netdev - RE: [PATCH net 2/3] net: cdc_mbim: send ZLP after max sized NTBs

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Message-ID: <2AC7D4AD8BA1C640B4C60C61C8E520154A8EE17D57@EXDCVYMBSTM006.EQ1STM.local>
Date:	Mon, 21 Jan 2013 17:31:37 +0100
From:	Alexey ORISHKO <alexey.orishko@...ricsson.com>
To:	Bjørn Mork <bjorn@...k.no>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc:	"linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
	Greg Suarez <gsuarez@...thmicro.com>,
	Oliver Neukum <oneukum@...e.de>,
	Alexey Orishko <alexey.orishko@...il.com>
Subject: RE: [PATCH net 2/3] net: cdc_mbim: send ZLP after max sized NTBs

> -----Original Message-----
> From: Bjørn Mork [mailto:bjorn@...k.no]
> 
> We normally avoid sending ZLPs by padding NTBs with a zero byte if the
> NTB is shorter than dwNtbOutMaxSize, resulting in a short USB packet
> instead of a ZLP.  But in the case where the NTB length is exactly
> dwNtbOutMaxSize and this is an exact multiplum of wMaxPacketSize, then
> we must send a ZLP.
> 
> This fixes an issue seen on a Sierra Wireless MC7710 device where the
> transmission would fail whenever we ended up padding the NTBs to max
> size.

If you add ZLP for NTBs of dwNtbOutMaxSize, you are heavily affecting CPU
load, increasing interrupt load by factor of 2 in high load traffic
scenario and possibly decreasing throughput for all other devices
which behaves correctly. 

Why should others pay for Sierra Wireless fault?

At least, do check VID/PID in usbnet and send ZLP only for malfunctioning 
devices.

Regards,
Alexey