Re: [PATCH] net: forcedeth: add xmit_more support

[Zhu Yanjun <yanjun.zhu@oracle.com>]

On 2019/10/22 23:40, Jakub Kicinski wrote:
> 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.

Then if igb problem is fixed, I will follow.;-)

Zhu Yanjun

>