[PATCH v2 net] net: mana: Fix doorbell out of order violation and avoid unnecessary doorbell rings

[longli@linuxonhyperv.com]

From: Long Li <longli@microsoft.com>

After napi_complete_done() is called when NAPI is polling in the current
process context, another NAPI may be scheduled and start running in
softirq on another CPU and may ring the doorbell before the current CPU
does. When combined with unnecessary rings when there is no need to arm
the CQ, it triggers error paths in the hardware.

This patch fixes this by calling napi_complete_done() after doorbell
rings. It limits the number of unnecessary rings when there is
no need to arm. MANA hardware specifies that there must be one doorbell
ring every 8 CQ wraparounds. This driver guarantees one doorbell ring as
soon as the number of consumed CQEs exceeds 4 CQ wraparounds. In pratical
workloads, the 4 CQ wraparounds proves to be big enough that it rarely
exceeds this limit before all the napi weight is consumed.

To implement this, add a per-CQ counter cq->work_done_since_doorbell,
and make sure the CQ is armed as soon as passing 4 wraparounds of the CQ.

Cc: stable@vger.kernel.org
Fixes: e1b5683ff62e ("net: mana: Move NAPI from EQ to CQ")

Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Long Li <longli@microsoft.com>
---

change in v2:
Added more details to comments to explain the patch

 drivers/net/ethernet/microsoft/mana/mana_en.c | 24 ++++++++++++-------
 include/net/mana/mana.h                       |  1 +
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/drivers/net/ethernet/microsoft/mana/mana_en.c b/drivers/net/ethernet/microsoft/mana/mana_en.c
index d2f07e179e86..f83211f9e737 100644
--- a/drivers/net/ethernet/microsoft/mana/mana_en.c
+++ b/drivers/net/ethernet/microsoft/mana/mana_en.c
@@ -1788,7 +1788,6 @@ static void mana_poll_rx_cq(struct mana_cq *cq)
 static int mana_cq_handler(void *context, struct gdma_queue *gdma_queue)
 {
 	struct mana_cq *cq = context;
-	u8 arm_bit;
 	int w;
 
 	WARN_ON_ONCE(cq->gdma_cq != gdma_queue);
@@ -1799,16 +1798,23 @@ static int mana_cq_handler(void *context, struct gdma_queue *gdma_queue)
 		mana_poll_tx_cq(cq);
 
 	w = cq->work_done;
-
-	if (w < cq->budget &&
-	    napi_complete_done(&cq->napi, w)) {
-		arm_bit = SET_ARM_BIT;
-	} else {
-		arm_bit = 0;
+	cq->work_done_since_doorbell += w;
+
+	if (w < cq->budget) {
+		mana_gd_ring_cq(gdma_queue, SET_ARM_BIT);
+		cq->work_done_since_doorbell = 0;
+		napi_complete_done(&cq->napi, w);
+	} else if (cq->work_done_since_doorbell >
+		   cq->gdma_cq->queue_size / COMP_ENTRY_SIZE * 4) {
+		/* MANA hardware requires at least one doorbell ring every 8
+		 * wraparounds of CQ even if there is no need to arm the CQ.
+		 * This driver rings the doorbell as soon as we have exceeded
+		 * 4 wraparounds.
+		 */
+		mana_gd_ring_cq(gdma_queue, 0);
+		cq->work_done_since_doorbell = 0;
 	}
 
-	mana_gd_ring_cq(gdma_queue, arm_bit);
-
 	return w;
 }
 
diff --git a/include/net/mana/mana.h b/include/net/mana/mana.h
index 6439fd8b437b..7caa334f4888 100644
--- a/include/net/mana/mana.h
+++ b/include/net/mana/mana.h
@@ -275,6 +275,7 @@ struct mana_cq {
 	/* NAPI data */
 	struct napi_struct napi;
 	int work_done;
+	int work_done_since_doorbell;
 	int budget;
 };
 
-- 
2.17.1