netdev - RE: [PATCH/RFC] net: ethernet: ravb: Try to wake subqueue instead of stop on timeout

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Message-ID: <TY2PR01MB3692A9F68ACC62E920EE2965D86F0@TY2PR01MB3692.jpnprd01.prod.outlook.com>
Date:   Tue, 30 Jun 2020 05:22:57 +0000
From:   Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
To:     Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        Sergei Shtylyov <sergei.shtylyov@...il.com>
CC:     "REE dirk.behme@...bosch.com" <dirk.behme@...bosch.com>,
        "Shashikant.Suguni@...bosch.com" <Shashikant.Suguni@...bosch.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-renesas-soc@...r.kernel.org" 
        <linux-renesas-soc@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>
Subject: RE: [PATCH/RFC] net: ethernet: ravb: Try to wake subqueue instead of
 stop on timeout

Hello!

> From: Sergei Shtylyov, Sent: Monday, June 29, 2020 5:40 PM
> 
> Hello!
> 
> On 29.06.2020 8:24, Yoshihiro Shimoda wrote:
> 
> >>>>> From: Yoshihiro Shimoda, Sent: Tuesday, May 26, 2020 6:47 PM
> >>>>>
> >>>>> According to the report of [1], this driver is possible to cause
> >>>>> the following error in ravb_tx_timeout_work().
> >>>>>
> >>>>> ravb e6800000.ethernet ethernet: failed to switch device to config mode
> >>>>>
> >>>>> This error means that the hardware could not change the state
> >>>>> from "Operation" to "Configuration" while some tx queue is operating.
> >>>>> After that, ravb_config() in ravb_dmac_init() will fail, and then
> >>>>> any descriptors will be not allocaled anymore so that NULL porinter
> >>>>> dereference happens after that on ravb_start_xmit().
> >>>>>
> >>>>> Such a case is possible to be caused because this driver supports
> >>>>> two queues (NC and BE) and the ravb_stop_dma() is possible to return
> >>>>> without any stopping process if TCCR or CSR register indicates
> >>>>> the hardware is operating for TX.
> >>>>>
> >>>>> To fix the issue, just try to wake the subqueue on
> >>>>> ravb_tx_timeout_work() if the descriptors are not full instead
> >>>>> of stop all transfers (all queues of TX and RX).
> >>>>>
> >>>>> [1]
> >>>>> https://lore.kernel.org/linux-renesas-soc/20200518045452.2390-1-dirk.behme@de.bosch.com/
> >>>>>
> >>>>> Reported-by: Dirk Behme <dirk.behme@...bosch.com>
> >>>>> Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
> >>>>> ---
> >>>>>    I'm guessing that this issue is possible to happen if:
> >>>>>    - ravb_start_xmit() calls netif_stop_subqueue(), and
> >>>>>    - ravb_poll() will not be called with some reason, and
> >>>>>    - netif_wake_subqueue() will be not called, and then
> >>>>>    - dev_watchdog() in net/sched/sch_generic.c calls ndo_tx_timeout().
> >>>>>
> >>>>>    However, unfortunately, I didn't reproduce the issue yet.
> >>>>>    To be honest, I'm also guessing other queues (SR) of this hardware
> >>>>>    which out-of tree driver manages are possible to reproduce this issue,
> >>>>>    but I didn't try such environment for now...
> >>>>>
> >>>>>    So, I marked RFC on this patch now.
> >>>>
> >>>> I'm afraid, but do you have any comments about this patch?
> >>>
> >>>      I agree that we should now reset only the stuck queue, not both but I
> >>> doubt your solution is good enough. Let me have another look...
> >>
> >> Thank you for your comment! I hope this solution is good enough...
> >
> > I'm sorry again and again. But, do you have any time to look this patch?
> 
>     Yes, in the sense of reviewing -- I don't consider it complete. And no, in
> the sense of looking into the issue myself... Can we do a per-queue tear-down
> and re-init (not necessarily all in 1 patch)?

Thank you for your comment! I'm not sure this "re-init" mean. But, we can do
a per-queue tear-down if DMAC is still working. And, we can prepare new descriptors
for the queue after tear-down.

< Tear-down >
1. Set DT_EOS to the desc_bat[queue].
2. Set DLR.LBAx to 1.
3. Check if DLA.LBAx is cleared.

< Prepare new descriptors and start again >
4. Prepare new descriptors.
5. Set DT_LINKFIX to the desc_bat[queue].
6. Set DLR.LBAx to 1.
7. Check if DLA.LBAx is cleared.


I'm thinking doing tear-down and "re-init" of the descriptors is better
than just try to wake the subqueue. But, what do you think?

Best regards,
Yoshihiro Shimoda

> > Best regards,
> > Yoshihiro Shimoda
> 
> MBR, Sergei