[RFC 5/6] vfs: Add timestamp_truncate() api

[Deepa Dinamani]

timespec_trunc() function is used to truncate a
filesystem timestamp to the right granularity.
But, the function does not clamp tv_sec part of the
timestamps according to the filesystem timestamp limits.

Also, timespec_trunc() is exclusively used for filesystem
timestamps. Move the api to be part of vfs.

The replacement api: timestamp_truncate() also alters the
signature of the function to accommodate filesystem
timestamp clamping according to flesystem limits.

Note that clamp_t macro is used for clamping here as vfs
is not yet using struct timespec64 internally. This is
only for compilation purposes.
The actual patch can only be merged after the vfs is
transitioned to use timespec64 for correct operation of
clamp macro. At which point, clamp_t() will be replaced
by clamp().

Signed-off-by: Deepa Dinamani <deepa.kernel@xxxxxxxxx>
---
 fs/inode.c | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/fs/inode.c b/fs/inode.c
index 7b2b78d..f9285f2 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -2106,6 +2106,36 @@ void inode_nohighmem(struct inode *inode)
 EXPORT_SYMBOL(inode_nohighmem);
 
 /**
+ * fs_timespec_trunc - Truncate timespec to a granularity
+ * @t: Timespec
+ * @gran: Granularity in ns.
+ *
+ * Truncate a timespec to a granularity. Always rounds down. gran must
+ * not be 0 nor greater than a second (NSEC_PER_SEC, or 10^9 ns).
+ */
+struct timespec timestamp_truncate(struct timespec t, struct inode *inode)
+{
+	struct super_block *sb = inode->i_sb;
+	unsigned int gran = sb->s_time_gran;
+
+	t.tv_sec = clamp_t(time64_t, t.tv_sec, sb->s_time_min, sb->s_time_max);
+
+	/* Avoid division in the common cases 1 ns and 1 s. */
+	if (gran == 1) {
+		/* nothing */
+	} else if (gran == NSEC_PER_SEC) {
+		t.tv_nsec = 0;
+	} else if (gran > 1 && gran < NSEC_PER_SEC) {
+		t.tv_nsec -= t.tv_nsec % gran;
+	} else {
+		WARN(1, "illegal file time granularity: %u", gran);
+	}
+	return t;
+}
+EXPORT_SYMBOL(timestamp_truncate);
+
+
+/**
  * current_time - Return FS time
  * @inode: inode.
  *
@@ -2124,6 +2154,6 @@ struct timespec current_time(struct inode *inode)
 		return now;
 	}
 
-	return timespec_trunc(now, inode->i_sb->s_time_gran);
+	return timestamp_truncate(now, inode);
 }
 EXPORT_SYMBOL(current_time);
-- 
2.7.4