NFC: trf7970a: Prefix TX data when refilling FIFO

When refilling the FIFO with more TX data (using a new
SPI transaction), the driver must prefix the TX data with
a write to the FIFO I/O Register.  This tells the trf7970a
that the following data is destined for the FIFO so it can
be transmitted.

To accomplish this, the driver cannot simply push the
prefix data just before the next set of TX data that
is to be transmitted because that will overwrite part
of the TX data provided by the digital layer.  Instead,
separate the prefix data and the TX data when calling
trf7970a_transmit().  trf7970a_transmit() can then send
the prefix and TX data from different memory locations
with one spi_sync() operation.  This also means that
the driver doesn't require any skb "tx_headroom" as
provided by the digital layer (see
nfc_digital_allocate_device() and digital_skb_alloc()).

Also ensure that the prefix is of type 'u8' and not
'char'.
Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/nfc/trf7970a.c

@@ -125,13 +125,6 @@
 
 #define TRF7970A_AUTOSUSPEND_DELAY		30000 /* 30 seconds */
 
-/* TX data must be prefixed with a FIFO reset cmd, a cmd that depends
- * on what the current framing is, the address of the TX length byte 1
- * register (0x1d), and the 2 byte length of the data to be transmitted.
- * That totals 5 bytes.
- */
-#define TRF7970A_TX_SKB_HEADROOM		5
-
 #define TRF7970A_RX_SKB_ALLOC_SIZE		256
 
 #define TRF7970A_FIFO_SIZE			127
@@ -515,15 +508,29 @@ static void trf7970a_send_err_upstream(struct trf7970a *trf, int errno)
 }
 
 static int trf7970a_transmit(struct trf7970a *trf, struct sk_buff *skb,
-		unsigned int len)
+		unsigned int len, u8 *prefix, unsigned int prefix_len)
 {
+	struct spi_transfer t[2];
+	struct spi_message m;
 	unsigned int timeout;
 	int ret;
 
 	print_hex_dump_debug("trf7970a tx data: ", DUMP_PREFIX_NONE,
 			16, 1, skb->data, len, false);
 
-	ret = spi_write(trf->spi, skb->data, len);
+	spi_message_init(&m);
+
+	memset(&t, 0, sizeof(t));
+
+	t[0].tx_buf = prefix;
+	t[0].len = prefix_len;
+	spi_message_add_tail(&t[0], &m);
+
+	t[1].tx_buf = skb->data;
+	t[1].len = len;
+	spi_message_add_tail(&t[1], &m);
+
+	ret = spi_sync(trf->spi, &m);
 	if (ret) {
 		dev_err(trf->dev, "%s - Can't send tx data: %d\n", __func__,
 				ret);
@@ -559,6 +566,7 @@ static void trf7970a_fill_fifo(struct trf7970a *trf)
 	unsigned int len;
 	int ret;
 	u8 fifo_bytes;
+	u8 prefix;
 
 	ret = trf7970a_read(trf, TRF7970A_FIFO_STATUS, &fifo_bytes);
 	if (ret) {
@@ -574,7 +582,9 @@ static void trf7970a_fill_fifo(struct trf7970a *trf)
 	len = TRF7970A_FIFO_SIZE - fifo_bytes;
 	len = min(skb->len, len);
 
-	ret = trf7970a_transmit(trf, skb, len);
+	prefix = TRF7970A_CMD_BIT_CONTINUOUS | TRF7970A_FIFO_IO_REGISTER;
+
+	ret = trf7970a_transmit(trf, skb, len, &prefix, sizeof(prefix));
 	if (ret)
 		trf7970a_send_err_upstream(trf, ret);
 }
@@ -1108,7 +1118,7 @@ static int trf7970a_in_send_cmd(struct nfc_digital_dev *ddev,
 		nfc_digital_cmd_complete_t cb, void *arg)
 {
 	struct trf7970a *trf = nfc_digital_get_drvdata(ddev);
-	char *prefix;
+	u8 prefix[5];
 	unsigned int len;
 	int ret;
 	u8 status;
@@ -1164,11 +1174,11 @@ static int trf7970a_in_send_cmd(struct nfc_digital_dev *ddev,
 	trf->ignore_timeout = false;
 
 	len = skb->len;
-	prefix = skb_push(skb, TRF7970A_TX_SKB_HEADROOM);
 
 	/* TX data must be prefixed with a FIFO reset cmd, a cmd that depends
 	 * on what the current framing is, the address of the TX length byte 1
 	 * register (0x1d), and the 2 byte length of the data to be transmitted.
+	 * That totals 5 bytes.
 	 */
 	prefix[0] = TRF7970A_CMD_BIT_CTRL |
 			TRF7970A_CMD_BIT_OPCODE(TRF7970A_CMD_FIFO_RESET);
@@ -1192,7 +1202,7 @@ static int trf7970a_in_send_cmd(struct nfc_digital_dev *ddev,
 	if (ret)
 		goto out_err;
 
-	ret = trf7970a_transmit(trf, skb, len);
+	ret = trf7970a_transmit(trf, skb, len, prefix, sizeof(prefix));
 	if (ret) {
 		kfree_skb(trf->rx_skb);
 		trf->rx_skb = NULL;
@@ -1377,8 +1387,7 @@ static int trf7970a_probe(struct spi_device *spi)
 
 	trf->ddev = nfc_digital_allocate_device(&trf7970a_nfc_ops,
 			TRF7970A_SUPPORTED_PROTOCOLS,
-			NFC_DIGITAL_DRV_CAPS_IN_CRC, TRF7970A_TX_SKB_HEADROOM,
-			0);
+			NFC_DIGITAL_DRV_CAPS_IN_CRC, 0, 0);
 	if (!trf->ddev) {
 		dev_err(trf->dev, "Can't allocate NFC digital device\n");
 		ret = -ENOMEM;