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

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Date:   Fri, 19 Jun 2020 05:46:15 +0000
From:   Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>
To:     Sergei Shtylyov <sergei.shtylyov@...il.com>,
        "sergei.shtylyov@...entembedded.com" 
        <sergei.shtylyov@...entembedded.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>
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>
Subject: RE: [PATCH/RFC] net: ethernet: ravb: Try to wake subqueue instead of
 stop on timeout

Hello!

> From: Sergei Shtylyov, Sent: Monday, June 15, 2020 5:13 PM
> 
> Hello!
> 
> On 15.06.2020 8:58, 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...


By the way, there is other topic though, I'm thinking we should not call
ravb_{close,open}() in ravb_{suspend,resume}() because this is possible to
a race condition between ifconfig up/down and system suspend/resume.
In other words, I'm thinking this should not free/reallocate
dma memory while netif_running is true. But, what do you think?

Best regards,
Yoshihiro Shimoda

> [...]
> 
> MBR, Sergei