spi/xilinx: Use polling mode on small transfers

Small transfers generally can be accomplished faster in polling mode.
This patch select the transfer which size is bellow the buffer size to
be done on polling mode

Suggested-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-xilinx.c

@@ -128,7 +128,6 @@ static void xilinx_spi_rx(struct xilinx_spi *xspi)
 static void xspi_init_hw(struct xilinx_spi *xspi)
 {
 	void __iomem *regs_base = xspi->regs;
-	u32 inhibit;
 
 	/* Reset the SPI device */
 	xspi->write_fn(XIPIF_V123B_RESET_MASK,
@@ -138,22 +137,15 @@ static void xspi_init_hw(struct xilinx_spi *xspi)
 	 */
 	xspi->write_fn(XSPI_INTR_TX_EMPTY,
 			regs_base + XIPIF_V123B_IIER_OFFSET);
-	/* Enable the global IPIF interrupt */
-	if (xspi->irq >= 0) {
-		xspi->write_fn(XIPIF_V123B_GINTR_ENABLE,
-			regs_base + XIPIF_V123B_DGIER_OFFSET);
-		inhibit = XSPI_CR_TRANS_INHIBIT;
-	} else {
-		xspi->write_fn(0, regs_base + XIPIF_V123B_DGIER_OFFSET);
-		inhibit = 0;
-	}
+	/* Disable the global IPIF interrupt */
+	xspi->write_fn(0, regs_base + XIPIF_V123B_DGIER_OFFSET);
 	/* Deselect the slave on the SPI bus */
 	xspi->write_fn(0xffff, regs_base + XSPI_SSR_OFFSET);
 	/* Disable the transmitter, enable Manual Slave Select Assertion,
 	 * put SPI controller into master mode, and enable it */
-	xspi->write_fn(inhibit | XSPI_CR_MANUAL_SSELECT |
-		XSPI_CR_MASTER_MODE | XSPI_CR_ENABLE | XSPI_CR_TXFIFO_RESET |
-		XSPI_CR_RXFIFO_RESET, regs_base + XSPI_CR_OFFSET);
+	xspi->write_fn(XSPI_CR_MANUAL_SSELECT |	XSPI_CR_MASTER_MODE |
+		XSPI_CR_ENABLE | XSPI_CR_TXFIFO_RESET |	XSPI_CR_RXFIFO_RESET,
+		regs_base + XSPI_CR_OFFSET);
 }
 
 static void xilinx_spi_chipselect(struct spi_device *spi, int is_on)
@@ -212,6 +204,8 @@ static int xilinx_spi_txrx_bufs(struct spi_device *spi, struct spi_transfer *t)
 {
 	struct xilinx_spi *xspi = spi_master_get_devdata(spi->master);
 	int remaining_words;	/* the number of words left to transfer */
+	bool use_irq = false;
+	u16 cr = 0;
 
 	/* We get here with transmitter inhibited */
 
@@ -220,8 +214,20 @@ static int xilinx_spi_txrx_bufs(struct spi_device *spi, struct spi_transfer *t)
 	remaining_words = t->len / xspi->bytes_per_word;
 	reinit_completion(&xspi->done);
 
+	if (xspi->irq >= 0 &&  remaining_words > xspi->buffer_size) {
+		use_irq = true;
+		xspi->write_fn(XSPI_INTR_TX_EMPTY,
+				xspi->regs + XIPIF_V123B_IISR_OFFSET);
+		/* Enable the global IPIF interrupt */
+		xspi->write_fn(XIPIF_V123B_GINTR_ENABLE,
+				xspi->regs + XIPIF_V123B_DGIER_OFFSET);
+		/* Inhibit irq to avoid spurious irqs on tx_empty*/
+		cr = xspi->read_fn(xspi->regs + XSPI_CR_OFFSET);
+		xspi->write_fn(cr | XSPI_CR_TRANS_INHIBIT,
+			       xspi->regs + XSPI_CR_OFFSET);
+	}
+
 	while (remaining_words) {
-		u16 cr = 0;
 		int n_words, tx_words, rx_words;
 
 		n_words = min(remaining_words, xspi->buffer_size);
@@ -234,9 +240,7 @@ static int xilinx_spi_txrx_bufs(struct spi_device *spi, struct spi_transfer *t)
 		 * longer
 		 */
 
-		if (xspi->irq >= 0) {
-			cr = xspi->read_fn(xspi->regs + XSPI_CR_OFFSET) &
-							~XSPI_CR_TRANS_INHIBIT;
+		if (use_irq) {
 			xspi->write_fn(cr, xspi->regs + XSPI_CR_OFFSET);
 			wait_for_completion(&xspi->done);
 		} else
@@ -249,7 +253,7 @@ static int xilinx_spi_txrx_bufs(struct spi_device *spi, struct spi_transfer *t)
 		 * transmitter while the Isr refills the transmit register/FIFO,
 		 * or make sure it is stopped if we're done.
 		 */
-		if (xspi->irq >= 0)
+		if (use_irq)
 			xspi->write_fn(cr | XSPI_CR_TRANS_INHIBIT,
 			       xspi->regs + XSPI_CR_OFFSET);
 
@@ -261,6 +265,9 @@ static int xilinx_spi_txrx_bufs(struct spi_device *spi, struct spi_transfer *t)
 		remaining_words -= n_words;
 	}
 
+	if (use_irq)
+		xspi->write_fn(0, xspi->regs + XIPIF_V123B_DGIER_OFFSET);
+
 	return t->len;
 }