Messages in this thread | | | Date | Fri, 4 Feb 2011 17:54:24 +0800 | From | Richard Zhao <> | Subject | Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare |
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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.
Thanks Richard > > 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. > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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