netdev - Packet loss when txqueuelen is zero

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Message-ID: <53AEA00F.6060500@lategoodbye.de>
Date:	Sat, 28 Jun 2014 12:59:27 +0200
From:	Stefan Wahren <info@...egoodbye.de>
To:	netdev@...r.kernel.org
CC:	arnd@...db.de
Subject: Packet loss when txqueuelen is zero

Hi,

i'm new to Linux network driver development and currently i want to port 
the QCA7000 network driver to mainline [1]. I concentrate my tests on tx 
buffering since my last QCA7000 patch RFC [2]. Now i've found a test 
scenario which leads to packet loss:

host A          Powerline       host B
(QCA7000)       Ethernet        Ethernet
192.168.1.3     adaptor         192.168.1.5
|------------------|---------------|
      Homeplug         Ethernet
      10 Mbit          100 Mbit

1. Reduce the txqueuelen from 100 (default value) to 0
2. Run ping in flood mode on host A to host B

ping -c 200 -s 10000 -f 192.168.1.5

3. ping reports a high packet loss

Additional information:
- QCA7000 network driver has a tx ring size of 10 packets
- the packet loss doesn't appear when txqueuelen is 100

Here are my questions:

Is the packet loss a expected result for this scenario?
If not what could be the cause of the packet loss?

[1] - https://github.com/I2SE/qca7000/tree/linux-mainline
[2] - http://www.spinics.net/lists/netdev/msg280751.html
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