linux-kernel - Re: 31-rc3-mmotm0716 - wonky wireless statistics..

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Message-Id: <1248851953.13742.12.camel@johannes.local>
Date:	Wed, 29 Jul 2009 09:19:12 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	Zhu Yi <yi.zhu@...el.com>
Cc:	"Valdis.Kletnieks@...edu" <Valdis.Kletnieks@...edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>
Subject: Re: 31-rc3-mmotm0716 - wonky wireless statistics..

On Wed, 2009-07-29 at 09:31 +0800, Zhu Yi wrote:

> > # cat /proc/net/wireless
> > Inter-| sta-|   Quality        |   Discarded packets               | Missed | WE
> >  face | tus | link level noise |  nwid  crypt   frag  retry   misc | beacon | 22
> >  wlan0: 0000   63.  -47.  -256        0      0      0      0      0        0
> >  wlan1: 0000    0     0     0        0      0      0      0      0        0
> >  wlan2: 0000    0     0     0        0      0      0      0      0        0
> > 
> > Where did that -256 for "noise" come from?  It ends up confusing the
> > gkrellm-wifi plugin, it reports a S/N of 219dB ;)
> 
> This should be related to http://git.kernel.org/?p=linux/kernel/git/linville/wireless-testing.git;a=commit;h=fdb897ea5c77836f890033d563336ab0c94c9222
> 
> But I don't think this is an ABI broken. User space should parse the
> trailing '.' to see if a field is updated or not because there are cards
> not capable to report hardware noise level anyway. Johannes?

So I had to look at the code to figure out wtf is going on.

Then the value shown is calculated as:
	((__s32) stats->qual.noise) - ((stats->qual.updated & IW_QUAL_DBM) ? 0x100 : 0)

and since we have dBm for the signal strength (and dBm is "all or
nothing") you get -256 since we don't fill noise right now.

However, it goes on like this:
	stats->qual.updated & IW_QUAL_NOISE_UPDATED ? '.' : ' '

so you're right -- the tool shouldn't be using the value unless followed
by a dot (".").

Thus, your analysis is correct as far as I can tell -- it's a userspace
bug and your userspace just happened to work correctly with this card,
but would have been broken with almost all other wireless cards.

johannes

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