spi/bcm63xx: work around inability to keep CS up

This SPI controller does not support keeping CS asserted after sending
a transfer.
Since messages expected on this SPI controller are rather short, we can
work around it for normal use cases by sending all transfers at once in
a big full duplex stream.

This means that we cannot change the speed between transfers if they
require CS to be kept asserted, but these would have been rejected
before anyway because of the inability of keeping CS asserted.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi-bcm63xx.c

@@ -37,6 +37,8 @@
 
 #define PFX		KBUILD_MODNAME
 
+#define BCM63XX_SPI_MAX_PREPEND		15
+
 struct bcm63xx_spi {
 	struct completion	done;
 
@@ -169,33 +171,63 @@ static int bcm63xx_spi_setup(struct spi_device *spi)
 	return 0;
 }
 
-static int bcm63xx_txrx_bufs(struct spi_device *spi, struct spi_transfer *t)
+static int bcm63xx_txrx_bufs(struct spi_device *spi, struct spi_transfer *first,
+				unsigned int num_transfers)
 {
 	struct bcm63xx_spi *bs = spi_master_get_devdata(spi->master);
 	u16 msg_ctl;
 	u16 cmd;
 	u8 rx_tail;
-	unsigned int timeout = 0;
+	unsigned int i, timeout = 0, prepend_len = 0, len = 0;
+	struct spi_transfer *t = first;
+	bool do_rx = false;
+	bool do_tx = false;
 
 	/* Disable the CMD_DONE interrupt */
 	bcm_spi_writeb(bs, 0, SPI_INT_MASK);
 
 	dev_dbg(&spi->dev, "txrx: tx %p, rx %p, len %d\n",
 		t->tx_buf, t->rx_buf, t->len);
 
-	if (t->tx_buf)
-		memcpy_toio(bs->tx_io, t->tx_buf, t->len);
+	if (num_transfers > 1 && t->tx_buf && t->len <= BCM63XX_SPI_MAX_PREPEND)
+		prepend_len = t->len;
+
+	/* prepare the buffer */
+	for (i = 0; i < num_transfers; i++) {
+		if (t->tx_buf) {
+			do_tx = true;
+			memcpy_toio(bs->tx_io + len, t->tx_buf, t->len);
+
+			/* don't prepend more than one tx */
+			if (t != first)
+				prepend_len = 0;
+		}
+
+		if (t->rx_buf) {
+			do_rx = true;
+			/* prepend is half-duplex write only */
+			if (t == first)
+				prepend_len = 0;
+		}
+
+		len += t->len;
+
+		t = list_entry(t->transfer_list.next, struct spi_transfer,
+			       transfer_list);
+	}
+
+	len -= prepend_len;
 
 	init_completion(&bs->done);
 
 	/* Fill in the Message control register */
-	msg_ctl = (t->len << SPI_BYTE_CNT_SHIFT);
+	msg_ctl = (len << SPI_BYTE_CNT_SHIFT);
 
-	if (t->rx_buf && t->tx_buf)
+	if (do_rx && do_tx && prepend_len == 0)
 		msg_ctl |= (SPI_FD_RW << bs->msg_type_shift);
-	else if (t->rx_buf)
+	else if (do_rx)
 		msg_ctl |= (SPI_HD_R << bs->msg_type_shift);
-	else if (t->tx_buf)
+	else if (do_tx)
 		msg_ctl |= (SPI_HD_W << bs->msg_type_shift);
 
 	switch (bs->msg_ctl_width) {
@@ -209,7 +241,7 @@ static int bcm63xx_txrx_bufs(struct spi_device *spi, struct spi_transfer *t)
 
 	/* Issue the transfer */
 	cmd = SPI_CMD_START_IMMEDIATE;
-	cmd |= (0 << SPI_CMD_PREPEND_BYTE_CNT_SHIFT);
+	cmd |= (prepend_len << SPI_CMD_PREPEND_BYTE_CNT_SHIFT);
 	cmd |= (spi->chip_select << SPI_CMD_DEVICE_ID_SHIFT);
 	bcm_spi_writew(bs, cmd, SPI_CMD);
 
@@ -223,9 +255,25 @@ static int bcm63xx_txrx_bufs(struct spi_device *spi, struct spi_transfer *t)
 	/* read out all data */
 	rx_tail = bcm_spi_readb(bs, SPI_RX_TAIL);
 
+	if (do_rx && rx_tail != len)
+		return -EIO;
+
+	if (!rx_tail)
+		return 0;
+
+	len = 0;
+	t = first;
 	/* Read out all the data */
-	if (rx_tail)
-		memcpy_fromio(t->rx_ptr, bs->rx_io, rx_tail);
+	for (i = 0; i < num_transfers; i++) {
+		if (t->rx_buf)
+			memcpy_fromio(t->rx_buf, bs->rx_io + len, t->len);
+
+		if (t != first || prepend_len == 0)
+			len += t->len;
+
+		t = list_entry(t->transfer_list.next, struct spi_transfer,
+			       transfer_list);
+	}
 
 	return 0;
 }
@@ -252,46 +300,76 @@ static int bcm63xx_spi_transfer_one(struct spi_master *master,
 					struct spi_message *m)
 {
 	struct bcm63xx_spi *bs = spi_master_get_devdata(master);
-	struct spi_transfer *t;
+	struct spi_transfer *t, *first = NULL;
 	struct spi_device *spi = m->spi;
 	int status = 0;
-
+	unsigned int n_transfers = 0, total_len = 0;
+	bool can_use_prepend = false;
+
+	/*
+	 * This SPI controller does not support keeping CS active after a
+	 * transfer.
+	 * Work around this by merging as many transfers we can into one big
+	 * full-duplex transfers.
+	 */
 	list_for_each_entry(t, &m->transfers, transfer_list) {
 		status = bcm63xx_spi_check_transfer(spi, t);
 		if (status < 0)
 			goto exit;
 
+		if (!first)
+			first = t;
+
+		n_transfers++;
+		total_len += t->len;
+
+		if (n_transfers == 2 && !first->rx_buf && !t->tx_buf &&
+		    first->len <= BCM63XX_SPI_MAX_PREPEND)
+			can_use_prepend = true;
+		else if (can_use_prepend && t->tx_buf)
+			can_use_prepend = false;
+
 		/* we can only transfer one fifo worth of data */
-		if (t->len > bs->fifo_size) {
+		if ((can_use_prepend &&
+		     total_len > (bs->fifo_size + BCM63XX_SPI_MAX_PREPEND)) ||
+		    (!can_use_prepend && total_len > bs->fifo_size)) {
 			dev_err(&spi->dev, "unable to do transfers larger than FIFO size (%i > %i)\n",
-				t->len, bs->fifo_size);
+				total_len, bs->fifo_size);
 			status = -EINVAL;
 			goto exit;
 		}
 
-		/* CS will be deasserted directly after transfer */
-		if (t->delay_usecs) {
-			dev_err(&spi->dev, "unable to keep CS asserted after transfer\n");
+		/* all combined transfers have to have the same speed */
+		if (t->speed_hz != first->speed_hz) {
+			dev_err(&spi->dev, "unable to change speed between transfers\n");
 			status = -EINVAL;
 			goto exit;
 		}
 
-		if (!t->cs_change &&
-		    !list_is_last(&t->transfer_list, &m->transfers)) {
-			dev_err(&spi->dev, "unable to keep CS asserted between transfers\n");
+		/* CS will be deasserted directly after transfer */
+		if (t->delay_usecs) {
+			dev_err(&spi->dev, "unable to keep CS asserted after transfer\n");
 			status = -EINVAL;
 			goto exit;
 		}
 
-		/* configure adapter for a new transfer */
-		bcm63xx_spi_setup_transfer(spi, t);
+		if (t->cs_change ||
+		    list_is_last(&t->transfer_list, &m->transfers)) {
+			/* configure adapter for a new transfer */
+			bcm63xx_spi_setup_transfer(spi, first);
 
-		/* send the data */
-		status = bcm63xx_txrx_bufs(spi, t);
-		if (status)
-			goto exit;
+			/* send the data */
+			status = bcm63xx_txrx_bufs(spi, first, n_transfers);
+			if (status)
+				goto exit;
+
+			m->actual_length += total_len;
 
-		m->actual_length += t->len;
+			first = NULL;
+			n_transfers = 0;
+			total_len = 0;
+			can_use_prepend = false;
+		}
 	}
 exit:
 	m->status = status;