can: mcp251x: avoid repeated frame bug

The MCP2515 has a silicon bug causing repeated frame transmission, see section
5 of MCP2515 Rev. B Silicon Errata Revision G (March 2007).

Basically, setting TXBnCTRL.TXREQ in either SPI mode (00 or 11) will eventually
cause the bug. The workaround proposed by Microchip is to use mode 00 and send
a RTS command on the SPI bus to initiate the transmission.

Cc: <stable@vger.kernel.org>
Signed-off-by: Benoît Locher <Benoit.Locher@skf.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/mcp251x.c

@@ -83,6 +83,11 @@
 #define INSTRUCTION_LOAD_TXB(n)	(0x40 + 2 * (n))
 #define INSTRUCTION_READ_RXB(n)	(((n) == 0) ? 0x90 : 0x94)
 #define INSTRUCTION_RESET	0xC0
+#define RTS_TXB0		0x01
+#define RTS_TXB1		0x02
+#define RTS_TXB2		0x04
+#define INSTRUCTION_RTS(n)	(0x80 | ((n) & 0x07))
+
 
 /* MPC251x registers */
 #define CANSTAT	      0x0e
@@ -397,6 +402,7 @@ static void mcp251x_hw_tx_frame(struct spi_device *spi, u8 *buf,
 static void mcp251x_hw_tx(struct spi_device *spi, struct can_frame *frame,
 			  int tx_buf_idx)
 {
+	struct mcp251x_priv *priv = dev_get_drvdata(&spi->dev);
 	u32 sid, eid, exide, rtr;
 	u8 buf[SPI_TRANSFER_BUF_LEN];
 
@@ -418,7 +424,10 @@ static void mcp251x_hw_tx(struct spi_device *spi, struct can_frame *frame,
 	buf[TXBDLC_OFF] = (rtr << DLC_RTR_SHIFT) | frame->can_dlc;
 	memcpy(buf + TXBDAT_OFF, frame->data, frame->can_dlc);
 	mcp251x_hw_tx_frame(spi, buf, frame->can_dlc, tx_buf_idx);
-	mcp251x_write_reg(spi, TXBCTRL(tx_buf_idx), TXBCTRL_TXREQ);
+
+	/* use INSTRUCTION_RTS, to avoid "repeated frame problem" */
+	priv->spi_tx_buf[0] = INSTRUCTION_RTS(1 << tx_buf_idx);
+	mcp251x_spi_trans(priv->spi, 1);
 }
 
 static void mcp251x_hw_rx_frame(struct spi_device *spi, u8 *buf,