spi/xilinx: Simplify spi_fill_tx_fifo

Instead of checking the TX_FULL flag for every transaction, find out the
size of the buffer at probe time and use it.

To avoid situations where the core had some data on the buffer before
initialization, the core is reseted before the buffer size is detected
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-xilinx.c

@@ -88,6 +88,7 @@ struct xilinx_spi {
 	const u8 *tx_ptr;	/* pointer in the Rx buffer */
 	int remaining_bytes;	/* the number of bytes left to transfer */
 	u8 bits_per_word;
+	int buffer_size;	/* buffer size in words */
 	unsigned int (*read_fn)(void __iomem *);
 	void (*write_fn)(u32, void __iomem *);
 	void (*tx_fn)(struct xilinx_spi *);
@@ -221,24 +222,16 @@ static int xilinx_spi_setup_transfer(struct spi_device *spi,
 	return 0;
 }
 
-static int xilinx_spi_fill_tx_fifo(struct xilinx_spi *xspi)
+static void xilinx_spi_fill_tx_fifo(struct xilinx_spi *xspi, int n_words)
 {
-	u8 sr;
-	int n_words = 0;
+	xspi->remaining_bytes -= n_words * xspi->bits_per_word / 8;
 
-	/* Fill the Tx FIFO with as many bytes as possible */
-	sr = xspi->read_fn(xspi->regs + XSPI_SR_OFFSET);
-	while ((sr & XSPI_SR_TX_FULL_MASK) == 0 && xspi->remaining_bytes > 0) {
+	while (n_words--)
 		if (xspi->tx_ptr)
 			xspi->tx_fn(xspi);
 		else
 			xspi->write_fn(0, xspi->regs + XSPI_TXD_OFFSET);
-		xspi->remaining_bytes -= xspi->bits_per_word / 8;
-		sr = xspi->read_fn(xspi->regs + XSPI_SR_OFFSET);
-		n_words++;
-	}
-
-	return n_words;
+	return;
 }
 
 static int xilinx_spi_txrx_bufs(struct spi_device *spi, struct spi_transfer *t)
@@ -265,7 +258,10 @@ static int xilinx_spi_txrx_bufs(struct spi_device *spi, struct spi_transfer *t)
 		u16 cr;
 		int n_words;
 
-		n_words = xilinx_spi_fill_tx_fifo(xspi);
+		n_words = (xspi->remaining_bytes * 8) / xspi->bits_per_word;
+		n_words = min(n_words, xspi->buffer_size);
+
+		xilinx_spi_fill_tx_fifo(xspi, n_words);
 
 		/* Start the transfer by not inhibiting the transmitter any
 		 * longer
@@ -322,6 +318,28 @@ static irqreturn_t xilinx_spi_irq(int irq, void *dev_id)
 	return IRQ_HANDLED;
 }
 
+static int xilinx_spi_find_buffer_size(struct xilinx_spi *xspi)
+{
+	u8 sr;
+	int n_words = 0;
+
+	/*
+	 * Before the buffer_size detection we reset the core
+	 * to make sure we start with a clean state.
+	 */
+	xspi->write_fn(XIPIF_V123B_RESET_MASK,
+		xspi->regs + XIPIF_V123B_RESETR_OFFSET);
+
+	/* Fill the Tx FIFO with as many words as possible */
+	do {
+		xspi->write_fn(0, xspi->regs + XSPI_TXD_OFFSET);
+		sr = xspi->read_fn(xspi->regs + XSPI_SR_OFFSET);
+		n_words++;
+	} while (!(sr & XSPI_SR_TX_FULL_MASK));
+
+	return n_words;
+}
+
 static const struct of_device_id xilinx_spi_of_match[] = {
 	{ .compatible = "xlnx,xps-spi-2.00.a", },
 	{ .compatible = "xlnx,xps-spi-2.00.b", },
@@ -413,6 +431,8 @@ static int xilinx_spi_probe(struct platform_device *pdev)
 		goto put_master;
 	}
 
+	xspi->buffer_size = xilinx_spi_find_buffer_size(xspi);
+
 	/* SPI controller initializations */
 	xspi_init_hw(xspi);