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

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Date:   Tue, 22 Oct 2019 13:32:35 +0800
From:   Zhu Yanjun <yanjun.zhu@...cle.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     rain.1986.08.12@...il.com, davem@...emloft.net,
        netdev@...r.kernel.org
Subject: Re: [PATCH] net: forcedeth: add xmit_more support


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.

>
>>> Also the labels should be lower case.
>> This patch passes checkpatch.pl. It seems that "not lower case" is not a
>> problem?
>>
>> If you think it is a problem, please show me where it is defined.
> Look at this driver and at other kernel code. Labels are lower case,
> upper case is for constants and macros.

It sounds reasonable. I will send V2 to fix this problem. Thanks.

Zhu Yanjun

>