Re: Locking in the clk API, part 2: clk_prepare/clk_unprepare

On Tue, Feb 01, 2011 at 05:06:37PM +0000, Russell King - ARM Linux wrote:

On Tue, Feb 01, 2011 at 04:53:44PM +0100, Uwe Kleine-König wrote:

quoted

On Tue, Feb 01, 2011 at 03:24:58PM +0000, Russell King - ARM Linux wrote:

quoted

On Tue, Feb 01, 2011 at 04:18:46PM +0100, Uwe Kleine-König wrote:

quoted

yeah, didn't thought about multiple consumers, so (as Jeremy suggested)
the right thing is to sleep until CLK_BUSY is cleared.

A simpler way to write this is:

int clk_prepare(struct clk *clk)
{
	int ret = 0;

	mutex_lock(&clk->mutex);
	if (clk->prepared = 0)
		ret = clk->ops->prepare(clk);
	if (ret = 0)
		clk->prepared++;
	mutex_unlock(&clk->mutex);

	return ret;
}

But you cannot call this in atomic context when you know the clock is
already prepared.

So?  You're not _supposed_ to call it from any atomic context ever.

My motivation for a more complicated clk_prepare was to make clk_prepare
atomic when that's possible (i.e. when the clk is already prepared) and
call it before the enable callback in clk_enable.  Then everything
behaves nicely even if clk_enable is called from atomic context provided
that the clock was prepared before (or doesn't need to).

If a driver writer doesn't know that a certain clock might need to sleep
at some point he runs into an atomic might_sleep with your approach and
with mine.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
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