netdev - Re: net: fec: rx descriptor ring out of order

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Message-ID: <CAMeyCbhFfdONLEDYtqHxVZ59kBsH6vEaDBsvc5dWRinNY7RSgA@mail.gmail.com>
Date:   Thu, 12 Nov 2020 07:52:18 +0100
From:   Kegl Rohit <keglrohit@...il.com>
To:     Fabio Estevam <festevam@...il.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: net: fec: rx descriptor ring out of order

On Wed, Nov 11, 2020 at 11:18 PM Fabio Estevam <festevam@...il.com> wrote:
>
> On Wed, Nov 11, 2020 at 11:27 AM Kegl Rohit <keglrohit@...il.com> wrote:
> >
> > Hello!
> >
> > We are using a imx6q platform.
> > The fec interface is used to receive a continuous stream of custom /
> > raw ethernet packets. The packet size is fixed ~132 bytes and they get
> > sent every 250µs.
> >
> > While testing I observed spontaneous packet delays from time to time.
> > After digging down deeper I think that the fec peripheral does not
> > update the rx descriptor status correctly.
>
> What is the kernel version that you are using?

Sadly stuck at 3.10.108.
https://github.com/gregkh/linux/blob/v3.10.108/drivers/net/ethernet/freescale/fec_main.c
The rx queue status handling did not change much compared to 5.x. Only
the NAPI handling / clearing IRQs was changed more than once.
I also backported the newer NAPI handling style / clearing irqs not in
the irq handler but in napi_poll() => same issue.
The issue is pretty rare => To reproduce i have to reboot the system
every 3 min. Sometimes after 1~2min on the first, sometimes on the
~10th reboot it will happen.