linux-kernel - Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio

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Date:	Tue, 13 Nov 2007 12:46:56 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Brownell <david-b@...bell.net>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Florian Fainelli <florian.fainelli@...ecomint.eu>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio_chip support

On Tue, 13 Nov 2007 11:22:45 -0800 David Brownell <david-b@...bell.net> wrote:

> On Tuesday 13 November 2007, Ingo Molnar wrote:
> > 
> > * David Brownell <david-b@...bell.net> wrote:
> > 
> > > > > I speculate that either the design has changed (without fanfare), 
> > > > > or else that stuff is in RT kernels and has not yet gone upstream.
> > > > 
> > > > Well whatever.  We shouldn't have to resort to caller-side party 
> > > > tricks like this to get acceptable performance.
> > > 
> > > I'd be happy if, as originally presented, it were possible to just 
> > > pass a raw_spinlock_t to spin_lock_irqsave() and friends.
> > 
> > that's a spinlock type abstraction of PREEMPT_RT, not of mainline.
> 
> Any reason that stuff shouldn't move into mainline?
> 
> 
> > 	 Why do you want to use raw_spinlock_t?
> 
> Already answered elsewhere in this thread ...

Can't say I really understood the answer.  I don't think we actually know
where all of this extra time is being spent?

> I'll highlight the
> point that such bitops shouldn't be preemption points.

Disagree.  *everything* should be a preemption point.  For
internal-implementation details we do need to disable preemtion sometimes
(to prevent deadlocks and to protect per-cpu resources).  But those
preemption-off periods should be minimised.

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