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SubjectRe: Locking in the clk API, part 2: clk_prepare/clk_unprepare
On Tue, Feb 01, 2011 at 01:15:12PM +0000, Russell King - ARM Linux wrote:
> 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.
Right, but I thought it a bit further than you did. Like the following:

int clk_prepare(struct clk *clk)
{
int ret = 0, first;
unsigned long flags;

spin_lock_irqsave(&clk->enable_lock, flags);
if (clk->flags & CLK_BUSY) {
/*
* this must not happen, please serialize calls to
* clk_prepare/clk_enable
*/
ret = -EBUSY;
goto out_unlock;
}
first = clk->prepare_count++ == 0;
if (first)
clk->flags |= CLK_BUSY;
spin_unlock_irqrestore(&clk->enable_lock, flags);

if (!first)
return 0;

if (clk->ops->prepare) {
might_sleep();
ret = clk->ops->prepare(clk);
}

spin_lock_irqsave(&clk->enable_lock, flags);
clk->flags &= ~CLK_BUSY;
if (ret)
clk->prepare_count--;
out_unlock:
spin_unlock_irqrestore(&clk->enable_lock, flags);

return ret;
}

If you now find a problem with that you can blame me not having thought
it to an end.

And note, this is only a suggestion. I.e. I don't know what is the best
to do in the case where I implemented returning -EBUSY above. BUG?
Wait for CLK_BUSY to be cleared?

I'm not sure I like "clk_prepare sleeps iff unprepared but preparable".
Still I think the approach is worth to be discussed.

Best regards
Uwe

--
Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | http://www.pengutronix.de/ |
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