Subject: Re: Setting negative value in dBm for power output (txpower)
From: Johannes Berg <johannes@sipsolutions.net>
To: =?ISO-8859-1?Q?G=E1bor?= Stefanik <netrolller.3d@gmail.com>
Cc: Patryk =?UTF-8?Q?=C5=BB=C3=B3=C5=82towski?=
	<patryk.zoltowski@gmail.com>, linux-wireless@vger.kernel.org
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Date: Sat, 22 Oct 2011 15:30:04 +0200
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On Sat, 2011-10-22 at 00:02 +0200, Gábor Stefanik wrote:
> 2011/10/21 Patryk Żółtowski <patryk.zoltowski@gmail.com>:
> > I'm doing a research and I'm wondering if it's possible to hack
> > wireless drivers to allow setting  txpower to smaller value than 1mW
> > (e.g. 0.1mW = -10dBM, or even 0.01mW). I've checked source code and in
> > net/mac80211/cfg.c there is the following check:
> >
> >  if (mbm < 0 || (mbm % 100))
> >                     return -EOPNOTSUPP;
> 
> The "mbm % 100" part is even weirder; why do we even use millibels if
> we only allow whole-decibel values?

This is really just historic -- nobody has so far cared about TX power
enough to clean up the APIs for it etc. The distinction between
"automatic", "fixed" and "limited" (auto <= some value) is also not
really done very well here.

johannes