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SubjectRe: Locking in the clk API, part 2: clk_prepare/clk_unprepare
On Tue, Feb 01, 2011 at 03:18:37PM +0100, Uwe Kleine-König wrote:
> 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
> */

How do different drivers serialize calls to clk_prepare? Are you
really suggesting that we should have a global mutex somewhere to
prevent this?

> 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?

So what're you proposing that a driver writer should do when he sees
-EBUSY returned from this function? Abandon the probe() returning -EBUSY
and hope the user retries later? Or maybe:

do {
err = clk_prepare(clk);
} while (err == -EBUSY);
?

I don't think that's reasonable to offload this onto driver writers, who
already have a big enough problem already. The less complexity that
driver writers have to deal with, the better.
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