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

[David Miller <davem@davemloft.net>]

From: Bjørn Mork <bjorn@mork.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@stericsson.com>
> Cc: Yauheni Kaliuta <y.kaliuta@gmail.com>
> Signed-off-by: Bjørn Mork <bjorn@mork.no>

Applied.