Date:	Fri, 15 Nov 2013 16:50:31 -0800
From:	Mike Turquette <mturquette@...aro.org>
To:	boris brezillon <b.brezillon@...rkiz.com>,
	Rob Landley <rob@...dley.net>,
	Rob Herring <rob.herring@...xeda.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Stephen Warren <swarren@...dotorg.org>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Russell King <linux@....linux.org.uk>
Cc:	devicetree@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH RESEND 1/2] clk: add clk accuracy retrieval support

Quoting boris brezillon (2013-11-08 00:54:45)
> Hello Mike,
> 
> On 08/11/2013 01:51, Mike Turquette wrote:
> > Quoting Boris BREZILLON (2013-10-13 10:17:10)
> >> +/**
> >> + * clk_get_accuracy - return the accuracy of clk
> >> + * @clk: the clk whose accuracy is being returned
> >> + *
> >> + * Simply returns the cached accuracy of the clk, unless
> >> + * CLK_GET_ACCURACY_NOCACHE flag is set, which means a recalc_rate will be
> >> + * issued.
> >> + * If clk is NULL then returns 0.
> >> + */
> >> +unsigned long clk_get_accuracy(struct clk *clk)
> >> +{
> >> +       unsigned long accuracy;
> >> +       clk_prepare_lock();
> >> +
> >> +       if (clk && (clk->flags & CLK_GET_ACCURACY_NOCACHE))
> >> +               __clk_recalc_accuracies(clk);
> > I think that there is some overlap between recalculating the accuracy
> > here and simply getting it. You only provide clk_get_accuracy and it
> > serves both purposes. It would be better if clk_recalc_accuracy walked
> > the subtree of children and if clk_get_accuracy simply returned a cached
> > value.
> 
> I'm not sure I get your point.
> 
> I used exactly the same model as for clk rate retrieval.

Not exactly the same model. For rates we support public clk_recalc_rate
and clk_get_rate functions.

> 
> Actually there's one flag (CLK_GET_ACCURACY_NOCACHE) which is
> checked to decide wether the accuracy should be recalculated each
> time the get_accuracy is called or not.
> 
> This means most of the time the clk_get_accuracy will return the cached
> value unless the clk provider explicitely ask for accuracy recalculation
> (e.g. a clk with dynamic accuracy according to temperature range ?).
> 
> Are you suggesting to expose 2 functions to clk users (clk_get_accuracy
> and clk_recalc_accuracy) ?
> Or is clk_recalc_accuracy an internal/private function used by the CCF ?

I was suggesting that in my previous email. However I've thought on it a
bit and I'm not sure there is any value to having a public
clk_recalc_accuracy right now.

The only reason to do so would be if the accuracy can change in such a
way that the clock framework is never aware of it.  This would mean that
somehow the accuracy changed "behind our back".

One hypothetical example is a PLL which Linux knows about, but perhaps
is controlled by some other co-processor (not Linux). In this case the
co-processor may reprogram/relock the PLL in a way that affects it's
accuracy, and a Linux driver that knows this may want to update the CCF
book-keeping with a call to clk_recalc_accuracy.

That's all hypothetical though and it can be added in later. Looking
over your change again I think that it is sufficient for now.

Can you respin this patch with fixes for the two nitpicks I outlined in
my previous mail and rebase it onto -rc1 when it drops? That should be
any day now.

Thanks!
Mike

> 
> >
> >> +
> >> +       accuracy = __clk_get_accuracy(clk);
> >> +       clk_prepare_unlock();
> >> +
> >> +       return accuracy;
> >> +}
> >> +EXPORT_SYMBOL_GPL(clk_get_accuracy);
> >> +
> >> +/**
> >>    * __clk_recalc_rates
> >>    * @clk: first clk in the subtree
> >>    * @msg: notification type (see include/linux/clk.h)
> >> @@ -1545,6 +1610,7 @@ void __clk_reparent(struct clk *clk, struct clk *new_parent)
> >>   {
> >>          clk_reparent(clk, new_parent);
> >>          clk_debug_reparent(clk, new_parent);
> >> +       __clk_recalc_accuracies(clk);
> > Similar to the above statement. Why do this here? We do this for rates
> > since calls to clk_get_rate will return the cached rate (unless the
> > NOCACHE flag is set). But since a call to clk_get_accuracy will always
> > recalculate it then there is no benefit to doing that here.
> 
> This is the same for clk_get_accuracies (it returns the cached
> accuracy unless CLK_GET_ACCURACY_NOCACHE is defined).
> 
> And changing parent of a clk will indirectly change the clk
> accuracy (clk accuracies are cumulative).
> 
> >
> >>          __clk_recalc_rates(clk, POST_RATE_CHANGE);
> >>   }
> >>   
> >> @@ -1615,6 +1681,9 @@ int clk_set_parent(struct clk *clk, struct clk *parent)
> >>          /* do the re-parent */
> >>          ret = __clk_set_parent(clk, parent, p_index);
> >>   
> >> +       /* propagate accuracy recalculation */
> >> +       __clk_recalc_accuracies(clk);
> > Ditto.
> Ditto. :)
> 
> 
> Please tell me if I misunderstood your requests.
> 
> Best Regards,
> 
> Boris
> 
> >
> > Regards,
> > Mike
> >
> >> +
> >>          /* propagate rate recalculation accordingly */
> >>          if (ret)
> >>                  __clk_recalc_rates(clk, ABORT_RATE_CHANGE);
> >> @@ -1724,6 +1793,21 @@ int __clk_init(struct device *dev, struct clk *clk)
> >>                  hlist_add_head(&clk->child_node, &clk_orphan_list);
> >>   
> >>          /*
> >> +        * Set clk's accuracy.  The preferred method is to use
> >> +        * .recalc_accuracy. For simple clocks and lazy developers the default
> >> +        * fallback is to use the parent's accuracy.  If a clock doesn't have a
> >> +        * parent (or is orphaned) then accuracy is set to zero (perfect
> >> +        * clock).
> >> +        */
> >> +       if (clk->ops->recalc_accuracy)
> >> +               clk->accuracy = clk->ops->recalc_accuracy(clk->hw,
> >> +                                       __clk_get_accuracy(clk->parent));
> >> +       else if (clk->parent)
> >> +               clk->accuracy = clk->parent->accuracy;
> >> +       else
> >> +               clk->accuracy = 0;
> >> +
> >> +       /*
> >>           * Set clk's rate.  The preferred method is to use .recalc_rate.  For
> >>           * simple clocks and lazy developers the default fallback is to use the
> >>           * parent's rate.  If a clock doesn't have a parent (or is orphaned)
> >> diff --git a/include/linux/clk-private.h b/include/linux/clk-private.h
> >> index 8138c94..accb517 100644
> >> --- a/include/linux/clk-private.h
> >> +++ b/include/linux/clk-private.h
> >> @@ -41,6 +41,7 @@ struct clk {
> >>          unsigned long           flags;
> >>          unsigned int            enable_count;
> >>          unsigned int            prepare_count;
> >> +       unsigned long           accuracy;
> >>          struct hlist_head       children;
> >>          struct hlist_node       child_node;
> >>          unsigned int            notifier_count;
> >> diff --git a/include/linux/clk-provider.h b/include/linux/clk-provider.h
> >> index 73bdb69..942811d 100644
> >> --- a/include/linux/clk-provider.h
> >> +++ b/include/linux/clk-provider.h
> >> @@ -29,6 +29,7 @@
> >>   #define CLK_IS_BASIC           BIT(5) /* Basic clk, can't do a to_clk_foo() */
> >>   #define CLK_GET_RATE_NOCACHE   BIT(6) /* do not use the cached clk rate */
> >>   #define CLK_SET_RATE_NO_REPARENT BIT(7) /* don't re-parent on rate change */
> >> +#define CLK_GET_ACCURACY_NOCACHE BIT(8) /* do not use the cached clk accuracy */
> >>   
> >>   struct clk_hw;
> >>   
> >> @@ -108,6 +109,13 @@ struct clk_hw;
> >>    *             which is likely helpful for most .set_rate implementation.
> >>    *             Returns 0 on success, -EERROR otherwise.
> >>    *
> >> + * @recalc_accuracy: Recalculate the accuracy of this clock. The clock accuracy
> >> + *             is expressed in ppb (parts per billion). The parent accuracy is
> >> + *             an input parameter.
> >> + *             Returns the calculated accuracy.  Optional - if this op is not
> >> + *             set then clock accuracy will be initialized to parent accuracy
> >> + *             or 0 (perfect clock) if clock has no parent.
> >> + *
> >>    * The clk_enable/clk_disable and clk_prepare/clk_unprepare pairs allow
> >>    * implementations to split any work between atomic (enable) and sleepable
> >>    * (prepare) contexts.  If enabling a clock requires code that might sleep,
> >> @@ -139,6 +147,8 @@ struct clk_ops {
> >>          u8              (*get_parent)(struct clk_hw *hw);
> >>          int             (*set_rate)(struct clk_hw *hw, unsigned long,
> >>                                      unsigned long);
> >> +       unsigned long   (*recalc_accuracy)(struct clk_hw *hw,
> >> +                                          unsigned long parent_accuracy);
> >>          void            (*init)(struct clk_hw *hw);
> >>   };
> >>   
> >> @@ -194,6 +204,7 @@ struct clk_hw {
> >>   struct clk_fixed_rate {
> >>          struct          clk_hw hw;
> >>          unsigned long   fixed_rate;
> >> +       unsigned long   fixed_accuracy;
> >>          u8              flags;
> >>   };
> >>   
> >> diff --git a/include/linux/clk.h b/include/linux/clk.h
> >> index 9a6d045..2fe3b54 100644
> >> --- a/include/linux/clk.h
> >> +++ b/include/linux/clk.h
> >> @@ -85,6 +85,23 @@ int clk_notifier_unregister(struct clk *clk, struct notifier_block *nb);
> >>   #endif
> >>   
> >>   /**
> >> + * clk_get_accuracy - obtain the clock accuracy in ppb (parts per billion)
> >> + *                   for a clock source.
> >> + * @clk: clock source
> >> + *
> >> + * This gets the clock source accuracy expressed in ppb.
> >> + * A perfect clock returns 0.
> >> + */
> >> +#ifdef CONFIG_HAVE_CLK_GET_ACCURACY
> >> +unsigned long clk_get_accuracy(struct clk *clk);
> >> +#else
> >> +static inline unsigned long clk_get_accuracy(struct clk *clk)
> >> +{
> >> +       return 0;
> >> +}
> >> +#endif
> >> +
> >> +/**
> >>    * clk_prepare - prepare a clock source
> >>    * @clk: clock source
> >>    *
> >> -- 
> >> 1.7.9.5
> >>
> >>
> >> _______________________________________________
> >> linux-arm-kernel mailing list
> >> linux-arm-kernel@...ts.infradead.org
> >> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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