aoe: use ktime_t instead of timeval

'struct frame' uses two variables to store the sent timestamp - 'struct
timeval' and jiffies. jiffies is used to avoid discrepancies caused by
updates to system time. 'struct timeval' is deprecated because it uses
32-bit representation for seconds which will overflow in year 2038.

This patch does the following:
- Replace the use of 'struct timeval' and jiffies with ktime_t, which
  is the recommended type for timestamping
- ktime_t provides both long range (like jiffies) and high resolution
  (like timeval). Using ktime_get (monotonic time) instead of wall-clock
  time prevents any discprepancies caused by updates to system time.

[updates by Arnd below]
The original patch from Tina never went anywhere as we discussed how
to keep the impact on performance minimal. I've started over now but
arrived at basically the same patch that she had originally, except for
an slightly improved tsince_hr() function. I'm making it more robust
against overflows, and also optimize explicitly for the common case
in which a frame is less than 4.2 seconds old, using only a 32-bit
division in that case.

This should make the new version more efficient than the old code,
since we replace the existing two 32-bit division in do_gettimeofday()
plus one multiplication with a single single 32-bit division in
tsince_hr() and drop the double bookkeeping. It's also more efficient
than the ktime_get_us() API we discussed before, since that would
also rely on multiple divisions.

Link: https://lists.linaro.org/pipermail/y2038/2015-May/000276.html
Signed-off-by: Tina Ruchandani <ruchandani.tina@gmail.com>
Cc: Ed Cashin <ed.cashin@acm.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/aoe/aoe.h

@@ -112,8 +112,7 @@ enum frame_flags {
 struct frame {
 	struct list_head head;
 	u32 tag;
-	struct timeval sent;	/* high-res time packet was sent */
-	u32 sent_jiffs;		/* low-res jiffies-based sent time */
+	ktime_t sent;			/* high-res time packet was sent */
 	ulong waited;
 	ulong waited_total;
 	struct aoetgt *t;		/* parent target I belong to */

drivers/block/aoe/aoecmd.c

@@ -398,8 +398,7 @@ aoecmd_ata_rw(struct aoedev *d)
 
 	skb = skb_clone(f->skb, GFP_ATOMIC);
 	if (skb) {
-		do_gettimeofday(&f->sent);
-		f->sent_jiffs = (u32) jiffies;
+		f->sent = ktime_get();
 		__skb_queue_head_init(&queue);
 		__skb_queue_tail(&queue, skb);
 		aoenet_xmit(&queue);
@@ -489,8 +488,7 @@ resend(struct aoedev *d, struct frame *f)
 	skb = skb_clone(skb, GFP_ATOMIC);
 	if (skb == NULL)
 		return;
-	do_gettimeofday(&f->sent);
-	f->sent_jiffs = (u32) jiffies;
+	f->sent = ktime_get();
 	__skb_queue_head_init(&queue);
 	__skb_queue_tail(&queue, skb);
 	aoenet_xmit(&queue);
@@ -499,33 +497,17 @@ resend(struct aoedev *d, struct frame *f)
 static int
 tsince_hr(struct frame *f)
 {
-	struct timeval now;
-	int n;
+	u64 delta = ktime_to_ns(ktime_sub(ktime_get(), f->sent));
 
-	do_gettimeofday(&now);
-	n = now.tv_usec - f->sent.tv_usec;
-	n += (now.tv_sec - f->sent.tv_sec) * USEC_PER_SEC;
+	/* delta is normally under 4.2 seconds, avoid 64-bit division */
+	if (likely(delta <= UINT_MAX))
+		return (u32)delta / NSEC_PER_USEC;
 
-	if (n < 0)
-		n = -n;
+	/* avoid overflow after 71 minutes */
+	if (delta > ((u64)INT_MAX * NSEC_PER_USEC))
+		return INT_MAX;
 
-	/* For relatively long periods, use jiffies to avoid
-	 * discrepancies caused by updates to the system time.
-	 *
-	 * On system with HZ of 1000, 32-bits is over 49 days
-	 * worth of jiffies, or over 71 minutes worth of usecs.
-	 *
-	 * Jiffies overflow is handled by subtraction of unsigned ints:
-	 * (gdb) print (unsigned) 2 - (unsigned) 0xfffffffe
-	 * $3 = 4
-	 * (gdb)
-	 */
-	if (n > USEC_PER_SEC / 4) {
-		n = ((u32) jiffies) - f->sent_jiffs;
-		n *= USEC_PER_SEC / HZ;
-	}
-
-	return n;
+	return div_u64(delta, NSEC_PER_USEC);
 }
 
 static int
@@ -589,7 +571,6 @@ reassign_frame(struct frame *f)
 	nf->waited = 0;
 	nf->waited_total = f->waited_total;
 	nf->sent = f->sent;
-	nf->sent_jiffs = f->sent_jiffs;
 	f->skb = skb;
 
 	return nf;
@@ -633,8 +614,7 @@ probe(struct aoetgt *t)
 
 	skb = skb_clone(f->skb, GFP_ATOMIC);
 	if (skb) {
-		do_gettimeofday(&f->sent);
-		f->sent_jiffs = (u32) jiffies;
+		f->sent = ktime_get();
 		__skb_queue_head_init(&queue);
 		__skb_queue_tail(&queue, skb);
 		aoenet_xmit(&queue);
@@ -1432,10 +1412,8 @@ aoecmd_ata_id(struct aoedev *d)
 	d->timer.function = rexmit_timer;
 
 	skb = skb_clone(skb, GFP_ATOMIC);
-	if (skb) {
-		do_gettimeofday(&f->sent);
-		f->sent_jiffs = (u32) jiffies;
-	}
+	if (skb)
+		f->sent = ktime_get();
 
 	return skb;
 }