spi: sirf: provide a shortcut for spi command-data mode

there are many SPI clients which use the following protocal:
step 1: send command bytes to clients(rx buffer is empty)
step 2: send data bytes to clients or receive data bytes from
clients.
SiRFprimaII provides a shortcut for this kind of SPI transfer.
when tx buf is less or equal than 4 bytes and rx buf is null
in a transfer, we think it as 'command' data and use hardware
command register for the transfer.
here we can save some CPU loading than doing both tx and rx
for a normal transfer.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi-sirf.c

@@ -131,6 +131,8 @@
 #define IS_DMA_VALID(x) (x && ALIGNED(x->tx_buf) && ALIGNED(x->rx_buf) && \
 	ALIGNED(x->len) && (x->len < 2 * PAGE_SIZE))
 
+#define SIRFSOC_MAX_CMD_BYTES	4
+
 struct sirfsoc_spi {
 	struct spi_bitbang bitbang;
 	struct completion rx_done;
@@ -161,6 +163,12 @@ struct sirfsoc_spi {
 	void *dummypage;
 	int word_width; /* in bytes */
 
+	/*
+	 * if tx size is not more than 4 and rx size is NULL, use
+	 * command model
+	 */
+	bool	tx_by_cmd;
+
 	int chipselect[0];
 };
 
@@ -259,6 +267,12 @@ static irqreturn_t spi_sirfsoc_irq(int irq, void *dev_id)
 
 	writel(spi_stat, sspi->base + SIRFSOC_SPI_INT_STATUS);
 
+	if (sspi->tx_by_cmd && (spi_stat & SIRFSOC_SPI_FRM_END)) {
+		complete(&sspi->tx_done);
+		writel(0x0, sspi->base + SIRFSOC_SPI_INT_EN);
+		return IRQ_HANDLED;
+	}
+
 	/* Error Conditions */
 	if (spi_stat & SIRFSOC_SPI_RX_OFLOW ||
 			spi_stat & SIRFSOC_SPI_TX_UFLOW) {
@@ -309,6 +323,34 @@ static int spi_sirfsoc_transfer(struct spi_device *spi, struct spi_transfer *t)
 
 	writel(SIRFSOC_SPI_INT_MASK_ALL, sspi->base + SIRFSOC_SPI_INT_STATUS);
 
+	/*
+	 * fill tx_buf into command register and wait for its completion
+	 */
+	if (sspi->tx_by_cmd) {
+		u32 cmd;
+		memcpy(&cmd, sspi->tx, t->len);
+
+		if (sspi->word_width == 1 && !(spi->mode & SPI_LSB_FIRST))
+			cmd = cpu_to_be32(cmd) >>
+				((SIRFSOC_MAX_CMD_BYTES - t->len) * 8);
+		if (sspi->word_width == 2 && t->len == 4 &&
+				(!(spi->mode & SPI_LSB_FIRST)))
+			cmd = ((cmd & 0xffff) << 16) | (cmd >> 16);
+
+		writel(cmd, sspi->base + SIRFSOC_SPI_CMD);
+		writel(SIRFSOC_SPI_FRM_END_INT_EN,
+			sspi->base + SIRFSOC_SPI_INT_EN);
+		writel(SIRFSOC_SPI_CMD_TX_EN,
+			sspi->base + SIRFSOC_SPI_TX_RX_EN);
+
+		if (wait_for_completion_timeout(&sspi->tx_done, timeout) == 0) {
+			dev_err(&spi->dev, "transfer timeout\n");
+			return 0;
+		}
+
+		return t->len;
+	}
+
 	if (sspi->left_tx_word == 1) {
 		writel(readl(sspi->base + SIRFSOC_SPI_CTRL) |
 			SIRFSOC_SPI_ENA_AUTO_CLR,
@@ -509,6 +551,14 @@ spi_sirfsoc_setup_transfer(struct spi_device *spi, struct spi_transfer *t)
 	writel(txfifo_ctrl, sspi->base + SIRFSOC_SPI_TXFIFO_CTRL);
 	writel(rxfifo_ctrl, sspi->base + SIRFSOC_SPI_RXFIFO_CTRL);
 
+	if (t && t->tx_buf && !t->rx_buf && (t->len <= SIRFSOC_MAX_CMD_BYTES)) {
+		regval |= (SIRFSOC_SPI_CMD_BYTE_NUM((t->len - 1)) |
+				SIRFSOC_SPI_CMD_MODE);
+		sspi->tx_by_cmd = true;
+	} else {
+		regval &= ~SIRFSOC_SPI_CMD_MODE;
+		sspi->tx_by_cmd = false;
+	}
 	writel(regval, sspi->base + SIRFSOC_SPI_CTRL);
 
 	if (IS_DMA_VALID(t)) {