lkml.org 
[lkml]   [2011]   [Feb]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: Locking in the clk API, part 2: clk_prepare/clk_unprepare
    Hello Richard,

    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:
    > > 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.
    > 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.
    I happily point out that the prepare_count needs to be protected by a
    spinlock and you need a flag that signals a prepare or unprepare is
    currently running.

    SCNR
    Uwe

    --
    Pengutronix e.K. | Uwe Kleine-König |
    Industrial Linux Solutions | http://www.pengutronix.de/ |
    --
    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: 2011-02-04 11:23    [W:0.024 / U:9.164 seconds]
    ©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site