Date:	Tue, 14 Oct 2014 15:27:33 +0200
From:	Tomeu Vizoso <tomeu.vizoso@...labora.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	Mike Turquette <mturquette@...aro.org>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	Russell King <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 8/8] clk: Add floor and ceiling constraints to clock rates

On 11 October 2014 01:55, Stephen Boyd <sboyd@...eaurora.org> wrote:
> On 10/09, Tomeu Vizoso wrote:
>> diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
>> index 4db918a..97cf1a3 100644
>> --- a/drivers/clk/clk.c
>> +++ b/drivers/clk/clk.c
>> @@ -1629,18 +1609,27 @@ int clk_set_rate(struct clk *clk, unsigned long rate)
>>       /* prevent racing with updates to the clock topology */
>>       clk_prepare_lock();
>>
>> +     hlist_for_each_entry(clk_user, &clk->per_user_clks, child_node) {
>> +             rate = max(rate, clk_user->floor_constraint);
>> +     }
>> +
>> +     hlist_for_each_entry(clk_user, &clk->per_user_clks, child_node) {
>> +             if (clk_user->ceiling_constraint > 0)
>> +                     rate = min(rate, clk_user->ceiling_constraint);
>> +     }
>> +
>>       /* bail early if nothing to do */
>> -     if (rate == clk_get_rate(clk))
>> +     if (rate == clk_core_get_rate(clk))
>
> Can we use the nolock variant here?

It's true that we have the lock already, but I'm not sure about the
implications of not calling __clk_recalc_rates. Nothing that cannot be
solved with one more function though...

> As an aside, this is going to
> make my per-clock locking series complicated. I'm not even sure
> how the two series will work together. In the per-clock locking
> series I was hoping we could check if rate == current rate
> without holding any locks. Otherwise we get into a recursive lock
> situation. Need to think more about that one.

Ok.

>>               goto out;
>>
>> -     if ((clk->core->flags & CLK_SET_RATE_GATE) &&
>> -         clk->core->prepare_count) {
>> +     if ((clk->flags & CLK_SET_RATE_GATE) &&
>> +         clk->prepare_count) {
>>               ret = -EBUSY;
>>               goto out;
>>       }
>>
>>       /* calculate new rates and get the topmost changed clock */
>> -     top = clk_calc_new_rates(clk->core, rate);
>> +     top = clk_calc_new_rates(clk, rate);
>>       if (!top) {
>>               ret = -EINVAL;
>>               goto out;
>> @@ -1664,8 +1653,69 @@ out:
> [...]
>> +int clk_set_rate(struct clk *clk, unsigned long rate)
>> +{
>> +     return clk_core_set_rate(clk->core, rate);
>> +}
>>  EXPORT_SYMBOL_GPL(clk_set_rate);
>>
>
> What about clk_round_rate()? That needs to tell us what the rate
> will be if we call clk_set_rate() with whatever value is passed
> here.

My thinking was that clock consumers would be setting constraints and
calling set_rate regardless of any constraints that may be set at the
moment.

Though in my particular use case (scaling of the external memory bus)
it shouldn't really matter as the clock would be given a suitably low
frequency at startup and from that moment on only the constraints
would be updated.

> I would expect that the rate aggregation logic would be
> somewhere in that codepath but I don't see it.

I can certainly make round_rate aware of constraints, no problem.
Mike, what do you think?

>> +int clk_set_floor_rate(struct clk *clk, unsigned long rate)
>
> We need some documentation for these new functions. I see we have
> them in the header, maybe we can copy that here.

Ok.

>> +{
>> +     int ret;
>> +
>> +     clk_prepare_lock();
>> +
>> +     clk->floor_constraint = rate;
>
> No check for ceiling being less than floor?

No, otherwise the calling code would have to be very careful to set
both constraints in the correct order based on the current and next
values. In practice I expect a given consumer to either set a floor or
a constraint, but not both.

>> +     ret = clk_set_rate(clk, clk_get_rate(clk));
>> +
>> +     clk_prepare_unlock();
>> +
>> +     return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(clk_set_floor_rate);
>> +
>> +int clk_set_ceiling_rate(struct clk *clk, unsigned long rate)
>> +{
>> +     int ret;
>> +
>> +     clk_prepare_lock();
>
> We don't need to grab the lock this early right? Presumably the
> caller is doing any required locking so they don't call different
> clk APIs for the same clk pointer at the same time. So we should
> be able to grab this lock after checking for this error
> condition.

Sounds good.

>> +
>> +     WARN(rate > 0 && rate < clk->floor_constraint,
>> +          "clk %s dev %s con %s: new ceiling %lu lower than existing floor %lu\n",
>> +          clk->core->name, clk->dev_id, clk->con_id, rate,
>> +          clk->floor_constraint);
>> +
>> +     clk->ceiling_constraint = rate;
>> +     ret = clk_set_rate(clk, clk_get_rate(clk));
>
> Why not just pass 0 as the second argument? The same comment
> applies in the floor function.

Both behaviours seem equally correct to me, but wonder if it wouldn't
be better to store the rate that was last set explicitly with set_rate
and try to reapply that one after every update to the constraints,
instead of the current rate, or 0/INT_MAX.

>> +
>> +     clk_prepare_unlock();
>> +
>> +     return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(clk_set_ceiling_rate);
>> +
>>  static struct clk_core *clk_core_get_parent(struct clk_core *clk)
>>  {
>>       struct clk_core *parent;
>> @@ -2143,6 +2195,12 @@ struct clk *__clk_register(struct device *dev, struct clk_hw *hw)
>>  }
>>  EXPORT_SYMBOL_GPL(__clk_register);
>>
>> +static void __clk_free_clk(struct clk *clk_user)
>> +{
>> +     hlist_del(&clk_user->child_node);
>> +     kfree(clk_user);
>
> We need to re-evaluate the rate when a user is removed, right?

Oops, you are right.

> This leads to another question though. What does it means for a
> per-user clock to be disabled and change it's floor or ceiling?
> Should that count in the aggregation logic?

Per-user clocks don't get disabled, the whole clock does (but we can
use per-user clk information to provide better debug information about
unbalanced calls to prepare and enable).

> So far we haven't required drivers to explicitly call
> clk_set_rate() with 0 so they can "drop" their rate request
> because there isn't any other user and disabling the clock is
> pretty much the same. With multiple users it looks like we're
> going to require them to set the floor or ceiling to 0 or INT_MAX
> if they want to remove their request. Alternatively we could
> track the prepare/unprepare state of the per-user clock and drop
> the constraint when that instance is unprepared (or reapply it
> when prepared). It's pretty much a semantic difference, but one
> benefit would be that we don't have to make two (or three?) calls
> to the clock framework if we want to drop the rate constraints
> and disable the clock.

In my mind this is not such an issue because I view clock constraints
as attributes of the per-user clks, while the enable and prepare
states and the actual rate are attributes of the global clock
instance.

>> +}
>> +
>>  /**
>>   * clk_register - allocate a new clock, register it and return an opaque cookie
>>   * @dev: device that is registering this clock
>> diff --git a/include/linux/clk-private.h b/include/linux/clk-private.h
>> index 1cdb727..025aca2 100644
>> --- a/include/linux/clk-private.h
>> +++ b/include/linux/clk-private.h
>> @@ -50,6 +50,7 @@ struct clk_core {
>>       struct hlist_head       children;
>>       struct hlist_node       child_node;
>>       struct hlist_node       debug_node;
>> +     struct hlist_head       per_user_clks;
>
> Can we just call it 'clks'?

Sure.

Thanks,

Tomeu

>>       unsigned int            notifier_count;
>>  #ifdef CONFIG_DEBUG_FS
>>       struct dentry           *dentry;
>> @@ -61,6 +62,10 @@ struct clk {
>>       struct clk_core *core;
>>       const char *dev_id;
>>       const char *con_id;
>> +
>> +     unsigned long   floor_constraint;
>> +     unsigned long   ceiling_constraint;
>> +     struct hlist_node child_node;
>>  };
>>
>
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