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SubjectRe: [PATCH v4 3/6] clk: introduce the common clock framework
On Wed, Dec 14, 2011 at 5:18 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Tue, 13 Dec 2011, Mike Turquette wrote:
>> +void __clk_unprepare(struct clk *clk)
>> +{
>> +     if (!clk)
>> +             return;
>> +
>> +     if (WARN_ON(clk->prepare_count == 0))
>> +             return;
>> +
>> +     if (--clk->prepare_count > 0)
>> +             return;
>> +
>> +     WARN_ON(clk->enable_count > 0);
>
> So this leaves the clock enable count set. I'm a bit wary about
> that. Shouldn't it either return (including bumping the prepare_count
> again) or call clk_disable() ?

I've hit this in my port of OMAP. It comes from this simple situation:

driver 1 (adapted for clk_prepare/clk_unprepare):
clk_prepare(clk);
clk_enable(clk);

...

driver2 (not adapted for clk_prepare/clk_unprepare):
clk_enable(clk);

...

driver1:
clk_disable(clk);
clk_unprepare(clk);

In such a case we don't want to bump the prepare_count, since the
offending driver2 won't decrement that count. Also we don't want to
shut down that clk since driver2 is using it.

Returning after the WARN is maybe OK, but the point of this check is
really to find code which hasn't yet been made prepare-aware, and the
current implementation is noisy enough about it.

>> +/**
>> + * clk_get_parent - return the parent of a clk
>> + * @clk: the clk whose parent gets returned
>> + *
>> + * Simply returns clk->parent.  It is up to the caller to hold the
>> + * prepare_lock, if desired.  Returns NULL if clk is NULL.
>
> Holding the prepare lock in the caller will deadlock. So it's not a
> real good idea to document it as an option :)

Oops. That comment is left over from before clk_get_parent held the
lock. Will remove.

>
>> + */
>> +struct clk *clk_get_parent(struct clk *clk)
>> +{
>> +     struct clk *parent;
>> +
>> +     if (!clk)
>> +             return NULL;
>> +
>> +     mutex_lock(&prepare_lock);
>
>> +/**
>> + * clk_init - initialize the data structures in a struct clk
>> + * @dev: device initializing this clk, placeholder for now
>> + * @clk: clk being initialized
>> + *
>> + * Initializes the lists in struct clk, queries the hardware for the
>> + * parent and rate and sets them both.  Adds the clk to the sysfs tree
>> + * topology.
>> + *
>> + * Caller must populate clk->name and clk->flags before calling
>
> I'm not too happy about this construct. That leaves struct clk and its
> members exposed to the world instead of making it a real opaque
> cookie. I know from my own painful experience, that this will lead to
> random fiddling in that data structure in drivers and arch code just
> because the core code has a shortcoming.
>
> Why can't we make struct clk a real cookie and confine the data
> structure to the core code ?
>
> That would change the init call to something like:
>
> struct clk *clk_init(struct device *dev, const struct clk_hw *hw,
>                     struct clk *parent)
>
> And have:
> struct clk_hw {
>       struct clk_hw_ops *ops;
>       const char        *name;
>       unsigned long     flags;
> };
>
> Implementers can do:
> struct my_clk_hw {
>       struct clk_hw    hw;
>       mydata;
> };
>
> And then change the clk ops callbacks to take struct clk_hw * as an
> argument.
>
> So the core code can allocate the clk data structure and you return a
> real opaque cookie. You do the same thing for the basic gated clock in
> one of the next patches, so doing it for struct clk itself is just
> consequent.

The opaque cookie would work if platform code never needs any
information outside of the data a single clk_hw_whatever provides.
Unfortunately hardware doesn't make things that easy on us and the
operations we do for a single clk are affected by the state of other
clks.

Here is an example I've been using a lot lately: on OMAP we have two
clock inputs to our PLLs: a bypass clk and reference clk (actually we
can have more than this). When changing the PLL rate both clks must
be enabled, regardless of which clk ends up driving the PLL. To
illustrate, the PLL may be driven by clk_ref at 400MHz, and then we
call clk_set_rate(pll_clk, 196000000); which will also leave it
clocked by clk_ref but at 196MHz. For this to happen however, the
clk_bypass had to be enabled during the rate change operation. Here
is the relevant .set_rate implementation:
http://git.linaro.org/gitweb?p=people/mturquette/linux.git;a=blob;f=arch/arm/mach-omap2/dpll3xxx.c;h=b2412574b63829944593c1a7a6eda5fa4350686f;hb=738bde65918ae1ac743b1498801da0b860e2ee32#l461

In order for the OMAP platform code to do this, we have to have two
things: firstly we need the struct clk so that we can call
clk_enable/disable and __clk_prepare/unprepare on the ref and bypass
clks from within .set_rate. The second thing is that we need
__clk_prepare and __clk_unprepare to be visible by this code so that
we don't nest the prepare_lock mutex.

Is there a good way to solve such problems if we hide struct clk from
the platform code/clk driver implementation?

Also note that if top-level clk APIs are going to be holding
prepare_lock (clk_get_rate, clk_get_parent, etc) then we can't call
these APIs to get the info we want from within the platform code. For
example, today most .recalc_rate implementations reference
clk->parent->rate and apply it to some divider or something. To get
around having an opaque cookie .recalc_rate would have to have
parent->rate passed into it since the implementation cannot call
clk_get_rate(clk_get_parent(clk)) due to not having 'clk' in the first
place, and also because of mutex nesting.

I agree that by not using an opaque cookie we open ourselves up to the
possibility of badness, but we'll just have to catch it in code
review.

>
>> + * clk_init.  A key assumption in the call to .get_parent is that clks
>> + * are being initialized in-order.
>
> We should not require that, see below.
>
>> + */
>
>> +void clk_init(struct device *dev, struct clk *clk)
>> +{
>> +     if (!clk)
>> +             return;
>> +
>> +     INIT_HLIST_HEAD(&clk->children);
>> +     INIT_HLIST_NODE(&clk->child_node);
>> +
>> +     mutex_lock(&prepare_lock);
>> +
>> +     /*
>> +      * .get_parent is mandatory for populating .parent for clks with
>> +      * multiple possible parents.  For clks with a fixed parent
>> +      * .get_parent is not mandatory and .parent can be statically
>> +      * initialized
>> +      */
>> +     if (clk->ops->get_parent)
>> +             clk->parent = clk->ops->get_parent(clk);
>> +
>> +     /* clks without a parent are considered root clks */
>
> I'd rather prefer that root clocks are explicitely marked with a flag
> instead of relying on clk->parent being NULL.

I originally had CLK_IS_ROOT flag but removed it after thinking that
some clks might be a root sometimes, or a child at other times and I
had considered the flags to be constant.

If the flags aren't going to be constant and can change at run-time
then I can move this back to a flag. Also if there are no use cases
where a clk can change from a root to a child then again this can
safely go into flags. Thoughts?

>
>> +     if (clk->parent)
>> +             hlist_add_head(&clk->child_node,
>> +                             &clk->parent->children);
>> +     else
>> +             hlist_add_head(&clk->child_node, &clk_root_list);
>
> You could put clocks which have no parent and are not marked as root
> clock onto a separate list clk_orphaned or such. You might need this
> for handling clocks which are disconnected from any parent runtime as
> well.

Agreed, that is a more well-defined approach.

> And to avoid the whole in-order initialization issue, you could change
> clk_init to:
>
> struct clk *clk_init(struct device *dev, const struct clk_hw *hw,
>                     unsigned long flags, const char *parent_name)
>
> Then you can lookup the requested parent by name. If that fails, you
> put the clock into the orphaned list. When a new clock is initialized,
> then you walk the orphaned list and check whether something is waiting
> for that clock.
>
> To handle multi-parent clocks you need to go one step further and add:
>
> struct clk_hw {
>       struct clk_hw_ops *ops;
>       const char        *name;
>       const char        **possible_parents;
> };
>
> You also require a get_parent_idx() function in clk_hw_ops. So when
> clk_init() is called with parent_name = NULL and get_parent_idx() is
> implemented, then you call it and the clock returns you the index of
> the possible_parents array. If that parent clock is not yet
> registered, you store the requested name and do the lookup when new
> clocks are registered.

This approach ends up having something like O(n^2) (similar to
discussion around driver re-probing).

However, I agree that forcing folks to register/init clks in-order is
asking for trouble. I also think that for groups of well defined clks
(SoC clks, or clks in the same device) that are statically initialized
we should allow for having clk->parent populated statically and leave
it at that (mux clks will still need to run clk->get_parent() in
clk_init of course, since we can't trust the bootloader).

>
> That has also the advantage, that you can validate parent settings
> upfront and for setting the parent during runtime, you don't have to
> acquire a pointer to the parent clock. It's enough to call
> clk_set_named_parent().

Right, but the usefulness of this last point hinges on whether or not
we want to hide struct clk from the rest of the world. I still think
that doing so will end up being a limitation that platforms end up
having to hack around. I'd like a lot more input from the folks
porting their platforms to this code on that point, as it is a
critical design element.

Thanks for reviewing,
Mike

>
> Thoughts ?
>
>         tglx
>
>
>
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