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    SubjectRe: Locking in the clk API, part 2: clk_prepare/clk_unprepare
    On Tue, Feb 01, 2011 at 12:59:11PM -0800, Stephen Boyd wrote:
    > On 02/01/2011 07:24 AM, Russell King - ARM Linux wrote:
    > > I'd also be tempted at this stage to build-in a no-op dummy clock,
    > > that being the NULL clk:
    > >
    > > int clk_prepare(struct clk *clk)
    > > {
    > > int ret = 0;
    > >
    > > if (clk) {
    > > mutex_lock(&clk->mutex);
    > > if (clk->prepared == 0)
    > > ret = clk->ops->prepare(clk);
    > > if (ret == 0)
    > > clk->prepared++;
    > > mutex_unlock(&clk->mutex);
    > > }
    > >
    > > return ret;
    > > }
    > I'm afraid this will hide enable/disable imbalances on some targets and
    > then expose them on others. Maybe its not a big problem though since
    > this also elegantly handles the root(s) of the tree.

    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.

    If you want tigher checking than that, you need to somehow identify and
    match up the clk_prepare/clk_enable/clk_disable/clk_unprepare calls from
    a particular driver instance. Addresses of the functions don't work as
    you can't be certain that driver code will be co-located within a certain
    range. Adding an additional argument to these functions which is driver
    instance specific seems to be horrible too.

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      Last update: 2011-02-01 22:27    [W:0.021 / U:11.300 seconds]
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