can: mcp251x: add support for half duplex controllers

Some SPI host controllers do not support full-duplex SPI and are
marked as such via the SPI_CONTROLLER_HALF_DUPLEX controller flag.

For such controllers use half duplex transactions but retain full
duplex transactions for the controllers that can handle those.
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Link: https://lore.kernel.org/r/1595516275-1179-1-git-send-email-tharvey@gateworks.comSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/spi/mcp251x.c

@@ -322,8 +322,12 @@ static u8 mcp251x_read_reg(struct spi_device *spi, u8 reg)
 	priv->spi_tx_buf[0] = INSTRUCTION_READ;
 	priv->spi_tx_buf[1] = reg;
 
+	if (spi->controller->flags & SPI_CONTROLLER_HALF_DUPLEX) {
+		spi_write_then_read(spi, priv->spi_tx_buf, 2, &val, 1);
+	} else {
 		mcp251x_spi_trans(spi, 3);
 		val = priv->spi_rx_buf[2];
+	}
 
 	return val;
 }
@@ -335,10 +339,18 @@ static void mcp251x_read_2regs(struct spi_device *spi, u8 reg, u8 *v1, u8 *v2)
 	priv->spi_tx_buf[0] = INSTRUCTION_READ;
 	priv->spi_tx_buf[1] = reg;
 
+	if (spi->controller->flags & SPI_CONTROLLER_HALF_DUPLEX) {
+		u8 val[2] = { 0 };
+
+		spi_write_then_read(spi, priv->spi_tx_buf, 2, val, 2);
+		*v1 = val[0];
+		*v2 = val[1];
+	} else {
 		mcp251x_spi_trans(spi, 4);
 
 		*v1 = priv->spi_rx_buf[2];
 		*v2 = priv->spi_rx_buf[3];
+	}
 }
 
 static void mcp251x_write_reg(struct spi_device *spi, u8 reg, u8 val)
@@ -657,9 +669,17 @@ 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);
+		if (spi->controller->flags & SPI_CONTROLLER_HALF_DUPLEX) {
+			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);
+		} else {
 			mcp251x_spi_trans(spi, SPI_TRANSFER_BUF_LEN);
 			memcpy(buf, priv->spi_rx_buf, SPI_TRANSFER_BUF_LEN);
 		}
+	}
 }
 
 static void mcp251x_hw_rx(struct spi_device *spi, int buf_idx)