ -111,6 +111,15 @@ tz=UTC        -- Interpret timestamps as UTC rather than local time.
                  useful when mounting devices (like digital cameras)
                  that are set to UTC in order to avoid the pitfalls of
                  local time.
+time_offset=minutes
+              -- Set offset for conversion of timestamps from local time
+                 used by FAT to UTC. I.e. <minutes> minutes will be subtracted
+                 from each timestamp to convert it to UTC used internally by
+                 Linux. This is useful when time zone set in sys_tz is
+                 not the time zone used by the filesystem. Note that this
+                 option still does not provide correct time stamps in all
+                 cases in presence of DST - time stamps in a different DST
+                 setting will be off by one hour.
 
 showexec      -- If set, the execute permission bits of the file will be
                  allowed only if the extension part of the name is .EXE,