linux-kernel - Re: [PATCH -tip] Switch arm defconfig files from CONFIG_CLASSIC_RCU to CONFIG_TREE

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Date:	Fri, 26 Dec 2008 19:35:12 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, akpm@...ux-foundation.org, ralf@...ux-mips.org,
	kernel@...tstofly.org, hskinnemoen@...el.com, cooloney@...nel.org,
	tony.luck@...el.com, geert@...ux-m68k.org, zippel@...ux-m68k.org,
	jwboyer@...ux.vnet.ibm.com, mporter@...nel.crashing.org,
	grant.likely@...retlab.ca, paulus@...ba.org,
	benh@...nel.crashing.org, dhowells@...hat.com,
	yasutake.koichi@...panasonic.com, lethal@...ux-sh.org,
	hpa@...or.com
Subject: Re: [PATCH -tip] Switch arm defconfig files from CONFIG_CLASSIC_RCU to CONFIG_TREE_RCU.

On Thu, Dec 25, 2008 at 11:23:32AM -0800, Paul E. McKenney wrote:
> On Thu, Dec 25, 2008 at 10:33:01AM +0100, Sam Ravnborg wrote:
> > On Thu, Dec 25, 2008 at 09:36:11AM +0100, Ingo Molnar wrote:
> > > 
> > > below is the fix for another small buglet that hits architectures that do 
> > > not include kernel/Kconfig.preempt.
> > 
> > What is needed to let all architectures include Kconfig.preempt?
> > We should not advertise preemption if not supported,
> > but we could do this by a simple Kconfig symbol:
> > 
> > config HAVE_PREEMPT
> > 	bool
> > 
> > And let all architectures that supports preemption do:
> > 
> > config "ARCH"
> > 	select HAVE_PREEMPT
> > 
> > But maybe there is a simpler solution
> 
> The idea being to make things like PREEMPT then depend on HAVE_PREEMPT?
Yes - exactly.

So an arch can say that is includes support for preemption
using the following Kconfig snippet:

config X86
	select HAVE_PREEMPT

And then in the spot where we have a prompt we do:

config PREEMPT
	bool "Do you want preemption?"
	depends on HAVE_PREEMPT


Thne we will only present this option to the user
for the architectures where it has indicated preemption
support by selecting HAVE_PREEMPT.

It is on purpose that HAVE_PREEMPT is a single boolean
with no prompt or anything so we do not hit
any of the pitfalls of using select.

We use a similar pattern in many places today.

	Sam
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