Date: Thu, 30 Jul 2015 12:17:47 +0100
From: Lee Jones <lee.jones@linaro.org>
To: Michael Turquette <mturquette@linaro.org>
Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        kernel@stlinux.com, sboyd@codeaurora.org, devicetree@vger.kernel.org,
        geert@linux-m68k.org, maxime.ripard@free-electrons.com,
        s.hauer@pengutronix.de, linux-clk@vger.kernel.org
Subject: Re: [PATCH v7 3/5] clk: Supply the critical clock {init, enable,
 disable} framework
Message-ID: <20150730111747.GF14642@x1>
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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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