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    SubjectRe: Locking in the clk API, part 2: clk_prepare/clk_unprepare
    On Fri, Feb 04, 2011 at 05:54:24PM +0800, Richard Zhao wrote:
    > On Tue, Feb 01, 2011 at 09:24:09PM +0000, Russell King - ARM Linux wrote:
    > > You can't catch enable/disable imbalances in the prepare code, and you
    > > can't really catch them in the unprepare code either.
    > >
    > > Consider two drivers sharing the same struct clk. When the second driver
    > > prepares the clock, the enable count could well be non-zero, caused by
    > > the first driver. Ditto for when the second driver is removed, and it
    > > calls unprepare - the enable count may well be non-zero.
    > >
    > > The only thing you can check is that when the prepare count is zero,
    > > the enable count is also zero. You can also check in clk_enable() and
    > > clk_disable() that the prepare count is non-zero.
    >
    > but how can we check prepare count without mutex lock? Even if prepare count
    > is atomic_t, it can not guarantee the clock is actually prepared or unprepared.
    > So it's important for driver writer to maintain the call sequence.

    Forget atomic_t - it's the most abused type in the kernel. Just because
    something says its atomic doesn't make it so. In a use like this,
    atomic_t just buys you additional needless complexity with no benefit.

    Of course we can check the prepared count. What we can't do is check
    that it doesn't change concurrently - but that's something we can't do
    anyway.

    int clk_enable(struct clk *clk)
    {
    unsigned long flags;
    int ret = 0;

    if (clk) {
    if (WARN_ON(!clk->prepare_count))
    return -EINVAL;

    spin_lock_irqsave(&clk->lock, flags);
    if (clk->enable_count++ == 0)
    ret = clk->ops->enable(clk);
    spin_unlock_irqrestore(&clk->lock, flags);
    }
    return ret;
    }

    is entirely sufficient to catch the case of a single-use clock not being
    prepared before clk_enable() is called.

    We're after detecting drivers missing calls to clk_prepare(), we're not
    after detecting concurrent calls to clk_prepare()/clk_unprepare().


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