RE: [PATCH] net: renesas: ravb: Fix a stuck issue when a lot of frames are received

[Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>]

Hello!

> From: Sergei Shtylyov, Sent: Tuesday, May 18, 2021 4:36 AM
> 
> On 5/10/21 1:29 PM, Yoshihiro Shimoda wrote:
> 
> >>>     Posting a review of the already commited (over my head) patch. It would have
> >>> been appropriate if the patch looked OK but it's not. :-/
> >>>
> >>>> When a lot of frames were received in the short term, the driver
> >>>> caused a stuck of receiving until a new frame was received. For example,
> >>>> the following command from other device could cause this issue.
> >>>>
> >>>>      $ sudo ping -f -l 1000 -c 1000 <this driver's ipaddress>
> >>>
> >>>     -l is essential here, right?
> >
> > Yes.
> >
> >>>     Have you tried testing sh_eth sriver like that, BTW?
> >>
> >>     It's driver! :-)
> >
> > I have not tried testing sh_eth driver yet. I'll test it after I got an actual board.
> 
>    Now you've got it, let's not rush forth with the fix this time.

I sent a report yesterday:
https://patchwork.kernel.org/project/linux-renesas-soc/patch/20210421045246.215779-1-yoshihiro.shimoda.uh@renesas.com/#24181167

> >>>> The previous code always cleared the interrupt flag of RX but checks
> >>>> the interrupt flags in ravb_poll(). So, ravb_poll() could not call
> >>>> ravb_rx() in the next time until a new RX frame was received if
> >>>> ravb_rx() returned true. To fix the issue, always calls ravb_rx()
> >>>> regardless the interrupt flags condition.
> >>>
> >>>     That bacially defeats the purpose of IIUC...
> >>                                            ^ NAPI,
> >>
> >>     I was sure I typed NAPI here, yet it got lost in the edits. :-)
> >
> > I could not understand "that" (calling ravb_rx() regardless the interrupt
> > flags condition) defeats the purpose of NAPI. According to an article on
> > the Linux Foundation wiki [1], one of the purpose of NAPI is "Interrupt mitigation".
> 
>    Thank you for the pointer, BTW! Would have helped me with enabling NAPI in sh_eth
> (and ravb) drivers...
> 
> > In poll(), the interrupts are already disabled, and ravb_rx() will check the
> > descriptor's status. So, this patch keeps the "Interrupt mitigation" IIUC.
> 
>    I think we'll still have the short race window, described in section 5.1
> of this doc. So perhaps what we should do is changing the order of the code in
> the poll() method, not eliminating the loops totally. Thoughts?

The ravb hardware acts as "non-level sensitive IRQs". However, fortunately,
the hardware can set an interrupt flag even if the interrupt is masked.
So, I don't think this patch have any race window.

Best regards,
Yoshihiro Shimoda

> > [1]
> >
> https://jpn01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwiki.linuxfoundation.org%2Fnetworking%2Fnapi&amp;d
> ata=04%7C01%7Cyoshihiro.shimoda.uh%40renesas.com%7C0102c1f2995947bcca1608d9196af978%7C53d82571da1947e49cb4625a166a4a
> 2a%7C0%7C0%7C637568769530134169%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6
> Mn0%3D%7C1000&amp;sdata=47kgAmI3d%2Fz%2BHunT0a8bzHRRQk1VdnxRETSExLkTrdI%3D&amp;reserved=0
> >
> > Best regards,
> > Yoshihiro Shimoda
> 
> MBR, Sergei