linux-kernel - [PATCH] e1000e: prevent division by zero if TIMINCA is zero

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Message-Id: <1462563711-30350-1-git-send-email-dvlasenk@redhat.com>
Date:	Fri,  6 May 2016 21:41:51 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	"Ruinskiy, Dima" <dima.ruinskiy@...el.com>,
	intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH] e1000e: prevent division by zero if TIMINCA is zero

Users report that under VMWare, er32(TIMINCA) returns zero.
This causes division by zero at init time as follows:

 ==>            incvalue = er32(TIMINCA) & E1000_TIMINCA_INCVALUE_MASK;
                for (i = 0; i < E1000_MAX_82574_SYSTIM_REREADS; i++) {
                        /* latch SYSTIMH on read of SYSTIML */
                        systim_next = (cycle_t)er32(SYSTIML);
                        systim_next |= (cycle_t)er32(SYSTIMH) << 32;

                        time_delta = systim_next - systim;
                        temp = time_delta;
 ====>                  rem = do_div(temp, incvalue);

This change makes kernel survive this, and users report that
NIC does work after this change.

Since on real hardware incvalue is never zero, this should not affect
real hardware use case.

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
CC: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
CC: "Ruinskiy, Dima" <dima.ruinskiy@...el.com>
CC: intel-wired-lan@...ts.osuosl.org
CC: netdev@...r.kernel.org
CC: LKML <linux-kernel@...r.kernel.org>
---
 drivers/net/ethernet/intel/e1000e/netdev.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/e1000e/netdev.c b/drivers/net/ethernet/intel/e1000e/netdev.c
index 269087c..0626935 100644
--- a/drivers/net/ethernet/intel/e1000e/netdev.c
+++ b/drivers/net/ethernet/intel/e1000e/netdev.c
@@ -4315,7 +4315,8 @@ static cycle_t e1000e_cyclecounter_read(const struct cyclecounter *cc)
 
 			time_delta = systim_next - systim;
 			temp = time_delta;
-			rem = do_div(temp, incvalue);
+			/* VMWare users have seen incvalue of zero, don't div / 0 */
+			rem = incvalue ? do_div(temp, incvalue) : (time_delta != 0);
 
 			systim = systim_next;
 
-- 
1.8.1.4