[PATCH iwl-net v3 5/6] ixgbe: fix ITR value overflow in adaptive interrupt throttling

[Aleksandr Loktionov <aleksandr.loktionov@intel.com>]

ixgbe_update_itr() packs a mode flag (IXGBE_ITR_ADAPTIVE_LATENCY,
bit 7) and a usecs delay (bits [6:0]) into an unsigned int, then
stores the combined value in ring_container->itr which is declared as
u8.  Values above 0xFF wrap on truncation, corrupting both the delay
and the mode flag on the next readback.

Keep the mode bit (IXGBE_ITR_ADAPTIVE_LATENCY) and the usec delay as
separate operands in the final store expression.  Clamp only the usecs
portion to [IXGBE_ITR_ADAPTIVE_MIN_USECS, IXGBE_ITR_ADAPTIVE_MAX_USECS]
using clamp_val() so that:
 - overflow cannot bleed into the mode bit (bit 7),
 - the delay cannot exceed 126 us (IXGBE_ITR_ADAPTIVE_MAX_USECS),
 - the delay cannot drop below 10 us (IXGBE_ITR_ADAPTIVE_MIN_USECS).

Fixes: b4ded8327fea ("ixgbe: Update adaptive ITR algorithm")
Cc: stable@vger.kernel.org
Signed-off-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
---
v2 -> v3:
 - Use clamp_val() instead of min_t() to also guard the lower bound
   (IXGBE_ITR_ADAPTIVE_MIN_USECS); keep mode and delay as separate
   operands until final store; use IXGBE_ITR_ADAPTIVE_MAX_USECS (126)
   as upper bound instead of IXGBE_ITR_ADAPTIVE_LATENCY - 1 (127)
   (Simon Horman).

v1 -> v2:
 - Add proper [N/M] numbering so patchwork tracks it as part of the set;
   no code change.

 drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 10 +++++++---
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
index 210c7b9..9f3ae21 100644
--- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
+++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
@@ -2886,11 +2886,17 @@ static void ixgbe_update_itr(struct ixgbe_q_vector *q_vector,
 				    IXGBE_ITR_ADAPTIVE_MIN_INC * 64) *
 		       IXGBE_ITR_ADAPTIVE_MIN_INC;
 		break;
 	}
 
 clear_counts:
-	/* write back value */
-	ring_container->itr = itr;
+	/* Separate mode bit (IXGBE_ITR_ADAPTIVE_LATENCY) from usec delay;
+	 * clamp delay to [MIN_USECS, MAX_USECS] before storing to prevent
+	 * u8 truncation from corrupting the mode flag or delay on readback.
+	 */
+	ring_container->itr = (itr & IXGBE_ITR_ADAPTIVE_LATENCY) |
+		clamp_val(itr & ~IXGBE_ITR_ADAPTIVE_LATENCY,
+			  IXGBE_ITR_ADAPTIVE_MIN_USECS,
+			  IXGBE_ITR_ADAPTIVE_MAX_USECS);
 
 	/* next update should occur within next jiffy */
 	ring_container->next_update = next_update + 1;
-- 
2.52.0