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

[Uwe Kleine-König]

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:
> > On Tue, Feb 01, 2011 at 03:24:58PM +0000, Russell King - ARM Linux wrote:
> > > On Tue, Feb 01, 2011 at 04:18:46PM +0100, Uwe Kleine-König wrote:
> > > > 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

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