Messages in this thread | | | Date | Tue, 1 Feb 2011 13:15:12 +0000 | From | Russell King - ARM Linux <> | Subject | Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare |
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On Tue, Feb 01, 2011 at 11:54:49AM +0100, Uwe Kleine-König wrote: > Alternatively don't force the sleep in clk_prepare (e.g. by protecting > prepare_count by a spinlock (probably enable_lock)) and call clk_prepare > before calling clk->ops->enable?
That's a completely bad idea. I assume you haven't thought about this very much.
There's two ways I can think of doing what you're suggesting:
int clk_prepare(struct clk *clk) { unsigned long flags; int ret = 0;
might_sleep();
spin_lock_irqsave(&clk->enable_lock, flags); if (clk->prepare_count++ == 0) ret = clk->ops->prepare(clk); spin_unlock_irqrestore(&clk->enable_clock, flags);
return ret; }
The problem is that clk->ops->prepare() is called in a non-sleepable context. So this breaks the whole idea of clk_prepare(), and so isn't a solution.
The other solution is:
int clk_prepare(struct clk *clk) { unsigned long flags; int ret = 0; bool first;
might_sleep();
spin_lock_irqsave(clk->enable_lock, flags); first = clk->prepare_count++ == 0; spin_unlock_irqrestore(clk->enable_clock, flags);
if (first) ret = clk->ops->prepare(clk);
return ret; }
The problem with this is that you now don't have any sane locking on the prepare callback, and the circumstances under which it's called are very indefinite. For example, consider a preempt-enabled system:
thread 1 thread 2 prepare_count clk_prepare 0 clk->prepare_count++ 1 <thread switch> clk_prepare 1 clk->prepare_count++ 2 clk_prepare returns 2
clk_enable 2 <explodes as clock is not prepared> <thread switch> clk->ops->prepare(clk)
So really, what you're suggesting is completely broken. -- 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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