can: flexcan: flexcan_chip_start: fix regression, mark one MB for TX and abort pending TX

In patch

    0d1862e can: flexcan: fix flexcan_chip_start() on imx6

the loop in flexcan_chip_start() that iterates over all mailboxes after the
soft reset of the CAN core was removed. This loop put all mailboxes (even the
ones marked as reserved 1...7) into EMPTY/INACTIVE mode. On mailboxes 8...63,
this aborts any pending TX messages.

After a cold boot there is random garbage in the mailboxes, which leads to
spontaneous transmit of CAN frames during first activation. Further if the
interface was disabled with a pending message (usually due to an error
condition on the CAN bus), this message is retransmitted after enabling the
interface again.

This patch fixes the regression by:
1) Limiting the maximum number of used mailboxes to 8, 0...7 are used by the RX
FIFO, 8 is used by TX.
2) Marking the TX mailbox as EMPTY/INACTIVE, so that any pending TX of that
mailbox is aborted.

Cc: linux-stable <stable@vger.kernel.org>
Cc: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/flexcan.c

@@ -62,7 +62,7 @@
 #define FLEXCAN_MCR_BCC			BIT(16)
 #define FLEXCAN_MCR_LPRIO_EN		BIT(13)
 #define FLEXCAN_MCR_AEN			BIT(12)
-#define FLEXCAN_MCR_MAXMB(x)		((x) & 0xf)
+#define FLEXCAN_MCR_MAXMB(x)		((x) & 0x1f)
 #define FLEXCAN_MCR_IDAM_A		(0 << 8)
 #define FLEXCAN_MCR_IDAM_B		(1 << 8)
 #define FLEXCAN_MCR_IDAM_C		(2 << 8)
@@ -735,9 +735,11 @@ static int flexcan_chip_start(struct net_device *dev)
 	 *
 	 */
 	reg_mcr = flexcan_read(&regs->mcr);
+	reg_mcr &= ~FLEXCAN_MCR_MAXMB(0xff);
 	reg_mcr |= FLEXCAN_MCR_FRZ | FLEXCAN_MCR_FEN | FLEXCAN_MCR_HALT |
 		FLEXCAN_MCR_SUPV | FLEXCAN_MCR_WRN_EN |
-		FLEXCAN_MCR_IDAM_C | FLEXCAN_MCR_SRX_DIS;
+		FLEXCAN_MCR_IDAM_C | FLEXCAN_MCR_SRX_DIS |
+		FLEXCAN_MCR_MAXMB(FLEXCAN_TX_BUF_ID);
 	netdev_dbg(dev, "%s: writing mcr=0x%08x", __func__, reg_mcr);
 	flexcan_write(reg_mcr, &regs->mcr);
 
@@ -771,6 +773,10 @@ static int flexcan_chip_start(struct net_device *dev)
 	netdev_dbg(dev, "%s: writing ctrl=0x%08x", __func__, reg_ctrl);
 	flexcan_write(reg_ctrl, &regs->ctrl);
 
+	/* Abort any pending TX, mark Mailbox as INACTIVE */
+	flexcan_write(FLEXCAN_MB_CNT_CODE(0x4),
+		      &regs->cantxfg[FLEXCAN_TX_BUF_ID].can_ctrl);
+
 	/* acceptance mask/acceptance code (accept everything) */
 	flexcan_write(0x0, &regs->rxgmask);
 	flexcan_write(0x0, &regs->rx14mask);