Messages in this thread | | | Date | Fri, 4 Feb 2011 10:48:32 +0000 | From | Russell King - ARM Linux <> | Subject | Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare |
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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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