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

[Michael Turquette <mturquette@baylibre.com>]

Quoting Lee Jones (2015-07-31 02:02:19)
> On Thu, 30 Jul 2015, Michael Turquette wrote:
> 
> > Quoting Lee Jones (2015-07-30 04:17:47)
> > > 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.
> > 
> > Then this check should be replaced with a generic approach that refuses
> > to honor imbalances anyways. Below are two patches that probably resolve
> > the issue of badly behaving drivers that cause enable imbalances.
> 
> Your patch should make the requirement for this check moot, so it can
> probably be removed.
> 
> > > 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.
> > 
> > This naive approach will not work with per-user imbalance tracking.
> 
> Steady on.  I said we "_could_", that that I think it's a good idea.
> 
> I think it's a bad idea, which is why I wrote this set. ;)
> 
> > > 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.
> > 
> > We'll need some type of api for sure for the handoff.
> 
> This set will not trigger your new checks.  The clocks will be in
> perfect ballance becuase a reference will be taken at start-up.
> 
> Again:
> 
> start-up:
>   clk_prepare_enable()
> 
> knowlegable_driver_probe:
>   clk_get()
> 
> knowlegable_driver_gate_clk:
>   clk_disable_critical()

The call to clk_disable() nested inside clk_disable_critical will fail
with the new checks. This is because the struct clk instance will be
different from one used in your "start-up" section above. clk_get()
creates a unique struct clk every time you call it.

Put another way, a unique user of a clock cannot call clk_disable() when
the per-user enable_count is 0.

Furthermore, there is no way that I will ever be happy with a technique
that requires calling disable prior to an enable within a driver. That
goes against a long-standing api designs and is confusing as hell to
driver authors.

Regards,
Mike

> 
> knowlegable_driver_ungate_clk:
>   clk_enable_critical()
> 
> knowlegable_driver_remove:
>   clk_put()
> 
> > From 3599ed206da9ce770bfafcfd95cbb9a03ac44473 Mon Sep 17 00:00:00 2001
> > From: Michael Turquette <mturquette@baylibre.com>
> > Date: Wed, 29 Jul 2015 18:22:45 -0700
> > Subject: [PATCH 1/2] clk: per-user clk prepare & enable ref counts
> > 
> > This patch adds prepare and enable reference counts for the per-user
> > handles that clock consumers have for a clock node. This patch warns if
> > an imbalance occurs while trying to disable or unprepare a clock and
> > aborts, leaving the hardware unaffected.
> > 
> > Signed-off-by: Michael Turquette <mturquette@baylibre.com>
> > ---
> >  drivers/clk/clk.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> > index 898052e..72feee9 100644
> > --- a/drivers/clk/clk.c
> > +++ b/drivers/clk/clk.c
> > @@ -84,6 +84,8 @@ struct clk {
> >       unsigned long min_rate;
> >       unsigned long max_rate;
> >       struct hlist_node clks_node;
> > +     unsigned int enable_count;
> > +     unsigned int prepare_count;
> >  };
> >  
> >  /***           locking             ***/
> > @@ -600,6 +602,9 @@ void clk_unprepare(struct clk *clk)
> >               return;
> >  
> >       clk_prepare_lock();
> > +     if (WARN_ON(clk->prepare_count == 0))
> > +             return;
> > +     clk->prepare_count--;
> >       clk_core_unprepare(clk->core);
> >       clk_prepare_unlock();
> >  }
> > @@ -657,6 +662,7 @@ int clk_prepare(struct clk *clk)
> >               return 0;
> >  
> >       clk_prepare_lock();
> > +     clk->prepare_count++;
> >       ret = clk_core_prepare(clk->core);
> >       clk_prepare_unlock();
> >  
> > @@ -707,6 +713,9 @@ void clk_disable(struct clk *clk)
> >               return;
> >  
> >       flags = clk_enable_lock();
> > +     if (WARN_ON(clk->enable_count == 0))
> > +             return;
> > +     clk->enable_count--;
> >       clk_core_disable(clk->core);
> >       clk_enable_unlock(flags);
> >  }
> > @@ -769,6 +778,7 @@ int clk_enable(struct clk *clk)
> >               return 0;
> >  
> >       flags = clk_enable_lock();
> > +     clk->enable_count++;
> >       ret = clk_core_enable(clk->core);
> >       clk_enable_unlock(flags);
> >  
> 
> -- 
> Lee Jones
> Linaro STMicroelectronics Landing Team Lead
> Linaro.org │ Open source software for ARM SoCs
> Follow Linaro: Facebook | Twitter | Blog
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