NFC: digital: Fix ACK & NACK PDUs handling in target mode

When the target receives a NACK PDU, it re-sends the last sent PDU.

ACK PDUs are received by the target as a reply from the initiator to
chained I-PDUs. There are 3 cases to handle:
- If the target has previously received 1 or more ATN PDUs and the PNI
  in the ACK PDU is equal to the target PNI - 1, then it means that the
  initiator did not received the last issued PDU from the target. In
  this case it re-sends this PDU.
- If the target has received 1 or more ATN PDUs but the ACK PNI is not
  the target PNI - 1, then this means that this ACK is the reply of the
  previous chained I-PDU sent by the target. The target did not received
  it on the first attempt and it is being re-sent by the initiator. The
  process continues as usual.
- No ATN PDU received before this ACK PDU. This is the reply of a
  chained I-PDU. The target keeps on processing its chained I-PDU.

The code has been refactored to avoid too many indentation levels.

Also, ACK and NACK PDUs were not freed. This is now fixed.
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

net/nfc/digital_dep.c

@@ -1141,49 +1141,64 @@ static void digital_tg_recv_dep_req(struct nfc_digital_dev *ddev, void *arg,
 		rc = 0;
 		break;
 	case DIGITAL_NFC_DEP_PFB_ACK_NACK_PDU:
-		if (!DIGITAL_NFC_DEP_NACK_BIT_SET(pfb)) { /* ACK */
-			if ((ddev->atn_count &&
-			     (DIGITAL_NFC_DEP_PFB_PNI(pfb - 1) !=
-						ddev->curr_nfc_dep_pni)) ||
-			    (DIGITAL_NFC_DEP_PFB_PNI(pfb) !=
-						ddev->curr_nfc_dep_pni) ||
-			    !ddev->chaining_skb || !ddev->saved_skb) {
+		if (DIGITAL_NFC_DEP_NACK_BIT_SET(pfb)) { /* NACK */
+			if (DIGITAL_NFC_DEP_PFB_PNI(pfb + 1) !=
+						ddev->curr_nfc_dep_pni) {
 				rc = -EIO;
 				goto exit;
 			}
 
-			if (ddev->atn_count) {
-				ddev->atn_count = 0;
+			ddev->atn_count = 0;
+
+			rc = digital_tg_send_saved_skb(ddev);
+			if (rc)
+				goto exit;
+
+			goto free_resp;
+		}
+
+		/* ACK */
+		if (ddev->atn_count) {
+			/* The target has previously recevied one or more ATN
+			 * PDUs.
+			 */
+			ddev->atn_count = 0;
 
+			/* If the ACK PNI is equal to the target PNI - 1 means
+			 * that the initiator did not receive the previous PDU
+			 * sent by the target so re-send it.
+			 */
+			if (DIGITAL_NFC_DEP_PFB_PNI(pfb + 1) ==
+						ddev->curr_nfc_dep_pni) {
 				rc = digital_tg_send_saved_skb(ddev);
 				if (rc)
 					goto exit;
 
-				return;
+				goto free_resp;
 			}
 
-			kfree_skb(ddev->saved_skb);
-			ddev->saved_skb = NULL;
+			/* Otherwise, the target did not receive the previous
+			 * ACK PDU from the initiator. Fallback to normal
+			 * processing of chained PDU then.
+			 */
+		}
 
-			rc = digital_tg_send_dep_res(ddev, ddev->chaining_skb);
-			if (rc)
-				goto exit;
-		} else { /* NACK */
-			if ((DIGITAL_NFC_DEP_PFB_PNI(pfb + 1) !=
-						ddev->curr_nfc_dep_pni) ||
-			    !ddev->saved_skb) {
-				rc = -EIO;
-				goto exit;
-			}
+		/* Keep on sending chained PDU */
+		if (!ddev->chaining_skb ||
+		    DIGITAL_NFC_DEP_PFB_PNI(pfb) !=
+					ddev->curr_nfc_dep_pni) {
+			rc = -EIO;
+			goto exit;
+		}
 
-			ddev->atn_count = 0;
+		kfree_skb(ddev->saved_skb);
+		ddev->saved_skb = NULL;
 
-			rc = digital_tg_send_saved_skb(ddev);
-			if (rc)
-				goto exit;
-		}
+		rc = digital_tg_send_dep_res(ddev, ddev->chaining_skb);
+		if (rc)
+			goto exit;
 
-		return;
+		goto free_resp;
 	case DIGITAL_NFC_DEP_PFB_SUPERVISOR_PDU:
 		if (DIGITAL_NFC_DEP_PFB_IS_TIMEOUT(pfb)) {
 			rc = -EINVAL;