ice: fix software generating extra interrupts

The driver tried to work around missing completion events that occurred
while interrupts are disabled, by triggering a software interrupt
whenever we exit polling (but we had to have polled at least once).

This was causing a *lot* of extra interrupts for some workloads like
NVMe over TCP, which resulted in regressions in performance. It was also
visible when polling didn't prevent interrupts when busy_poll was
enabled.

Fix the extra interrupts by utilizing our previously unused 3rd ITR
(interrupt throttle) index and set it to 20K interrupts per second, and
then trigger a software interrupt within that rate limit.

While here, slightly refactor the code to avoid an overwrite of a local
variable in the case of wb_en = true.

Fixes: b7306b42 ("ice: manage interrupts during poll exit")
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/ice/ice_hw_autogen.h

@@ -182,6 +182,7 @@
 #define GLINT_DYN_CTL_INTERVAL_S		5
 #define GLINT_DYN_CTL_INTERVAL_M		ICE_M(0xFFF, 5)
 #define GLINT_DYN_CTL_SW_ITR_INDX_ENA_M		BIT(24)
+#define GLINT_DYN_CTL_SW_ITR_INDX_S		25
 #define GLINT_DYN_CTL_SW_ITR_INDX_M		ICE_M(0x3, 25)
 #define GLINT_DYN_CTL_WB_ON_ITR_M		BIT(30)
 #define GLINT_DYN_CTL_INTENA_MSK_M		BIT(31)

drivers/net/ethernet/intel/ice/ice_txrx.c

@@ -1361,21 +1361,25 @@ static void ice_enable_interrupt(struct ice_q_vector *q_vector)
 	if (test_bit(ICE_DOWN, vsi->state))
 		return;
 
-	/* When exiting WB_ON_ITR, let ITR resume its normal
-	 * interrupts-enabled path.
+	/* trigger an ITR delayed software interrupt when exiting busy poll, to
+	 * make sure to catch any pending cleanups that might have been missed
+	 * due to interrupt state transition. If busy poll or poll isn't
+	 * enabled, then don't update ITR, and just enable the interrupt.
 	 */
-	if (wb_en)
+	if (!wb_en) {
+		itr_val = ice_buildreg_itr(ICE_ITR_NONE, 0);
+	} else {
 		q_vector->wb_on_itr = false;
 
-	itr_val = ice_buildreg_itr(ICE_ITR_NONE, 0);
-	/* trigger an immediate software interrupt when exiting
-	 * busy poll, to make sure to catch any pending cleanups
-	 * that might have been missed due to interrupt state
-	 * transition.
-	 */
-	if (wb_en) {
+		/* do two things here with a single write. Set up the third ITR
+		 * index to be used for software interrupt moderation, and then
+		 * trigger a software interrupt with a rate limit of 20K on
+		 * software interrupts, this will help avoid high interrupt
+		 * loads due to frequently polling and exiting polling.
+		 */
+		itr_val = ice_buildreg_itr(ICE_IDX_ITR2, ICE_ITR_20K);
 		itr_val |= GLINT_DYN_CTL_SWINT_TRIG_M |
-			   GLINT_DYN_CTL_SW_ITR_INDX_M |
+			   ICE_IDX_ITR2 << GLINT_DYN_CTL_SW_ITR_INDX_S |
 			   GLINT_DYN_CTL_SW_ITR_INDX_ENA_M;
 	}
 	wr32(&vsi->back->hw, GLINT_DYN_CTL(q_vector->reg_idx), itr_val);