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SubjectRE: [PATCH] net: fec: Detect and recover receive queue hangs
Date
From: Chris Lesiak <chris.lesiak@licor.com> Sent: Friday, November 18, 2016 10:37 PM
>To: Andy Duan <fugang.duan@nxp.com>
>Cc: netdev@vger.kernel.org; linux-kernel@vger.kernel.org; Jaccon
>Bastiaansen <jaccon.bastiaansen@gmail.com>
>Subject: Re: [PATCH] net: fec: Detect and recover receive queue hangs
>
>On 11/18/2016 12:44 AM, Andy Duan wrote:
>> From: Chris Lesiak <chris.lesiak@licor.com> Sent: Friday, November 18,
>> 2016 5:15 AM
>> >To: Andy Duan <fugang.duan@nxp.com>
>> >Cc: netdev@vger.kernel.org; linux-kernel@vger.kernel.org; Jaccon
>> >Bastiaansen <jaccon.bastiaansen@gmail.com>; chris.lesiak@licor.com
>> >Subject: [PATCH] net: fec: Detect and recover receive queue hangs >
>> >This corrects a problem that appears to be similar to ERR006358. But
>> while
>> >ERR006358 is a race when the tx queue transitions from empty to not
>> empty, >this problem is a race when the rx queue transitions from full to
>not full.
>> >
>> >The symptom is a receive queue that is stuck. The ENET_RDAR
>> register will >read 0, indicating that there are no empty receive
>> descriptors in the receive >ring. Since no additional frames can be queued,
>no RXF interrupts occur.
>> >
>> >This problem can be triggered with a 1 Gb link and about 400 Mbps of
>traffic.
>
>I can cause the error by running the following on an imx6q: iperf -s -u And
>sending packets from the other end of a 1 Gbps link:
>iperf -c $IPADDR -u -b40000pps
>
>A few others have seen this problem.
>See: https://community.nxp.com/thread/322882
>
>> >
>> >This patch detects this condition, sets the work_rx bit, and
>> reschedules the >poll method.
>> >
>> >Signed-off-by: Chris Lesiak <chris.lesiak@licor.com>
>> >---
>> > drivers/net/ethernet/freescale/fec_main.c | 31
>> >+++++++++++++++++++++++++++++++ > 1 file changed, 31 insertions(+)
>> > Firstly, how to reproduce the issue, pls list the reproduce steps.
>> Thanks.
>> Secondly, pls check below comments.
>>
>> >diff --git a/drivers/net/ethernet/freescale/fec_main.c
>> >b/drivers/net/ethernet/freescale/fec_main.c
>> >index fea0f33..8a87037 100644
>> >--- a/drivers/net/ethernet/freescale/fec_main.c
>> >+++ b/drivers/net/ethernet/freescale/fec_main.c
>> >@@ -1588,6 +1588,34 @@ fec_enet_interrupt(int irq, void *dev_id)
>> > return ret;
>> > }
>> >
>> >+static inline bool
>> >+fec_enet_recover_rxq(struct fec_enet_private *fep, u16 queue_id) {
>> >+ int work_bit = (queue_id == 0) ? 2 : ((queue_id == 1) ? 0 : 1);
>> >+
>> >+ if (readl(fep->rx_queue[queue_id]->bd.reg_desc_active))
>> If rx ring is really empty in slight throughput cases, rdar is always cleared,
>then there always do napi reschedule.
>
>I think that you are concerned that if rdar is zero due to this hardware
>problem, but the rx ring is actually empty, then fec_enet_rx_queue will
>never do a write to rdar so that it can be non-zero. That will cause napi to
>always be resceduled.
>
>I suppose that might be the case with zero rx traffic, and I was concerned
>that it might be true even when there was rx traffic. I suspected that the
>hardware, seeing that rdar is zero, would never queue another packet, even
>if there were in fact empty descriptors. But it doesn't seem to be the case. It
>does reschedule multiple times, but eventually sees some packets in the rx
>ring and recovers.
>
>I admit that I do not completely understand how that can happen. I did
>confirm that fec_enet_active_rxring is not being called.
>
>Maybe someone with a deeper understanding of the fec than I can provide
>an explanation.
>
The patch needs to hold on for some time (days), I will reserve time to investigate the issue.
Thanks.

>>
>> >+ return false;
>> >+
>> >+ dev_notice_once(&fep->pdev->dev, "Recovered rx queue\n");
>> >+
>> >+ fep->work_rx |= 1 << work_bit;
>> >+
>> >+ return true;
>> >+}
>> >+
>> >+static inline bool fec_enet_recover_rxqs(struct fec_enet_private
>> *fep) >+{
>> >+ unsigned int q;
>> >+ bool ret = false;
>> >+
>> >+ for (q = 0; q < fep->num_rx_queues; q++) {
>> >+ if (fec_enet_recover_rxq(fep, q))
>> >+ ret = true;
>> >+ }
>> >+
>> >+ return ret;
>> >+}
>> >+
>> > static int fec_enet_rx_napi(struct napi_struct *napi, int budget) {
>> > struct net_device *ndev = napi->dev;
>> >@@ -1601,6 +1629,9 @@ static int fec_enet_rx_napi(struct napi_struct
>> *napi, >int budget)
>> > if (pkts < budget) {
>> > napi_complete(napi);
>> > writel(FEC_DEFAULT_IMASK, fep->hwp + FEC_IMASK);
>> >+
>> >+ if (fec_enet_recover_rxqs(fep) && napi_reschedule(napi))
>> >+ writel(FEC_NAPI_IMASK, fep->hwp + FEC_IMASK);
>> > }
>> > return pkts;
>> > }
>> >--
>> >2.5.5
>>
>
>
>--
>Chris Lesiak
>Principal Design Engineer, Software
>LI-COR Biosciences
>chris.lesiak@licor.com
>
>Any opinions expressed are those of the author and do not necessarily
>represent those of his employer.
>

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