On Tue, Jan 11, 2011 at 11:39:29AM +0100, Uwe Kleine-K?nig wrote:

A quick look into Digi's BSP (digiEL-5.0) shows they implemented
something I suggested earlier here:

	static int clk_enable_haslocknchange(struct clk *clk)
	{
		int ret = 0;

		assert_spin_locked(&clk_lock);
		BUG_ON(!test_bit(CLK_FLAG_CHANGESTATE, &clk->flags));

		if (clk->usage++ == 0) {
			if (clk->parent) {
				ret = clk_enable_haslock(clk->parent);
				if (ret)
					goto err_enable_parent;
			}

			spin_unlock(&clk_lock);

			if (clk->endisable)
				ret = clk->endisable(clk, 1);

			spin_lock(&clk_lock);

			if (ret) {
				clk_disable_parent_haslock(clk);
	err_enable_parent:
				clk->usage = 0;
			}
		}

		return ret;
	}

	static int clk_enable_haslock(struct clk *clk)
	{
		int ret;
		assert_spin_locked(&clk_lock);
		if (__test_and_set_bit(CLK_FLAG_CHANGESTATE, &clk->flags))
			return -EBUSY;

		ret = clk_enable_haslocknchange(clk);

		clear_bit(CLK_FLAG_CHANGESTATE, &clk->flags);

		return ret;
	}

	int clk_enable(struct clk *clk)
	{
		...
		spin_lock_irqsave(&clk_lock, flags);
		ret = clk_enable_haslock(clk);
		spin_unlock_irqrestore(&clk_lock, flags);
		return ret;
	}


I think the idea is nice.  At least it allows with a single lock to
implement both, sleeping and atomic clks without the need to mark the
atomicity in a global flag.

It doesn't.  clk_enable() here can still end up trying to sleep when
it's called from IRQ context - the code doesn't solve that.  All it
means is that the intermediate code doesn't care whether clk->endisable
ends up sleeping or not.

What it does do is return -EBUSY if there are two concurrent attempts
to enable the same clock.  How many drivers today deal sanely with
such an error from clk_enable(), and how many would just fail their
probe() call on such an occurance?