NFC: digital: Add NFC-DEP Target-side NACK Support

When an NFC-DEP Target receives a NACK PDU with
a PNI equal to 1 less than the current PNI, it
is supposed to re-send the last PDU.  This is
implied in section 14.12.5.4 of the NFC Digital
Protocol Spec.

The digital layer's NFC-DEP code doesn't implement
Target-side NACK handing so add it.  The last PDU
that was sent is saved in the 'nfc_digital_dev'
structure's 'saved_skb' member.  The skb will have
an additional reference taken to ensure that the skb
isn't freed when the driver performs a kfree_skb()
on the skb.  The length of the skb/PDU is also saved
so the length can be restored when re-sending the PDU
in the skb (the driver will perform an skb_pull() so
an skb_push() needs to be done to restore the skb's
data pointer/length).
Reviewed-by: NThierry Escande <thierry.escande@linux.intel.com>
Tested-by: NThierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: NMark A. Greer <mgreer@animalcreek.com>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>

include/net/nfc/digital.h

@@ -235,6 +235,9 @@ struct nfc_digital_dev {
 
 	int nack_count;
 
+	struct sk_buff *saved_skb;
+	unsigned int saved_skb_len;
+
 	u16 target_fsc;
 
 	int (*skb_check_crc)(struct sk_buff *skb);

net/nfc/digital_dep.c

@@ -864,14 +864,29 @@ static int digital_tg_send_ack(struct nfc_digital_dev *ddev,
 
 	ddev->skb_add_crc(skb);
 
+	ddev->saved_skb = skb_get(skb);
+	ddev->saved_skb_len = skb->len;
+
 	rc = digital_tg_send_cmd(ddev, skb, 1500, digital_tg_recv_dep_req,
 				 data_exch);
-	if (rc)
+	if (rc) {
 		kfree_skb(skb);
+		kfree_skb(ddev->saved_skb);
+		ddev->saved_skb = NULL;
+	}
 
 	return rc;
 }
 
+static int digital_tg_send_saved_skb(struct nfc_digital_dev *ddev)
+{
+	skb_get(ddev->saved_skb);
+	skb_push(ddev->saved_skb, ddev->saved_skb_len);
+
+	return digital_tg_send_cmd(ddev, ddev->saved_skb, 1500,
+				   digital_tg_recv_dep_req, NULL);
+}
+
 static void digital_tg_recv_dep_req(struct nfc_digital_dev *ddev, void *arg,
 				    struct sk_buff *resp)
 {
@@ -948,6 +963,9 @@ static void digital_tg_recv_dep_req(struct nfc_digital_dev *ddev, void *arg,
 			goto exit;
 		}
 
+		kfree_skb(ddev->saved_skb);
+		ddev->saved_skb = NULL;
+
 		resp = digital_recv_dep_data_gather(ddev, pfb, resp,
 						    digital_tg_send_ack, NULL);
 		if (IS_ERR(resp)) {
@@ -966,23 +984,36 @@ 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_PFB_PNI(pfb) != ddev->curr_nfc_dep_pni) {
-			PROTOCOL_ERR("14.12.3.4");
-			rc = -EIO;
-			goto exit;
-		}
+		if (!DIGITAL_NFC_DEP_NACK_BIT_SET(pfb)) { /* ACK */
+			if ((DIGITAL_NFC_DEP_PFB_PNI(pfb) !=
+						ddev->curr_nfc_dep_pni) ||
+			    !ddev->chaining_skb || !ddev->saved_skb) {
+				rc = -EIO;
+				goto exit;
+			}
+
+			kfree_skb(ddev->saved_skb);
+			ddev->saved_skb = NULL;
 
-		if (ddev->chaining_skb && !DIGITAL_NFC_DEP_NACK_BIT_SET(pfb)) {
 			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;
+			}
 
-			return;
+			rc = digital_tg_send_saved_skb(ddev);
+			if (rc) {
+				kfree_skb(ddev->saved_skb);
+				goto exit;
+			}
 		}
 
-		pr_err("Received a ACK/NACK PDU\n");
-		rc = -EINVAL;
-		goto exit;
+		return;
 	case DIGITAL_NFC_DEP_PFB_SUPERVISOR_PDU:
 		pr_err("Received a SUPERVISOR PDU\n");
 		rc = -EINVAL;
@@ -995,6 +1026,9 @@ static void digital_tg_recv_dep_req(struct nfc_digital_dev *ddev, void *arg,
 	kfree_skb(ddev->chaining_skb);
 	ddev->chaining_skb = NULL;
 
+	kfree_skb(ddev->saved_skb);
+	ddev->saved_skb = NULL;
+
 	if (rc)
 		kfree_skb(resp);
 }
@@ -1033,6 +1067,9 @@ int digital_tg_send_dep_res(struct nfc_digital_dev *ddev, struct sk_buff *skb)
 
 	ddev->skb_add_crc(tmp_skb);
 
+	ddev->saved_skb = skb_get(tmp_skb);
+	ddev->saved_skb_len = tmp_skb->len;
+
 	rc = digital_tg_send_cmd(ddev, tmp_skb, 1500, digital_tg_recv_dep_req,
 				 NULL);
 	if (rc) {
@@ -1041,6 +1078,9 @@ int digital_tg_send_dep_res(struct nfc_digital_dev *ddev, struct sk_buff *skb)
 
 		kfree_skb(chaining_skb);
 		ddev->chaining_skb = NULL;
+
+		kfree_skb(ddev->saved_skb);
+		ddev->saved_skb = NULL;
 	}
 
 	return rc;