mcp251x.c : patch to avoid repeated frame bug

[Benoit <benoit@pouic.eu>]

Hello,

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 an RTS command on the SPI bus to initiate the 
transmission. Accordingly, I have modified the transmission routine as 
follows in file mcp251x.c:


--- drivers/net/can/mcp251x.c-original	2012-08-26 15:00:28.000000000 +0200
+++ drivers/net/can/mcp251x.c	2012-08-26 20:42:58.000000000 +0200
@@ -83,6 +83,10 @@
  #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(txb)	(0x80 | ((txb) & 0x07))
  
  /* MPC251x registers */
  #define CANSTAT	      0x0e
@@ -394,9 +398,20 @@
  	}
  }
  
+
+/*
+ * CAN frame is now sent using instruction RTS over SPI to work around
+ * repeated frame problem as per MCP2515 Rev B. Silicon Errata revision G
+ * (March 2007) about repeated frames (see section 5 of document).
+ * Note that repeated frames can still occur if using SPI mode 11. It seems
+ * however that spi mode is forcibly set to 0, so it looks safe.
+ *
+ */
  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);
+	int ret;
  	u32 sid, eid, exide, rtr;
  	u8 buf[SPI_TRANSFER_BUF_LEN];
  
@@ -418,7 +433,12 @@
  	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);
+
+	priv->spi_tx_buf[0] = INSTRUCTION_RTS(1 << tx_buf_idx);
+	ret = spi_write(spi, priv->spi_tx_buf, 1);
+	if (ret) {
+		dev_err(&spi->dev, "RTS failed: ret = %d\n", ret);
+	}
  }
  
  static void mcp251x_hw_rx_frame(struct spi_device *spi, u8 *buf,



Would it be possible to integrate this patch into mainstream mcp251x.c?

Kind Regards,

Benoît.