Patch "e1000e: Fix tight loop implementation of systime read algorithm" has been added to the 4.2-stable tree

Patch "e1000e: Fix tight loop implementation of systime read algorithm" has been added to the 4.2-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    e1000e: Fix tight loop implementation of systime read algorithm

to the 4.2-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     e1000e-fix-tight-loop-implementation-of-systime-read-algorithm.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From 37b12910dd11d9ab969f2c310dc9160b7f3e3405 Mon Sep 17 00:00:00 2001
From: Raanan Avargil <raanan.avargil@intel.com>
Date: Sun, 19 Jul 2015 16:33:20 +0300
Subject: e1000e: Fix tight loop implementation of systime read algorithm

From: Raanan Avargil <raanan.avargil@intel.com>

commit 37b12910dd11d9ab969f2c310dc9160b7f3e3405 upstream.

Change the algorithm. Read systimel twice and check for overflow.
If there was no overflow, use the first value.
If there was an overflow, read systimeh again and use the second
systimel value.

Signed-off-by: Raanan Avargil <raanan.avargil@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/net/ethernet/intel/e1000e/netdev.c |   31 +++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

--- a/drivers/net/ethernet/intel/e1000e/netdev.c
+++ b/drivers/net/ethernet/intel/e1000e/netdev.c
@@ -4280,18 +4280,29 @@ static cycle_t e1000e_cyclecounter_read(
 	struct e1000_adapter *adapter = container_of(cc, struct e1000_adapter,
 						     cc);
 	struct e1000_hw *hw = &adapter->hw;
+	u32 systimel_1, systimel_2, systimeh;
 	cycle_t systim, systim_next;
-	/* SYSTIMH latching upon SYSTIML read does not work well. To fix that
-	 * we don't want to allow overflow of SYSTIML and a change to SYSTIMH
-	 * to occur between reads, so if we read a vale close to overflow, we
-	 * wait for overflow to occur and read both registers when its safe.
+	/* SYSTIMH latching upon SYSTIML read does not work well.
+	 * This means that if SYSTIML overflows after we read it but before
+	 * we read SYSTIMH, the value of SYSTIMH has been incremented and we
+	 * will experience a huge non linear increment in the systime value
+	 * to fix that we test for overflow and if true, we re-read systime.
 	 */
-	u32 systim_overflow_latch_fix = 0x3FFFFFFF;
-
-	do {
-		systim = (cycle_t)er32(SYSTIML);
-	} while (systim > systim_overflow_latch_fix);
-	systim |= (cycle_t)er32(SYSTIMH) << 32;
+	systimel_1 = er32(SYSTIML);
+	systimeh = er32(SYSTIMH);
+	systimel_2 = er32(SYSTIML);
+	/* Check for overflow. If there was no overflow, use the values */
+	if (systimel_1 < systimel_2) {
+		systim = (cycle_t)systimel_1;
+		systim |= (cycle_t)systimeh << 32;
+	} else {
+		/* There was an overflow, read again SYSTIMH, and use
+		 * systimel_2
+		 */
+		systimeh = er32(SYSTIMH);
+		systim = (cycle_t)systimel_2;
+		systim |= (cycle_t)systimeh << 32;
+	}
 
 	if ((hw->mac.type == e1000_82574) || (hw->mac.type == e1000_82583)) {
 		u64 incvalue, time_delta, rem, temp;


Patches currently in stable-queue which might be from raanan.avargil@intel.com are

queue-4.2/e1000e-fix-tight-loop-implementation-of-systime-read-algorithm.patch