[PATCH net v2 5/5] ionic: fix completion descriptor access with 2x desc size

[Eric Joyner <eric.joyner@amd.com>]

From: Prabu Thayalan <prabu.ponrajthayalan@amd.com>

The old ionic_rx_service() and ionic_tx_service() used array
indexing to access completion descriptors:

    comp = &((struct ionic_rxq_comp *)cq->base)[cq->tail_idx];

This assumes the stride is sizeof(struct ionic_rxq_comp) = 16 bytes.
However, when the IONIC_Q_F_2X_CQ_DESC flag is set, the actual
completion descriptor size is 32 bytes (2 * sizeof(comp)), and the
completion itself is located at the end of that 32-byte slot. Array
indexing with a 16-byte stride would access the wrong offset.

Use pointer arithmetic that accounts for the actual descriptor size
from cq->desc_size:

    comp = cq->base +
           cq->desc_size * cq->tail_idx +
           cq->desc_size - sizeof(*comp);

This correctly calculates the completion location regardless of
descriptor size. For the common case where desc_size equals
sizeof(*comp), use array indexing in a likely() fast path to avoid
performance regression.

Fixes: 0ec9f6669a7d ("ionic: add handling of larger descriptors")
Signed-off-by: Prabu Thayalan <prabu.ponrajthayalan@amd.com>
Signed-off-by: Eric Joyner <eric.joyner@amd.com>
---
 .../net/ethernet/pensando/ionic/ionic_txrx.c  | 27 ++++++++++---------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
index 301ebee2fdc5..27a113d63d28 100644
--- a/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
+++ b/drivers/net/ethernet/pensando/ionic/ionic_txrx.c
@@ -701,11 +701,7 @@ static void ionic_rx_clean(struct ionic_queue *q,
 		__le64 *cq_desc_hwstamp;
 		u64 hwstamp;
 
-		cq_desc_hwstamp =
-			(void *)comp +
-			qcq->cq.desc_size -
-			sizeof(struct ionic_rxq_comp) -
-			IONIC_HWSTAMP_CQ_NEGOFFSET;
+		cq_desc_hwstamp = (void *)comp - IONIC_HWSTAMP_CQ_NEGOFFSET;
 
 		hwstamp = le64_to_cpu(*cq_desc_hwstamp);
 
@@ -729,7 +725,12 @@ static bool __ionic_rx_service(struct ionic_cq *cq, struct bpf_prog *xdp_prog)
 	struct ionic_queue *q = cq->bound_q;
 	struct ionic_rxq_comp *comp;
 
-	comp = &((struct ionic_rxq_comp *)cq->base)[cq->tail_idx];
+	if (likely(cq->desc_size == sizeof(*comp)))
+		comp = &((struct ionic_rxq_comp *)cq->base)[cq->tail_idx];
+	else
+		comp = cq->base +
+		       cq->desc_size * cq->tail_idx +
+		       cq->desc_size - sizeof(*comp);
 
 	if (!color_match(comp->pkt_type_color, cq->done_color))
 		return false;
@@ -1180,7 +1181,6 @@ static void ionic_tx_clean(struct ionic_queue *q,
 			   bool in_napi)
 {
 	struct ionic_tx_stats *stats = q_to_tx_stats(q);
-	struct ionic_qcq *qcq = q_to_qcq(q);
 	struct sk_buff *skb;
 
 	if (desc_info->xdpf) {
@@ -1205,11 +1205,7 @@ static void ionic_tx_clean(struct ionic_queue *q,
 			__le64 *cq_desc_hwstamp;
 			u64 hwstamp;
 
-			cq_desc_hwstamp =
-				(void *)comp +
-				qcq->cq.desc_size -
-				sizeof(struct ionic_txq_comp) -
-				IONIC_HWSTAMP_CQ_NEGOFFSET;
+			cq_desc_hwstamp = (void *)comp - IONIC_HWSTAMP_CQ_NEGOFFSET;
 
 			hwstamp = le64_to_cpu(*cq_desc_hwstamp);
 
@@ -1244,7 +1240,12 @@ static bool ionic_tx_service(struct ionic_cq *cq,
 	unsigned int pkts = 0;
 	u16 index;
 
-	comp = &((struct ionic_txq_comp *)cq->base)[cq->tail_idx];
+	if (likely(cq->desc_size == sizeof(*comp)))
+		comp = &((struct ionic_txq_comp *)cq->base)[cq->tail_idx];
+	else
+		comp = cq->base +
+		       cq->desc_size * cq->tail_idx +
+		       cq->desc_size - sizeof(*comp);
 
 	if (!color_match(comp->color, cq->done_color))
 		return false;
-- 
2.17.1