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

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Date:   Mon, 21 Oct 2019 08:33:00 -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
Subject: Re: [PATCH] net: forcedeth: add xmit_more support

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.

> > 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.