Re: [PATCH] can: fix loss of frames due to wrong assumption in raw_rcv

From: Oliver Hartkopp
Date: Sat Jun 20 2015 - 18:42:52 EST

Hello Manfred,

On 06/20/2015 07:21 PM, Manfred Schlaegl wrote:
> I've detected a massive loss of can frames on i.MX6 using flexcan
> driver with 4.1-rc8 and tracked this down to following commit:
> 514ac99c64b22d83b52dfee3b8becaa69a92bc4a - "can: fix multiple delivery
> of a single CAN frame for overlapping CAN filters"

thanks for detecting this issue!

> 514ac99c64b22d83b52dfee3b8becaa69a92bc4a introduces a frame equality
> check. Since the sk_buff pointer is not sufficient to do this (buffers
> are reused), the check also compares time stamps.
> In short: pointer+time stamp was assumed as unique key to a specific
> frame.
> The problem with this is, that the time stamp is an optional property
> and not set per default.
> In our case (flexcan) the time stamp is always zero, so the equality
> check is reduced to equality of buffer pointers, resulting in a lot of
> dropped frames.

The question is why your system did not generate a timestamp at the time of
skb reception.

Usually when netif_rx(), netif_rx_ni() is invoked the timestamp is set in the
following reception process.

flexcan.c only uses netif_receive_skb() - but all theses functions set the

net_timestamp_check(netdev_tstamp_prequeue, skb);

depending on netdev_tstamp_prequeue which is configured by


See the idea of netdev_tstamp_prequeue here:

Can you tell me the output of /proc/sys/net/core/netdev_tstamp_prequeue on
your machine?

If it's not '1' can you set it to '1' for a test?

> Possible solutions I thought of:
> 1. Every driver has to set a time stamp
> (possibly error prone and hard to enforce?)
> 2. Change the equality check
> 3. Fulfil the requirements of the equality check by setting a
> time stamp per default.
> This patch fixes the problem with solution 3. A time stamp is set at
> time of allocation in alloc_can_skb.

That's a feasible way if won't find a better way to make sure the timestamps
are generally set before the skb is processed in the NET_RX softirq.

> The time stamp may be overridden later, but the function of the equality
> check is ensured.
> I'm not really deep in linux network subsystem, so there may exists
> more elegant solutions for the problem.
> Signed-off-by: Manfred Schlaegl <manfred.schlaegl@xxxxxx>
> ---
> drivers/net/can/dev.c | 1 +
> 1 file changed, 1 insertion(+)
> diff --git a/drivers/net/can/dev.c b/drivers/net/can/dev.c
> index b0f6924..282e2e7 100644
> --- a/drivers/net/can/dev.c
> +++ b/drivers/net/can/dev.c
> @@ -575,6 +575,7 @@ struct sk_buff *alloc_can_skb(struct net_device *dev, struct can_frame **cf)
> if (unlikely(!skb))
> return NULL;
> + __net_timestamp(skb);
> skb->protocol = htons(ETH_P_CAN);
> skb->pkt_type = PACKET_BROADCAST;
> skb->ip_summed = CHECKSUM_UNNECESSARY;

Please check the netdev_tstamp_prequeue value first.

If we would need solution 3 the __net_timestamp(skb) should be placed in
alloc_canfd_skb() too.

Thanks again for your investigation!

Best regards,

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