Re: [PATCH v7 3/5] clk: Supply the critical clock {init, enable, disable} framework

Michael Turquette
2015-07-30T01:21:44Z

Quoting Lee Jones (2015-07-27 01:53:38)
> On Mon, 27 Jul 2015, Maxime Ripard wrote:
>
> > On Wed, Jul 22, 2015 at 02:04:13PM +0100, Lee Jones wrote:
> > > These new API calls will firstly provide a mechanisms to tag a clock as
> > > critical and secondly allow any knowledgeable driver to (un)gate clocks,
> > > even if they are marked as critical.
> > >
> > > Suggested-by: Maxime Ripard <maxime.ripard@free-electrons.com>
> > > Signed-off-by: Lee Jones <lee.jones@linaro.org>
> > > ---
> > > drivers/clk/clk.c | 45 ++++++++++++++++++++++++++++++++++++++++++++
> > > include/linux/clk-provider.h | 2 ++
> > > include/linux/clk.h | 30 +++++++++++++++++++++++++++++
> > > 3 files changed, 77 insertions(+)
> > >
> > > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> > > index 61c3fc5..486b1da 100644
> > > --- a/drivers/clk/clk.c
> > > +++ b/drivers/clk/clk.c
> > > @@ -46,6 +46,21 @@ static struct clk_core *clk_core_lookup(const char *name);
> > >
> > > /*** private data structures ***/
> > >
> > > +/**
> > > + * struct critical - Provides 'play' over critical clocks. A clock can be
> > > + * marked as critical, meaning that it should not be
> > > + * disabled. However, if a driver which is aware of the
> > > + * critical behaviour wants to control it, it can do so
> > > + * using clk_enable_critical() and clk_disable_critical().
> > > + *
> > > + * @enabled Is clock critical? Once set, doesn't change
> > > + * @leave_on Self explanatory. Can be disabled by knowledgeable drivers
> > > + */
> > > +struct critical {
> > > + bool enabled;
> > > + bool leave_on;
> > > +};
> > > +
> > > struct clk_core {
> > > const char *name;
> > > const struct clk_ops *ops;
> > > @@ -75,6 +90,7 @@ struct clk_core {
> > > struct dentry *dentry;
> > > #endif
> > > struct kref ref;
> > > + struct critical critical;
> > > };
> > >
> > > struct clk {
> > > @@ -995,6 +1011,10 @@ static void clk_core_disable(struct clk_core *clk)
> > > if (WARN_ON(clk->enable_count == 0))
> > > return;
> > >
> > > + /* Refuse to turn off a critical clock */
> > > + if (clk->enable_count == 1 && clk->critical.leave_on)
> > > + return;
> > > +
> >
> > I think it should be handled by a separate counting. Otherwise, if you
> > have two users that marked the clock as critical, and then one of them
> > disable it...
> >
> > > if (--clk->enable_count > 0)
> > > return;
> > >
> > > @@ -1037,6 +1057,13 @@ void clk_disable(struct clk *clk)
> > > }
> > > EXPORT_SYMBOL_GPL(clk_disable);
> > >
> > > +void clk_disable_critical(struct clk *clk)
> > > +{
> > > + clk->core->critical.leave_on = false;
> >
> > .. you just lost the fact that it was critical in the first place.
>
> I thought about both of these points, which is why I came up with this
> strategy.
>
> Any device which uses the *_critical() API should a) have knowledge of
> what happens when a particular critical clock is gated and b) have
> thought about the consequences.

If this statement above is true then I fail to see the need for a new
api. A driver which has a really great idea of when it is safe or unsafe
to gate a clock should call clk_prepare_enable at probe and then only
call clk_disable_unprepare once it is safe to do so.

The existing bookkeeping in the clock framework will do the rest.

Regards,
Mike

> I don't think we can use reference
> counting, because we'd need as many critical clock owners as there are
> critical clocks. Cast your mind back to the reasons for this critical
> clock API. One of the most important intentions of this API is the
> requirement mitigation for each of the critical clocks to have an owner
> (driver).
>
> With regards to your second point, that's what 'critical.enabled'
> is for. Take a look at clk_enable_critical().
>
> --
> Lee Jones
> Linaro STMicroelectronics Landing Team Lead
> Linaro.org │ Open source software for ARM SoCs
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