adfs: improve timestamp precision

ADFS (FileCore) storage complies with the RISC OS timestamp specification
(40-bit centiseconds since 01 Jan 1900 00:00:00).  It is desirable that
stored timestamp precision be maintained to facilitate a precise copy of
data and metadata from a hard disc (or image thereof) into a RISC OS
emulator (such as RPCEmu).

This patch implements a full-precision conversion from ADFS to Unix
timestamp as the existing driver, for ease of calculation with old 32-bit
compilers, uses the common trick of shifting the 40-bits representing
centiseconds around into 32-bits representing seconds thereby losing
precision.

Signed-off-by: Stuart Swales<stuart.swales.croftnuisk@gmail.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 21 deletions

diff --git a/fs/adfs/inode.c b/fs/adfs/inode.c
index 09fe40198d1c..16d7ef2dffe1 100644
--- a/fs/adfs/inode.c
+++ b/fs/adfs/inode.c
@@ -174,50 +174,48 @@ adfs_mode2atts(struct super_block *sb, struct inode *inode)
 
 /*
  * Convert an ADFS time to Unix time.  ADFS has a 40-bit centi-second time
- * referenced to 1 Jan 1900 (til 2248)
+ * referenced to 1 Jan 1900 (til 2248) so we need to discard 2208988800 seconds
+ * of time to convert from RISC OS epoch to Unix epoch.
  */
 static void
 adfs_adfs2unix_time(struct timespec *tv, struct inode *inode)
 {
         unsigned int high, low;
+        /* 01 Jan 1970 00:00:00 (Unix epoch) as nanoseconds since
+         * 01 Jan 1900 00:00:00 (RISC OS epoch)
+         */
+        static const s64 nsec_unix_epoch_diff_risc_os_epoch =
+                                                        2208988800000000000LL;
+        s64 nsec;
 
         if (ADFS_I(inode)->stamped == 0)
                 goto cur_time;
 
-        high = ADFS_I(inode)->loadaddr << 24;
-        low  = ADFS_I(inode)->execaddr;
+        high = ADFS_I(inode)->loadaddr & 0xFF; /* top 8 bits of timestamp */
+        low  = ADFS_I(inode)->execaddr;    /* bottom 32 bits of timestamp */
 
-        high |= low >> 8;
-        low  &= 255;
+        /* convert 40-bit centi-seconds to 32-bit seconds
+         * going via nanoseconds to retain precision
+         */
+        nsec = (((s64) high << 32) | (s64) low) * 10000000; /* cs to ns */
 
         /* Files dated pre  01 Jan 1970 00:00:00. */
-        if (high < 0x336e996a)
+        if (nsec < nsec_unix_epoch_diff_risc_os_epoch)
                 goto too_early;
 
-        /* Files dated post 18 Jan 2038 03:14:05. */
-        if (high >= 0x656e9969)
-                goto too_late;
-
-        /* discard 2208988800 (0x336e996a00) seconds of time */
-        high -= 0x336e996a;
+        /* convert from RISC OS to Unix epoch */
+        nsec -= nsec_unix_epoch_diff_risc_os_epoch;
 
-        /* convert 40-bit centi-seconds to 32-bit seconds */
-        tv->tv_sec = (((high % 100) << 8) + low) / 100 + (high / 100 << 8);
-        tv->tv_nsec = 0;
+        *tv = ns_to_timespec(nsec);
         return;
 
  cur_time:
-        *tv = CURRENT_TIME_SEC;
+        *tv = CURRENT_TIME;
         return;
 
  too_early:
         tv->tv_sec = tv->tv_nsec = 0;
         return;
-
- too_late:
-        tv->tv_sec = 0x7ffffffd;
-        tv->tv_nsec = 0;
-        return;
 }
 
 /*