netdev - Re: [PATCH] net: forcedeth: add xmit

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Date:   Tue, 22 Oct 2019 08:40:19 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Zhu Yanjun <yanjun.zhu@...cle.com>
Cc:     rain.1986.08.12@...il.com, davem@...emloft.net,
        netdev@...r.kernel.org, Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH] net: forcedeth: add xmit_more support

On Tue, 22 Oct 2019 13:32:35 +0800, Zhu Yanjun wrote:
> On 2019/10/21 23:33, Jakub Kicinski wrote:
> > On Mon, 21 Oct 2019 17:56:06 +0800, Zhu Yanjun wrote:  
> >> On 2019/10/19 6:48, Jakub Kicinski wrote:  
> >>> On Fri, 18 Oct 2019 06:01:25 -0400, Zhu Yanjun wrote:  
> >>>> This change adds support for xmit_more based on the igb commit 6f19e12f6230
> >>>> ("igb: flush when in xmit_more mode and under descriptor pressure") and
> >>>> commit 6b16f9ee89b8 ("net: move skb->xmit_more hint to softnet data") that
> >>>> were made to igb to support this feature. The function netif_xmit_stopped
> >>>> is called to check if transmit queue on device is currently unable to send
> >>>> to determine if we must write the tail because we can add no further
> >>>> buffers.
> >>>> When normal packets and/or xmit_more packets fill up tx_desc, it is
> >>>> necessary to trigger NIC tx reg.  
> >>> Looks broken. You gotta make sure you check the kick on _every_ return
> >>> path. There are 4 return statements in each function, you only touched
> >>> 2.  
> >> In nv_start_xmit,
> >>
> >> [...]
> >>
> >> The above are dma_mapping_error. It seems that triggering NIC HW xmit is
> >> not needed.
> >>
> >> So when "tx_desc full" error, HW NIC xmit is triggerred. When
> >> dma_mapping_error,
> >>
> >> NIC HW xmit is not triggerred.
> >>
> >> That is why only 2 "return" are touched.  
> > Imagine you have the following sequence of frames:
> >
> > 	skbA  | xmit_more() == true
> > 	skbB  | xmit_more() == true
> > 	skbC  | xmit_more() == true
> > 	skbD  | xmit_more() == false
> >
> > A, B, and C got queued successfully but the driver didn't kick the
> > queue because of xmit_more(). Now D gets dropped due to a DMA error.
> > Queue never gets kicked.  
> 
> DMA error is a complicated problem. We will delve into this problem later.
> 
>  From the above commit log, this commit is based on the igb commit 
> 6f19e12f6230
> ("igb: flush when in xmit_more mode and under descriptor pressure") and
> commit 6b16f9ee89b8 ("net: move skb->xmit_more hint to softnet data").
> 
> It seems that the 2 commits did not consider the DMA errors that you 
> mentioned.

Then igb is buggy, too.