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

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Date:	Wed, 14 Nov 2007 22:50:17 -0800
From:	David Brownell <david-b@...bell.net>
To:	Haavard Skinnemoen <hskinnemoen@...el.com>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Florian Fainelli <florian.fainelli@...ecomint.eu>
Subject: Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio_chip support

On Wednesday 14 November 2007, Haavard Skinnemoen wrote:
> On Wed, 14 Nov 2007 00:37:57 -0800
> David Brownell <david-b@...bell.net> wrote:
> 
> > Although another point is related to "trivial":  the data
> > is being protected through an operation too trivial to be
> > worth paying for any of that priority logic.
> 
> But isn't there any way we can remove the lock from the fast path
> altogether? What is it really protecting?

The integrity of the table.  Entries can be added and removed
(both operations being *RARE* which is good!) at any time.

> Since this is the code that runs under the lock

No, there's more than that.  This is what runs under it in
the hot paths, yes, but the gpio request/free paths do
more work than this.  (That includes direction setting,
since that can be an implicit request.)

> (excluding the "extra checks" case):
> 
> +static inline struct gpio_chip *gpio_to_chip(unsigned gpio)
> +{
> +	return chips[gpio / ARCH_GPIOS_PER_CHIP];
> +}
> 
> I'd say it protects against chips being removed in the middle of the
> operation. However, this comment says that chips cannot be removed
> while any gpio on it is requested:
> 
> +/* gpio_lock protects the table of chips and to gpio_chip->requested.
> + * While any gpio is requested, its gpio_chip is not removable.  It's
> + * a raw spinlock to ensure safe access from hardirq contexts, and to
> + * shrink bitbang overhead:  per-bit preemption would be very wrong.
> + */
>
> And since we drop the lock before calling the actual get/set bit
> operation, we would be screwed anyway if the chip was removed during
> the call to __gpio_set_value(). So what does the lock really buy us?

The get/set bit calls are the hot paths.  Locking on those paths
buys us a consistent locking policy, which is obviously correct.
It's consistent with the request/free paths.

But I think what you're suggesting is that the "requested" flag
is effectively a long-term lock, so grabbing the spinlock on
those paths is not necessary.  Right?

Hmm ... that makes some sense.  I hadn't started out thinking of
that "requested" flag as a lock bit, but in fact that's what it
ended up becoming.

Removing the spinlock from those paths -- at least in the "no
extra checks case" -- would let us avoid all this flamage about
whether raw spinlocks are ever OK.

I think I forsee a patch coming...

- Dave
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