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

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Date:	Tue, 13 Nov 2007 23:25:53 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	David Brownell <david-b@...bell.net>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Florian Fainelli <florian.fainelli@...ecomint.eu>,
	Haavard Skinnemoen <hskinnemoen@...el.com>
Subject: Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio_chip support

On Wednesday 14 November 2007 06:22, David Brownell 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.

Even when you're talking about the -rt tree, I suspect you really
shouldn't be using raw spinlocks, right? I mean, if you have a
timing critical operation, then you should ensure you have priorities
set correctly so that you simply don't get preempted.

By using a raw_spinlock_t, you're saying that you're more important
than anyone else (for the period of the critical section) including
processes which the user has explicitly set to a higher priority.

If you get preempted, then you should be happy, because <standard
RTOS nuclear power plant scenario> didn't occur.

> Any reason that stuff shouldn't move into mainline?

This sort of raw_spinlock_t arms race throughout drivers/ would be
a huge reason not to move it into mainline.
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