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SubjectRe: Locking in the clk API, part 2: clk_prepare/clk_unprepare
On Tue, Feb 01, 2011 at 09:24:09PM +0000, Russell King - ARM Linux wrote:
> On Tue, Feb 01, 2011 at 12:59:11PM -0800, Stephen Boyd wrote:
> > On 02/01/2011 07:24 AM, Russell King - ARM Linux wrote:
> > > I'd also be tempted at this stage to build-in a no-op dummy clock,
> > > that being the NULL clk:
> > >
> > > int clk_prepare(struct clk *clk)
> > > {
> > > int ret = 0;
> > >
> > > if (clk) {
> > > mutex_lock(&clk->mutex);
> > > if (clk->prepared == 0)
> > > ret = clk->ops->prepare(clk);
> > > if (ret == 0)
> > > clk->prepared++;
> > > mutex_unlock(&clk->mutex);
> > > }
> > >
> > > return ret;
> > > }
> >
> > I'm afraid this will hide enable/disable imbalances on some targets and
> > then expose them on others. Maybe its not a big problem though since
> > this also elegantly handles the root(s) of the tree.
>
> You can't catch enable/disable imbalances in the prepare code, and you
> can't really catch them in the unprepare code either.
>
> Consider two drivers sharing the same struct clk. When the second driver
> prepares the clock, the enable count could well be non-zero, caused by
> the first driver. Ditto for when the second driver is removed, and it
> calls unprepare - the enable count may well be non-zero.
>
> The only thing you can check is that when the prepare count is zero,
> the enable count is also zero. You can also check in clk_enable() and
> clk_disable() that the prepare count is non-zero.
but how can we check prepare count without mutex lock? Even if prepare count
is atomic_t, it can not guarantee the clock is actually prepared or unprepared.
So it's important for driver writer to maintain the call sequence.

Thanks
Richard
>
> If you want tigher checking than that, you need to somehow identify and
> match up the clk_prepare/clk_enable/clk_disable/clk_unprepare calls from
> a particular driver instance. Addresses of the functions don't work as
> you can't be certain that driver code will be co-located within a certain
> range. Adding an additional argument to these functions which is driver
> instance specific seems to be horrible too.
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel


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