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

When an NFC-DEP Initiator receives a frame with
an incorrect CRC or with a parity error, and the
frame is at least 4 bytes long, its supposed to
send a NACK to the Target.  The Initiator can
send up to 'N(retry,nack)' consecutive NACKs
where 2 <= 'N(retry,nack)' <= 5.  When the limit
is exceeded, a PROTOCOL EXCEPTION is raised.
Any other type of transmission error is to be
ignored and the Initiator should continue
waiting for a new frame.  This is described
in section 14.12.5.4 of the NFC Digital Protocol
Spec.

The digital layer's NFC-DEP code doesn't implement
any of this so add it.  This support diverges from
the spec in two significant ways:

a) NACKs will be sent for ANY error reported by the
   driver except a timeout.  This is done because
   there is currently no way for the digital layer
   to distinguish a CRC or parity error from any
   other type of error reported by the driver.

b) All other errors will cause a PROTOCOL EXCEPTION
   even frames with CRC errors that are less than 4
   bytes.

The value chosen for 'N(retry,nack)' is 2.

Targets do not send NACK PDUs.

Reviewed-by: Thierry Escande <thierry.escande@linux.intel.com>
Tested-by: Thierry Escande <thierry.escande@linux.intel.com>
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

include/net/nfc/digital.h

@@ -233,6 +233,8 @@ struct nfc_digital_dev {
 	struct sk_buff *chaining_skb;
 	struct digital_data_exch *data_exch;
 
+	int nack_count;
+
 	u16 target_fsc;
 
 	int (*skb_check_crc)(struct sk_buff *skb);

net/nfc/digital_dep.c

@@ -17,6 +17,8 @@
 
 #include "digital.h"
 
+#define DIGITAL_NFC_DEP_N_RETRY_NACK	2
+
 #define DIGITAL_NFC_DEP_FRAME_DIR_OUT 0xD4
 #define DIGITAL_NFC_DEP_FRAME_DIR_IN  0xD5
 
@@ -529,6 +531,38 @@ static int digital_in_send_ack(struct nfc_digital_dev *ddev,
 	return rc;
 }
 
+static int digital_in_send_nack(struct nfc_digital_dev *ddev,
+				struct digital_data_exch *data_exch)
+{
+	struct digital_dep_req_res *dep_req;
+	struct sk_buff *skb;
+	int rc;
+
+	skb = digital_skb_alloc(ddev, 1);
+	if (!skb)
+		return -ENOMEM;
+
+	skb_push(skb, sizeof(struct digital_dep_req_res));
+
+	dep_req = (struct digital_dep_req_res *)skb->data;
+
+	dep_req->dir = DIGITAL_NFC_DEP_FRAME_DIR_OUT;
+	dep_req->cmd = DIGITAL_CMD_DEP_REQ;
+	dep_req->pfb = DIGITAL_NFC_DEP_PFB_ACK_NACK_PDU |
+		       DIGITAL_NFC_DEP_PFB_NACK_BIT | ddev->curr_nfc_dep_pni;
+
+	digital_skb_push_dep_sod(ddev, skb);
+
+	ddev->skb_add_crc(skb);
+
+	rc = digital_in_send_cmd(ddev, skb, 1500, digital_in_recv_dep_res,
+				 data_exch);
+	if (rc)
+		kfree_skb(skb);
+
+	return rc;
+}
+
 static int digital_in_send_rtox(struct nfc_digital_dev *ddev,
 				struct digital_data_exch *data_exch, u8 rtox)
 {
@@ -575,13 +609,17 @@ static void digital_in_recv_dep_res(struct nfc_digital_dev *ddev, void *arg,
 	if (IS_ERR(resp)) {
 		rc = PTR_ERR(resp);
 		resp = NULL;
-		goto exit;
-	}
 
-	rc = ddev->skb_check_crc(resp);
+		if ((rc != -ETIMEDOUT) &&
-	if (rc) {
+		    (ddev->nack_count++ < DIGITAL_NFC_DEP_N_RETRY_NACK)) {
-		PROTOCOL_ERR("14.4.1.6");
+			rc = digital_in_send_nack(ddev, data_exch);
-		goto error;
+			if (rc)
+				goto error;
+
+			return;
+		}
+
+		goto exit;
 	}
 
 	rc = digital_skb_pull_dep_sod(ddev, resp);
@@ -590,6 +628,25 @@ static void digital_in_recv_dep_res(struct nfc_digital_dev *ddev, void *arg,
 		goto exit;
 	}
 
+	rc = ddev->skb_check_crc(resp);
+	if (rc) {
+		if ((resp->len >= 4) &&
+		    (ddev->nack_count++ < DIGITAL_NFC_DEP_N_RETRY_NACK)) {
+			rc = digital_in_send_nack(ddev, data_exch);
+			if (rc)
+				goto error;
+
+			kfree_skb(resp);
+
+			return;
+		}
+
+		PROTOCOL_ERR("14.4.1.6");
+		goto error;
+	}
+
+	ddev->nack_count = 0;
+
 	if (resp->len > ddev->local_payload_max) {
 		rc = -EMSGSIZE;
 		goto exit;
@@ -721,6 +778,8 @@ int digital_in_send_dep_req(struct nfc_digital_dev *ddev,
 	dep_req->cmd = DIGITAL_CMD_DEP_REQ;
 	dep_req->pfb = ddev->curr_nfc_dep_pni;
 
+	ddev->nack_count = 0;
+
 	chaining_skb = ddev->chaining_skb;
 
 	tmp_skb = digital_send_dep_data_prep(ddev, skb, dep_req, data_exch);