linux-kernel - FlexCAN on i.MX28 interrupt flooding retrying send

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Date:	Fri, 07 Mar 2014 09:08:42 +0100
From:	Stanislav Meduna <stano@...una.org>
To:	wg@...ndegger.com, mkl@...gutronix.de, linux-can@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-rt-users@...r.kernel.org" <linux-rt-users@...r.kernel.org>
Subject: FlexCAN on i.MX28 interrupt flooding retrying send

Hi,

I am using a FlexCAN CAN controller on a Freescale i.MX28 platform [1].
If a packet is being sent when the bus is disconnected, I am getting
an interrupt flooed that basically kills the machine.

This is _not_ the same problem as [2] - my kernel already has
the fix.

The first interrupt comes with ESR 0x00028652, i.e.

TXWRN_INT
BIT1_ERR
STF_ERR
TX_WRN
TXRX
FLT_CONF error passive
ERR_INT

The next ones come the same without the acked TXWRN_INT.
Reading the ESR again immediately after acking gives
0x00000250, i.e.

TX_WRN
TXRX
FLT_CONF error passive

so everything ackable has actually been acked.

I think that the problem is that the FlexCAN tries to retransmit
the frame indefinitely. Each retry senses the bus in the invalid
state (BIT1_ERR) and immediately fires a new ERR_INT. To verify
this I aborted the transmitted frame in the error state in the
interrupt handler

#define FLEXCAN_ESR_ERR_TRANSMIT \
	(FLEXCAN_ESR_BIT1_ERR | FLEXCAN_ESR_BIT0_ERR | FLEXCAN_ESR_ACK_ERR)

if (reg_esr & FLEXCAN_ESR_ERR_TRANSMIT) {

	/* In case of a transmission error the packet is retried and
	 * if the error persists, we will get another interrupt right
	 * away. Abort the transmission - a lost packet is better than
	 * an irq storm.
	 */
	if(printk_ratelimit())
		netdev_err(dev, "Aborted transmission, ESR %08x\n", reg_esr);

	can_get_echo_skb(dev, 0);
	flexcan_write(FLEXCAN_MB_CNT_CODE(0x4),
		&regs->cantxfg[FLEXCAN_TX_BUF_ID].can_ctrl);
	netif_wake_queue(dev);
}

and the problem disappeared as expected. However, the correct
way is probably to retry during some reasonable (configurable?)
time interval.

What puzzles me is that I did not found any other instance
of this problem in the relevant mailing lists, only the original [2].

I am using the 3.4.77 kernel with the realtime patches, but the
code in the latest mainline looks the same in this respect.
Maybe the realtime patches change some bevaviour, but I don't
think they affect the core problem. I am not really an expert
in the network devices, NAPI etc - maybe in that case the error
interrupt should be disabled and re-enabled only if the
error condition goes away? - I don't know...

Please Cc: me when answering to the list.

[1] http://www.tq-group.com/en/products/product-details/prod/embedded-modul-tqma28/extb/Main/productdetail/
[2] https://gitorious.org/linux-can/wg-linux-can-next/commit/8ad94fa

Thanks
-- 
                                           Stano
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