Message-ID: <20150731085656.GD3208@x1>
Date:	Fri, 31 Jul 2015 09:56:56 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Michael Turquette <mturquette@...aro.org>
Cc:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kernel@...inux.com, sboyd@...eaurora.org,
	devicetree@...r.kernel.org, geert@...ux-m68k.org,
	s.hauer@...gutronix.de
Subject: Re: [PATCH v7 3/5] clk: Supply the critical clock {init, enable,
 disable} framework

On Thu, 30 Jul 2015, Michael Turquette wrote:

> Quoting Lee Jones (2015-07-30 02:21:39)
> > 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@...e-electrons.com>
> > > > > > Signed-off-by: Lee Jones <lee.jones@...aro.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.
> 
> Right. So I'll ask the same question here that I asked in the other
> patch: is there ever a case where a clock consumer driver would want to
> call clk_enable_critical? It seems to me that in you usage of it, that
> call would only ever be made by the core framework code (e.g. clk-conf.c
> or perhaps somewhere in drivers/clk/clk.c).

Yes, _after_ it has called clk_disable_critical(), when it has
finished fiddling with it.  clk_enable_critical() simply resets the
clock back to an enabled/critical state (how the knowledgeable driver
found it).

> > 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".
> 
> OK, so this almost answers my question above. You have a driver that may
> finish using a clock for a while (ie, rmmod knowledgeable_driver), and
> you want it (critically) enabled again. Is this a real use case? Who
> would come along and disable this clock later on? If the driver is to be
> re-loaded then I would suggest never unloading it in the first place.

This has nothing to do with modules.  I believe the knowledgeable
consumer should only gate the clock (steal a reference) when the clock
is actually gated.  The rest of the time the framework will have it
marked as critical "do not turn me off".

> (Oh and bear in mind when answering my question above that imbalanced
> clk_enable/clk_disable calls will not succeed thanks to the vaporware
> patch that I have in my local tree)

They won't be imbalanced, because clk_enable() would have been called
first during start-up (__set_critical_clocks()).

> If you have a second knowledgeable_driver_2 that shares that clock and
> can be trusted to manage it (critically) then I would need to see that
> example, as it doesn't feel like a real use to me.

Nor me, that's why this impementation doesn't handle that use-case,
however Maxime thinks it is one, so we can solve that with reference
counting.

> > 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.a
> 
> I like the first part. Makes sense and fills a real need. I am fully
> on-board with a provider-handoff-to-consumer solution. It is all the
> weird stuff that comes after it that I'm unsure of.

I don't think there should be hand-off.  I think the
{enable,disable}_critical() should "momentarily" take the last
reference, then put everything back as it found it when it's finished
disabling the clock.

> > 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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