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

Lee Jones
2015-07-30T11:17:56Z

On Wed, 29 Jul 2015, Michael Turquette wrote:

> Hi Lee,
>
> + linux-clk ml
>
> Quoting Lee Jones (2015-07-22 06:04:13)
> > 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
>
> Not self explanatory. I need this explained to me. What does leave_on
> do? Better yet, what would happen if leave_on did not exist?
>
> > + */
> > +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;
>
> How do we get to this point? clk_enable_critical actually calls
> clk_enable, thus incrementing the enable_count. The only time that we
> could hit the above case is if,
>
> a) there is an imbalance in clk_enable and clk_disable calls. If this is
> the case then the drivers need to be fixed. Or better yet some
> infrastructure to catch that, now that we have per-user struct clk
> cookies.
>
> b) a driver knowingly calls clk_enable_critical(foo) and then regular,
> old clk_disable(foo). But why would a driver do that?
>
> It might be that I am missing the point here, so please feel free to
> clue me in.

This check behaves in a very similar to the WARN() above. It's more
of a fail-safe. If all drivers are behaving properly, then it
shouldn't ever be true. If they're not, it prevents an incorrectly
written driver from irrecoverably crippling the system.

As I said in the other mail. We can do without these 3 new wrappers.
We _could_ just write a driver which only calls clk_enable() _after_
it calls clk_disable(), a kind of intentional unbalance and it would
do that same thing. However, what we're trying to do here is provide
a proper API, so we can see at first glance what the 'knowledgeable'
driver is trying to do and not have someone attempt to submit a 'fix'
which calls clk_enable() or something.

> > +
> > 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;
> > + clk_disable(clk);
> > +}
> > +EXPORT_SYMBOL_GPL(clk_disable_critical);
> > +
> > static int clk_core_enable(struct clk_core *clk)
> > {
> > int ret = 0;
> > @@ -1100,6 +1127,15 @@ int clk_enable(struct clk *clk)
> > }
> > EXPORT_SYMBOL_GPL(clk_enable);
> >
> > +int clk_enable_critical(struct clk *clk)
> > +{
> > + if (clk->core->critical.enabled)
> > + clk->core->critical.leave_on = true;
> > +
> > + return clk_enable(clk);
> > +}
> > +EXPORT_SYMBOL_GPL(clk_enable_critical);
> > +
> > static unsigned long clk_core_round_rate_nolock(struct clk_core *clk,
> > unsigned long rate,
> > unsigned long min_rate,
> > @@ -2482,6 +2518,15 @@ fail_out:
> > }
> > EXPORT_SYMBOL_GPL(clk_register);
> >
> > +void clk_init_critical(struct clk *clk)
> > +{
> > + struct critical *critical = &clk->core->critical;
> > +
> > + critical->enabled = true;
> > + critical->leave_on = true;
> > +}
> > +EXPORT_SYMBOL_GPL(clk_init_critical);
> > +
> > /*
> > * Free memory allocated for a clock.
> > * Caller must hold prepare_lock.
> > diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h
> > index 5591ea7..15ef8c9 100644
> > --- a/include/linux/clk-provider.h
> > +++ b/include/linux/clk-provider.h
> > @@ -563,6 +563,8 @@ struct clk *devm_clk_register(struct device *dev, struct clk_hw *hw);
> > void clk_unregister(struct clk *clk);
> > void devm_clk_unregister(struct device *dev, struct clk *clk);
> >
> > +void clk_init_critical(struct clk *clk);
> > +
> > /* helper functions */
> > const char *__clk_get_name(struct clk *clk);
> > struct clk_hw *__clk_get_hw(struct clk *clk);
> > diff --git a/include/linux/clk.h b/include/linux/clk.h
> > index 8381bbf..9807f3b 100644
> > --- a/include/linux/clk.h
> > +++ b/include/linux/clk.h
> > @@ -231,6 +231,19 @@ struct clk *devm_clk_get(struct device *dev, const char *id);
> > int clk_enable(struct clk *clk);
> >
> > /**
> > + * clk_enable_critical - inform the system when the clock source should be
> > + * running, even if clock is critical.
> > + * @clk: clock source
> > + *
> > + * If the clock can not be enabled/disabled, this should return success.
> > + *
> > + * May be called from atomic contexts.
> > + *
> > + * Returns success (0) or negative errno.
> > + */
> > +int clk_enable_critical(struct clk *clk);
> > +
> > +/**
> > * clk_disable - inform the system when the clock source is no longer required.
> > * @clk: clock source
> > *
> > @@ -247,6 +260,23 @@ int clk_enable(struct clk *clk);
> > void clk_disable(struct clk *clk);
> >
> > /**
> > + * clk_disable_critical - inform the system when the clock source is no
> > + * longer required, even if clock is critical.
> > + * @clk: clock source
> > + *
> > + * Inform the system that a clock source is no longer required by
> > + * a driver and may be shut down.
> > + *
> > + * May be called from atomic contexts.
> > + *
> > + * Implementation detail: if the clock source is shared between
> > + * multiple drivers, clk_enable_critical() calls must be balanced
> > + * by the same number of clk_disable_critical() calls for the clock
> > + * source to be disabled.
> > + */
> > +void clk_disable_critical(struct clk *clk);
> > +
> > +/**
> > * clk_get_rate - obtain the current clock rate (in Hz) for a clock source.
> > * This is only valid once the clock source has been enabled.
> > * @clk: clock source

--
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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