Messages in this thread | | | Date | Fri, 4 Feb 2011 11:21:20 +0100 | From | Uwe Kleine-König <> | Subject | Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare |
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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/
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