can: mcp251x: convert driver to half-duplex SPI

Some SPI host controllers such as the Cavium Thunder TX do not support
full-duplex SPI. Using half-duplex transfers allows the driver to work
with those host controllers.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

drivers/net/can/spi/mcp251x.c

@@ -291,23 +291,23 @@ static u8 mcp251x_read_reg(struct spi_device *spi, u8 reg)
 	priv->spi_tx_buf[0] = INSTRUCTION_READ;
 	priv->spi_tx_buf[1] = reg;
 
-	mcp251x_spi_trans(spi, 3);
-	val = priv->spi_rx_buf[2];
+	spi_write_then_read(spi, priv->spi_tx_buf, 2, &val, 1);
 
 	return val;
 }
 
 static void mcp251x_read_2regs(struct spi_device *spi, u8 reg, u8 *v1, u8 *v2)
 {
+	u8 val[4] = {0};
 	struct mcp251x_priv *priv = spi_get_drvdata(spi);
 
 	priv->spi_tx_buf[0] = INSTRUCTION_READ;
 	priv->spi_tx_buf[1] = reg;
 
-	mcp251x_spi_trans(spi, 4);
+	spi_write_then_read(spi, priv->spi_tx_buf, 2, val, 2);
 
-	*v1 = priv->spi_rx_buf[2];
-	*v2 = priv->spi_rx_buf[3];
+	*v1 = val[0];
+	*v2 = val[1];
 }
 
 static void mcp251x_write_reg(struct spi_device *spi, u8 reg, u8 val)
@@ -398,8 +398,9 @@ static void mcp251x_hw_rx_frame(struct spi_device *spi, u8 *buf,
 			buf[i] = mcp251x_read_reg(spi, RXBCTRL(buf_idx) + i);
 	} else {
 		priv->spi_tx_buf[RXBCTRL_OFF] = INSTRUCTION_READ_RXB(buf_idx);
-		mcp251x_spi_trans(spi, SPI_TRANSFER_BUF_LEN);
-		memcpy(buf, priv->spi_rx_buf, SPI_TRANSFER_BUF_LEN);
+		spi_write_then_read(spi, priv->spi_tx_buf, 1, priv->spi_rx_buf,
+				    SPI_TRANSFER_BUF_LEN);
+		memcpy(buf + 1, priv->spi_rx_buf, SPI_TRANSFER_BUF_LEN - 1);
 	}
 }