netdev - Re: [PATCH net] net: cdc_mbim: send ZLP only for the specific buggy device

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Message-Id: <20130123.134726.112187775032509711.davem@davemloft.net>
Date:	Wed, 23 Jan 2013 13:47:26 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	bjorn@...k.no
Cc:	netdev@...r.kernel.org, linux-usb@...r.kernel.org, oneukum@...e.de,
	gsuarez@...thmicro.com, alexey.orishko@...ricsson.com,
	y.kaliuta@...il.com
Subject: Re: [PATCH net] net: cdc_mbim: send ZLP only for the specific
 buggy device

From: Bjørn Mork <bjorn@...k.no>
Date: Wed, 23 Jan 2013 11:57:02 +0100

> Reverting 328d7b8 and instead adding an exception for the
> Sierra Wireless MC7710.
> 
> commit 328d7b8 (net: cdc_mbim: send ZLP after max sized NTBs)
> added a workaround for an issue observed on one specific device.
> Concerns were raised that this workaround adds a performance
> penalty to all devices based on questionable, if not buggy,
> behaviour of a single device:
> 
>  "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."
> 
>  "The idea of NCM was to avoid extra ZLPs. If your transfer is exactly
>   dwNtbOutMaxSize, it's known, you can submit such request on the receiver
>   side and you do not need any EOT indicatation, so the frametime can be
>   used for useful data."
> 
> Adding a device specific exception to prevent the workaround from
> affecting well behaved devices.
> 
> The assumption here is that needing a ZLP is truly an *exception*.
> We do not yet have enough data to verify this.  The generic
> workaround in commit 328d7b8 should be considered acceptable despite
> the performance penalty if the exception list becomes a maintainance
> hassle.
> 
> Cc: Alexey ORISHKO <alexey.orishko@...ricsson.com>
> Cc: Yauheni Kaliuta <y.kaliuta@...il.com>
> Signed-off-by: Bjørn Mork <bjorn@...k.no>

Applied.
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