lkml.org 
[lkml]   [2007]   [Nov]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [patch 2.6.24-rc2 1/3] generic gpio -- gpio_chip support
Date
On Wednesday 14 November 2007, Haavard Skinnemoen wrote:
> On Wed, 14 Nov 2007 00:37:57 -0800
> David Brownell <david-b@pacbell.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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-11-15 08:21    [W:0.203 / U:0.248 seconds]
©2003-2014 Jasper Spaans. Advertise on this site