linux-kernel - Re: yield() in i2c non-happy paths hits BUG under -rt patch

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Date:	Wed, 18 Nov 2009 21:36:48 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jean Delvare <khali@...ux-fr.org>
cc:	Leon Woestenberg <leon.woestenberg@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	Sven-Thorsten Dietrich <sven@...bigcorporation.com>,
	linux-i2c@...r.kernel.org,
	rt-users <linux-rt-users@...r.kernel.org>,
	"Ben Dooks (embedded platforms)" <ben-linux@...ff.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: yield() in i2c non-happy paths hits BUG under -rt patch

On Wed, 18 Nov 2009, Jean Delvare wrote:

> On Wed, 18 Nov 2009 17:28:53 +0100, Leon Woestenberg wrote:
> > On Wed, Nov 18, 2009 at 2:05 AM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> > > Our timers are very efficient and some day we will need to make jiffies a
> > > function and stop the timer ticking for best performance. At that point
> > > timers are probably the most efficient way to do much of this.
> >
> > The problem with I2C bitbanged is the stringent timing, we need a way
> > to have fine-grained sleeping
> > mixed with real-time tasks in order to make this work.
> 
> FWIW, the problem that was initially reported has nothing to do with
> this. i2c-algo-bit used mdelay() during transactions, not yield().
> yield() is used only in once place, _between_ transactions attempts.
> There are no strict timing constraints there.

That still does not explain why yield() is necessary _between_ the
transaction attempts.

That code is fully preemptible, otherwise you could not call
yield(). And as I said before nobody even noticed that the yield()
default implementation was changed to a NOOP by default in the
scheduler.

Thanks,

	tglx
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