netdev - Re: [PATCH net,stable] net: huawei_cdc_ncm: increase command buffer size

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Message-Id: <20140621.193404.1524985182957166239.davem@davemloft.net>
Date:	Sat, 21 Jun 2014 19:34:04 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	bjorn@...k.no
Cc:	netdev@...r.kernel.org, linux-usb@...r.kernel.org,
	mrkiko.rs@...il.com
Subject: Re: [PATCH net,stable] net: huawei_cdc_ncm: increase command
 buffer size

From: Bjørn Mork <bjorn@...k.no>
Date: Wed, 18 Jun 2014 14:21:24 +0200

> Messages from the modem exceeding 256 bytes cause communication
> failure.
> 
> The WDM protocol is strictly "read on demand", meaning that we only
> poll for unread data after receiving a notification from the modem.
> Since we have no way to know how much data the modem has to send,
> we must make sure that the buffer we provide is "big enough".
> Message truncation does not work. Truncated messages are left unread
> until the modem has another message to send.  Which often won't
> happen until the userspace application has given up waiting for the
> final part of the last message, and therefore sends another command.
> 
> With a proper CDC WDM function there is a descriptor telling us
> which buffer size the modem uses. But with this vendor specific
> implementation there is no known way to calculate the exact "big
> enough" number.  It is an unknown property of the modem firmware.
> Experience has shown that 256 is too small.  The discussion of
> this failure ended up concluding that 512 might be too small as
> well. So 1024 seems like a reasonable value for now.
> 
> Fixes: 41c47d8cfd68 ("net: huawei_cdc_ncm: Introduce the huawei_cdc_ncm driver")
> Cc: Enrico Mioso <mrkiko.rs@...il.com>
> Reported-by: Dan Williams <dcbw@...hat.com>
> Signed-off-by: Bjørn Mork <bjorn@...k.no>

Applied and queued up for -stable, thanks!
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