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    SubjectRe: [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 <> 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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