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

[Lee Jones <lee.jones-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

On Wed, 29 Jul 2015, Michael Turquette wrote:
> 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-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>
> > > > Signed-off-by: Lee Jones <lee.jones-QSEj5FYQhm4dnm+yROfE0A@public.gmane.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.

I think you are viewing this particular API back-to-front.  The idea
is to mark all of the critical clocks at start-up by taking a
reference.  Then, if there are no knowledgable drivers who wish to
turn the clock off, the CCF will leave the clock ungated becuase the
reference count will always be >0.

The clk_{disable,enable}_critical() calls are to be used by
knowledgable drivers to say "[disable] I know what I'm doing and it's
okay for this clock to be turned off" and "[enable] right I'm done
with this clock now, let's turn it back on and mark it back as
critical, so no one else can turn it off".

To put things simply, the knowledgable driver will _not_ be enabling
the clock in the first place.  The first interaction it has with it is
to gate it.  Then, once it's done, it will enable it again and mark it
back up as critical.

Still confused ... let's go on another Q in one of your other emails
for another way of putting it.

> > 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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