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

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Date:   Sun, 5 Jul 2020 23:28:39 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...il.com>
To:     Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.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!

On 30.06.2020 8:22, 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

    Pointer. :-)

>>>>>>> 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.

    Maybe we should just fix those blind assumptions?

>>>>>>> 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)?

    In fact, it would ensue many changes...

> Thank you for your comment! I'm not sure this "re-init" mean. But, we can do

    Well, I meant the ring re-allocation and re-formatting... but (looking at 
sh_eth) the former is not really necessary, it's enough to just stop the TX 
ring and then re-format it and re-start... Well, unfortunately, the way I 
structured the code, we can't do *just* that...

> 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.

    DLR.LBAx, you mean?
    Well, I was thinking of polling TCCR and CSR like the current 
ravb_stop_dma() does, but if that works...

> < Prepare new descriptors and start again >
> 4. Prepare new descriptors.

    That's where the cause for using the workqueue lies -- the descriptors are 
allocated with GFP_KERNEL, not GFP_ATOMIC... if you have time/desire to 
untangle all this, I'd appreciate it; else I'd have to work on this in my 
copious free time... :-)

> 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?

    Definitely better, yes...

> Best regards,
> Yoshihiro Shimoda

MBR, Sergei