From owner-freebsd-current@FreeBSD.ORG  Thu Aug 23 17:21:29 2012
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: current@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52])
	by hub.freebsd.org (Postfix) with ESMTP id 0BD49106566B;
	Thu, 23 Aug 2012 17:21:29 +0000 (UTC)
	(envelope-from luigi@onelab2.iet.unipi.it)
Received: from onelab2.iet.unipi.it (onelab2.iet.unipi.it [131.114.59.238])
	by mx1.freebsd.org (Postfix) with ESMTP id B9E5C8FC08;
	Thu, 23 Aug 2012 17:21:28 +0000 (UTC)
Received: by onelab2.iet.unipi.it (Postfix, from userid 275)
	id 11C177300A; Thu, 23 Aug 2012 19:40:37 +0200 (CEST)
Date: Thu, 23 Aug 2012 19:40:37 +0200
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: Attilio Rao <attilio@freebsd.org>
Message-ID: <20120823174037.GA4820@onelab2.iet.unipi.it>
References: <20120823145420.GB3103@onelab2.iet.unipi.it>
	<CAJ-FndCSfXvEmYjuEi985JPoiZYStOaqdeYsjyepHS8C55Szkw@mail.gmail.com>
	<20120823160543.GD3391@onelab2.iet.unipi.it>
	<CAJ-FndB1PQ+8rQLS+_Ac-i=xOckTv+7aV4Ma9KzJuhUe-DT8Pg@mail.gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAJ-FndB1PQ+8rQLS+_Ac-i=xOckTv+7aV4Ma9KzJuhUe-DT8Pg@mail.gmail.com>
User-Agent: Mutt/1.4.2.3i
Cc: current@freebsd.org
Subject: Re: TUNABLE_INT vs TUNABLE_INT_FETCH
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
	<freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>, 
	<mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 23 Aug 2012 17:21:29 -0000

On Thu, Aug 23, 2012 at 04:55:05PM +0100, Attilio Rao wrote:
> On Thu, Aug 23, 2012 at 5:05 PM, Luigi Rizzo <rizzo@iet.unipi.it> wrote:
> > On Thu, Aug 23, 2012 at 03:52:56PM +0100, Attilio Rao wrote:
> >> On 8/23/12, Luigi Rizzo <rizzo@iet.unipi.it> wrote:
> >> > Hi,
> >> > I am a bit unclear on what are the pros and cons of using
> >> > TUNABLE_INT vs TUNABLE_INT_FETCH within a device driver.
> >>
> >> TUNABLE_INT is basically the "statically initializer" version of
> >> TUNABLE_INT_FETCH.
> >> In short terms, you will use TUNABLE_INT_FETCH() in normal functions,
> >> while TUNABLE_INT() in data declaration.
> >
> > The thing is, do we need the data declaration at all ?
> 
> What do you mean with "data declaration"?

i am using your words :)

> We need to mimic a "static initialization" usage, so what we do is to
> use the first SYSINIT() family available (SI_SUB_TUNABLES). You also
> need the env to look for and the static variable to initialize, so for
> SYSINIT's sake you need to pack them up in a single argument.

To explain: as i understand it, kenv variables are created and stored
(presumably as strings) even if not explicitly declared as
TUNABLE_*(). The role of the SYSINIT() block is presumably to
copy the values of interesting entries into C variables
(i suppose at boot time, and perhaps even when kenv runs).
This should be the 'static initialization' you mention.

I think there is only a limited number of cases where this makes sense,
in most circumstances the variables passed through the environment
should be read explictly via TUNABLE_INT_FETCH() to make sure that
they do not change in unexpected moments.

This is why in the documentation I'd probably suggest to use
the TUNABLE_*_FETCH() variant unless you are really really
sure that the variable can change at any time as a result of
a kenv call (or make it clear that it *will not* reflect
the kenv result, i am not sure how it works).

Anyways, thanks for the clarificaiton, now i know what i need to use.

cheers
luigi