netdev - Re: [RFC][PATCH] usbnet: Fix tx_bytes statistic running backward in cdc

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Message-ID: <1425028679.12236.6.camel@linux-0dmf.site>
Date:	Fri, 27 Feb 2015 10:17:59 +0100
From:	Oliver Neukum <oliver@...kum.org>
To:	Bjørn Mork <bjorn@...k.no>
Cc:	Ben Hutchings <ben.hutchings@...ethink.co.uk>,
	netdev@...r.kernel.org,
	ct-linux-kernel <linux-kernel@...ethink.co.uk>,
	linux-usb@...r.kernel.org, Sami Farin <hvtaifwkbgefbaei@...il.com>,
	Aleksander Morgado <aleksander@...ksander.es>
Subject: Re: [RFC][PATCH] usbnet: Fix tx_bytes statistic running backward in
 cdc_ncm

On Fri, 2015-02-27 at 10:12 +0100, Bjørn Mork wrote:
> Ben Hutchings <ben.hutchings@...ethink.co.uk> writes:
> 
> > cdc_ncm disagrees with usbnet about how much framing overhead should
> > be counted in the tx_bytes statistics, and tries 'fix' this by
> > decrementing tx_bytes on the transmit path.  But statistics must never
> > be decremented except due to roll-over; this will thoroughly confuse
> > user-space.  Also, tx_bytes is only incremented by usbnet in the
> > completion path.
> >
> > Fix this by requiring drivers that set FLAG_MULTI_FRAME to set a
> > tx_bytes delta along with the tx_packets count.
> >
> > Signed-off-by: Ben Hutchings <ben.hutchings@...ethink.co.uk>
> > ---
> > I noticed this bug while trying to fix the tx_packets statistic in asix.
> > It depends on the patch I just sent for that.  I don't have any hardware
> > to test this with, or any need to make it work.  If you want this fix,
> > please test and re-submit it yoursef.
> 
> I tested this on an MBIM device, and it worked perfectly as-is.  Please
> submit it without the RFC prefix.  This fixes a real and reported
> problem with the cdc_ncm driver, so I'd claim it's "net" material along
> with a stable Cc and
> 
> Fixes: beeecd42c3b4 ("net: cdc_ncm/cdc_mbim: adding NCM protocol statistics")
> 
> But you and David decide that, of course...
> 
> 
> Fixing this was actually on my TODO-list after a recent report showing
> one of the problems with the previous hack: The decremented tx_bytes
> counter was never corrected if the URB transmission failed, possibly
> ending up with a negative (i.e. very large since it is unsigned)
> tx_bytes counter.
> 
> Thanks a lot.  I didn't know how to do this without introducing a new
> callback or something.  Your solutions is very nice, and so obvious when
> I see it.  Just brilliant :-)
> 
> Tested-by: Bjørn Mork <bjorn@...k.no>
> 
> And you might also want (if this is OK for Sami):
> 
> Reported-by: Sami Farin <hvtaifwkbgefbaei@...il.com>
Acked-by: Oliver Neukum <oliver@...kum.org>

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