Subject: [PATCH RFC v1 4/5] clk: core: add CLK_SET_PARENT_ON flags to support clocks require parent on

From: [email-masked] (Dong Aisheng)
Date: Wed, 13 May 2015 17:20:10 +0800

On Wed, May 06, 2015 at 04:34:47PM -0700, Stephen Boyd wrote:
> On 05/04, Dong Aisheng wrote:
> > On Thu, Apr 30, 2015 at 06:09:38PM -0700, Stephen Boyd wrote:
> > > On 04/15, Dong Aisheng wrote:
> > > > diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
> > > > index 7af553d..f2470e5 100644
> > > > --- a/drivers/clk/clk.c
> > > > +++ b/drivers/clk/clk.c
> > > > @@ -43,6 +43,11 @@ static int clk_core_get_phase(struct clk_core *clk);
> > > >  static bool clk_core_is_prepared(struct clk_core *clk);
> > > >  static bool clk_core_is_enabled(struct clk_core *clk);
> > > >  static struct clk_core *clk_core_lookup(const char *name);
> > > > +static struct clk *clk_core_get_parent(struct clk_core *clk);
> > > > +static int clk_core_prepare(struct clk_core *clk);
> > > > +static void clk_core_unprepare(struct clk_core *clk);
> > > > +static int clk_core_enable(struct clk_core *clk);
> > > > +static void clk_core_disable(struct clk_core *clk);
> > > 
> > > Let's avoid adding more here if we can.
> > > 
> > 
> > Yes, also don't want to add these pre-declarations, but needed by the following
> > changes...
> > 
> > > >  
> > > >  /***    private data structures    ***/
> > > >  
> > > > @@ -508,6 +513,7 @@ static void clk_unprepare_unused_subtree(struct clk_core *clk)
> > > >  static void clk_disable_unused_subtree(struct clk_core *clk)
> > > >  {
> > > >  	struct clk_core *child;
> > > > +	struct clk *parent = clk_core_get_parent(clk);
> > > >  	unsigned long flags;
> > > >  
> > > >  	lockdep_assert_held(&prepare_lock);
> > > > @@ -515,6 +521,13 @@ static void clk_disable_unused_subtree(struct clk_core *clk)
> > > >  	hlist_for_each_entry(child, &clk->children, child_node)
> > > >  		clk_disable_unused_subtree(child);
> > > >  
> > > > +	if (clk->flags & CLK_SET_PARENT_ON && parent) {
> > > > +		clk_core_prepare(parent->core);
> > > > +		flags = clk_enable_lock();
> > > > +		clk_core_enable(parent->core);
> > > > +		clk_enable_unlock(flags);
> > > > +	}
> > > 
> > > If there's a parent and this clock is on, why wouldn't the parent
> > > also be on? It doesn't seem right to have a clock that's on
> > > without it's parent on that we're trying to turn off. Put another
> > > way, how is this fixing anything?
> > > 
> > 
> > Well, this is caused by a state mis-align between HW and SW in clock tree
> > during booting.
> > Usually in uboot, we may enable all clocks in HW by default.
> > And during kernel booting time, the parent clock could be disabled in its driver
> > probe due to calling clk_prepare_enable and clk_disable_unprepare.
> > Because it's child clock is only enabled in HW while its SW usecount in
> > clock tree is still 0, so clk_disable of parent clock will gate the parent clock
> > in both HW and SW usecount ultimately.
> > Then there will be a clock is on in HW while its parent is disabled.
> > 
> > Later when clock core does clk_disable_unused, this clock disable
> > will cause system hang due to the limitation of operation requiring its parent
> > clock on.
> > 
> 
> Very good. Now we understand the real problem; the framework
> doesn't keep parent clocks on for child clocks that are already
> on in the bootloader when those child clocks haven't been
> acquired by a driver. How about we solve that problem?
> 

Yes, actually we have already tried this way before to fix the issue
that don't gate parent clock in HW if any child is on during booting phase.
e.g. Clock A and B, B is child of A.
Both A and B clock are enabled in HW in bootloader.

Driver A probe:
clk_prepare_enable(A) -> clk_disable_unprepare(A)
In disable operation, clock framework finds there's a child B is on,
so framework will not gate clock A by calling ops->disable(), but instead,
only de-reference the enable_count, although it's enable_count becomes 0.
Then both A and B are still kept on in HW but not in clock tree state.

Later the clk_disable_unused feature in clock framework will gate those unused
clocks, both A and B, both HW and SW clock tree state are consistent.

We've tested this approach can work, but comparing to the approach we
finally used in this patch series, it has a few disadvantages:
1) The definition of the API clk_disable and clk_unprepare becomes a bit
slightly different from current one, it needs take count of the child clock
state while currently not.
2) It's more complicated and require more code changes.
3) more power consumption although not big deal
4) Manually create inconsistent state between HW and clock tree during
booting may be not good choice.

So we intend to the approach in this patch series that enable parent clock
during clk_disable_unused and disable it again if the clock type requires.
It's simple and easy understand.

> > > > @@ -1483,12 +1524,22 @@ static void __clk_set_parent_after(struct clk_core *clk,
> > > >  	 * Finish the migration of prepare state and undo the changes done
> > > >  	 * for preventing a race with clk_enable().
> > > >  	 */
> > > > -	if (clk->prepare_count) {
> > > > +	if (clk->prepare_count || clk->flags & CLK_SET_PARENT_ON) {
> > > >  		flags = clk_enable_lock();
> > > > -		clk_core_disable(clk);
> > > >  		clk_core_disable(old_parent);
> > > >  		clk_enable_unlock(flags);
> > > >  		clk_core_unprepare(old_parent);
> > > > +
> > > > +		if (clk->prepare_count) {
> > > > +			flags = clk_enable_lock();
> > > > +			clk_core_disable(clk);
> > > > +			clk_enable_unlock(flags);
> > > > +		} else {
> > > > +			flags = clk_enable_lock();
> > > > +			clk_core_disable(parent);
> > > > +			clk_enable_unlock(flags);
> > > > +			clk_core_unprepare(parent);
> > > > +		}
> > > 
> > > Is there a reason why the clk itself can't be on when we switch
> > > parents? 
> > 
> > The CLK_SET_PARENT_ON only indicates that it's parent clocks should
> > be on, not itself clock.
> > And we don't want to break CLK_SET_PARENT_GATE clocks.
> > So if the clk is not prepared before, we only enable its old parent,
> > not including itself.
> > 
> > > It seems that if the clk was on during the parent
> > > switch, then it should be possible to just add a flag check on
> > > both these if conditions and be done. It may be possible to
> > > change the behavior here and not enable the clk in hardware, just
> > > up the count and turn on both the parents. I'm trying to recall
> > > why we enable the clk itself across the switch.
> > > 
> > 
> > Seems it's needed by this fix:
> > commit f8aa0b clk: Fix race condition between clk_set_parent and clk_enable()
> 
> I'm saying I don't recall why that commit needed to turn on the
> clock itself. It certainly doesn't seem to be required at first
> glance. I hope we can get away with just upping the count on the
> clock and enable both parents to avoid the race condition so that
> we don't have to worry about CLK_SET_PARENT_GATE.
> 
> > 
> > > >  		}
> > > >  		return ret;
> > > >  	}
> > > > @@ -1735,13 +1797,18 @@ static void clk_change_rate(struct clk_core *clk)
> > > >  	unsigned long best_parent_rate = 0;
> > > >  	bool skip_set_rate = false;
> > > >  	struct clk_core *old_parent;
> > > > +	struct clk_core *parent = NULL;
> > > > +	unsigned long flags;
> > > >  
> > > >  	old_rate = clk->rate;
> > > >  
> > > > -	if (clk->new_parent)
> > > > +	if (clk->new_parent) {
> > > > +		parent = clk->new_parent;
> > > >  		best_parent_rate = clk->new_parent->rate;
> > > > -	else if (clk->parent)
> > > > +	} else if (clk->parent) {
> > > > +		parent = clk->parent;
> > > >  		best_parent_rate = clk->parent->rate;
> > > > +	}
> > > >  
> > > >  	if (clk->new_parent && clk->new_parent != clk->parent) {
> > > >  		old_parent = __clk_set_parent_before(clk, clk->new_parent);
> > > > @@ -1762,6 +1829,13 @@ static void clk_change_rate(struct clk_core *clk)
> > > >  
> > > >  	trace_clk_set_rate(clk, clk->new_rate);
> > > >  
> > > > +	if (clk->flags & CLK_SET_PARENT_ON && parent) {
> > > > +		clk_core_prepare(parent);
> > > > +		flags = clk_enable_lock();
> > > > +		clk_core_enable(parent);
> > > > +		clk_enable_unlock(flags);
> > > > +	}
> > > 
> > > I can understand the case where clk_set_parent() can't switch the
> > > mux because it needs the source and destination parents to be
> > > clocking. I have that hardware design on my desk. But to change
> > > the rate of a clock? The name of the flag, CLK_SET_PARENT_ON,
> > > leads me to believe we don't really need to do this if we're
> > > changing the rate, unless we're also switching the parents. Care
> > > to explain why the hardware requires this?
> > > 
> > 
> > What i know is that it's designed for eliminate glitch internally.
> > Will find more information for you.
> 
> On my hardware we need to have both parents on to switch the mux
> in a glitch free manner. Using the .set_rate_and_parent op we can
> change the mux value and the rate (i.e. divider) at the same time
> with one register write. And we only need to worry about having
> both parents on if the clock itself is on, which the framework
> handles for us today.
> 

Our hardware needs the parent clock on for all clock settings,
parent switch, rate change and even clock enable/disable, that's
why we met issues that system will hang when disabling clock B
while its parent A is off in clk_disable_unused during booting.

The .set_rate_and_parent won't work for our issue, it's only used
if reparent during change rate. And it only enables both parents
if the clock itself is on.

But you reminder me that the MX7D clock setting registers in target
interface has clock enable/mux/divider in one regiser.
That means we do can implement .set_rate_and_parent later.
It can be an improvement.

> > 
> > > If we actually do need to keep the parent clock on when the rate
> > > is switching the name of the flag could be better and not have
> > > "set parent" in the name.
> > > 
> > 
> > Actually it's meaning is the clock set(e.g. rate, gate/ungate)
> > needs its parent clocks on, not set parent...
> > But Yes, i fully agree the naming is a bit misleading.
> > 
> > It just follows the exist CLK_SET_X convention..
> > Seems not easy find a better name.
> > What's your suggestion?
> > 
> > How about CLK_OPS_PARENT_ON?
> 
> Sure.
> 

Okay, Will use it.

> > 
> > > > +
> > > >  	if (!skip_set_rate && clk->ops->set_rate)
> > > >  		clk->ops->set_rate(clk->hw, clk->new_rate, best_parent_rate);
> > > >  
> > > > @@ -1769,6 +1843,13 @@ static void clk_change_rate(struct clk_core *clk)
> > > >  
> > > >  	clk->rate = clk_recalc(clk, best_parent_rate);
> > > >  
> > > > +	if (clk->flags & CLK_SET_PARENT_ON && parent) {
> > > > +		flags = clk_enable_lock();
> > > > +		clk_core_disable(parent);
> > > > +		clk_enable_unlock(flags);
> > > > +		clk_core_unprepare(parent);
> > > > +	}
> > > 
> > > Why not just call clk_prepare_enable()?
> > 
> > Caused by two reasons initially:
> > 1) no one uses clk_prepare_enable in clk core, so i guess it may be better
> > not use it :-)
> > 2) clk_prepare_enable includes one more unneeded prepare lock acquire,
> > lines exchange efficiency.
> > 
> > > Or add a clk_core
> > > specific function, clk_core_prepare_enable() that we can call
> > > here. We could put the parent pointer check in there too so that
> > > it's just
> > > 
> > >  if (clk->flags & CLK_SET_PARENT_ON)
> > >  	clk_core_prepare_enable(parent);
> > 
> > I'm ok if you like this. :)
> > 
> 
> Yes the helper is better than calling clk_prepare_enable()
> directly.
> 

Got it.

> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> a Linux Foundation Collaborative Project

Regards
Dong Aisheng