Re: [PATCH 1/4] clk: Add a generic clock infrastructure

[Richard Zhao]

On Tue, May 24, 2011 at 10:38:04AM +0200, Sascha Hauer wrote:
> On Tue, May 24, 2011 at 12:51:10AM -0700, Colin Cross wrote:
> > On Tue, May 24, 2011 at 12:02 AM, Sascha Hauer <s.hauer@xxxxxxxxxxxxxx> wrote:
> > > On Mon, May 23, 2011 at 04:55:15PM -0700, Colin Cross wrote:
> > >> On Fri, May 20, 2011 at 12:27 AM, Jeremy Kerr <jeremy.kerr@xxxxxxxxxxxxx> wrote:
> > >> >
> > >> >  struct clk_hw_ops {
> > >> >        int             (*prepare)(struct clk_hw *);
> > >> >        void            (*unprepare)(struct clk_hw *);
> > >> >        int             (*enable)(struct clk_hw *);
> > >> >        void            (*disable)(struct clk_hw *);
> > >> >        unsigned long   (*recalc_rate)(struct clk_hw *);
> > >> >        int             (*set_rate)(struct clk_hw *,
> > >> >                                        unsigned long, unsigned long *);
> > >> >        long            (*round_rate)(struct clk_hw *, unsigned long);
> > >> >        int             (*set_parent)(struct clk_hw *, struct clk *);
> > >> >        struct clk *    (*get_parent)(struct clk_hw *);
> > >> >  };
> > >>
> > >> You might want to split these into three separate structs, for mux
> > >> ops, rate ops, and gate ops.  That way, multiple building blocks (a
> > >> gate and a divider, for example), can be easily combined into a single
> > >> clock node.  Also, an init op that reads the clock tree state from the
> > >> hardware has been very useful on Tegra - there are often clocks that
> > >> you can't or won't change, and being able to view their state as
> > >> configured by the bootloader, and base other clocks off of them, is
> > >> helpful.
> > >
> > > The clock state is read initially from the hardware with the
> > > recalc_rate/get_parent ops. What do we need an additional init op for?
> > 
> > I see - I would rename them to make it clear they are for init, maybe
> > init_rate and init_parent, and not call them later - reading clock
> > state can be very expensive on some platforms, if not impossible -
> > Qualcomm requires RPCs to the modem to get clock state.  If every
> > clk_set_rate triggers state reads all the way through the descendants,
> > that could be a huge performance hit.  If you separate init and
> > recalculate, mux implementations can store their divider settings and
> > very easily recalculate their rate.
> 
> Even without additional hooks divider and muxes can decide to cache
> the actual register values.
> 
> > 
> > >> It also allows you to turn off clocks that are enabled by
> > >> the bootloader, but never enabled by the kernel (enabled=1,
> > >> enable_count=0).
> > >
> > > The enable count indeed is a problem. I don't think an init hook
> > > would be the right solution for this though as this sounds platform
> > > specific. struct clk_hw_ops should be specific to the type of clock
> > > (mux, divider, gate) and should be present only once per type.
> > >
> > > For the enable count (and whether a clock should initially be enabled or
> > > not) I can think of something like this:
> > >
> > > - A platform/SoC registers all clocks.
> > > - It then calls clk_prepare/enable for all vital core clocks
> > >  (SDRAM, CPU,...). At this time the enable counters are correct.
> > > - Then some hook in the generic clock layer is called. This hook
> > >  iterates over the tree and disables all clocks in hardware which
> > >  have a enable count of 0.
> > 
> > Is it always safe to disable a clock that is already disabled?
> 
> I'm not aware of any clock where that's not the case, but your mileage
> may vary. At least the implementation should be able to determine
> whether a clock already is disabled and just skip another disable.
Might be one possible case: arm AP and modem boot simultaneously, you can not
disable modem clock before modem driver increase the enable_count.
But nice to have a helper function to disable zero enable_count clocks.
> 
> >  An
> > init_enable hook that set an enabled flag would let you only disable
> > clocks that were actually left on by the bootloader, and report to the
> > user which ones are actually being turned off (which has caught a lot
> > of problems on Tegra).
> > 
> > >> > +
> > >> > +struct clk {
> > >> > +       const char              *name;
> > >> > +       struct clk_hw_ops       *ops;
> > >> > +       struct clk_hw           *hw;
> > >> > +       unsigned int            enable_count;
> > >> > +       unsigned int            prepare_count;
> > >> > +       struct clk              *parent;
> > >>
> > >> Storing the list of possible parents at this level can help abstract
> > >> some common code from mux ops if you pass the index into the list of
> > >> the new parent into the op - most mux ops only need to know which of
> > >> their mux inputs needs to be enabled.
> > >
> > > Please don't. Only muxes have more than one possible parent, so this
> > > should be handled there.
> > 
> > The cost is one pointer per clock that is not actually a mux, and the
> > benefit is that you can move a very common search loop out of every
> > mux implementation into the framework.  It also lets you determine
> > which clocks are connected, which becomes necessary if you try to do
> > per-tree locking or sysfs controls for clocks.
> 
> I agree that some sort of possible_parents iteration would be nice.
> 
> > 
> > >>
> > >> > +
> > >> > +       if (WARN_ON(clk->prepare_count == 0))
> > >> > +               return;
> > >> > +
> > >> > +       if (--clk->prepare_count > 0)
> > >> > +               return;
> > >> > +
> > >> > +       WARN_ON(clk->enable_count > 0);
> > >> > +
> > >> > +       if (clk->ops->unprepare)
> > >> > +               clk->ops->unprepare(clk->hw);
> > >> > +
> > >> > +       __clk_unprepare(clk->parent);
> > >> > +}
> > >> Are there any cases where the unlocked versions of the clk calls need
> > >> to be exposed to the ops implementations?  For example, a set_rate op
> > >> may want to call clk_set_parent on itself to change its parent to a
> > >> better source, and then set its rate.  It would need to call
> > >> __clk_set_parent to avoid deadlocking on the prepare_lock.
> > >
> > > I hope we can avoid that. The decision of the best parent should be left
> > > up to the user. Doing this in the mux/divider implementation would
> > > torpedo attempts to implement generic building blocks.
I believe some day, when we have to call child clock consumers' on-rate-change
hooks, we will need it.

Thanks
Richard
> > 
> > I agree it would be nice, but it does push some knowledge of the clock
> > tree into device drivers.  For example, on Tegra the display driver
> > may need to change the source pll of the display clock to get the
> > required pclk, which requires passing all the possible parents of the
> > display clock into the display driver.  If this is a common usage
> > pattern, there needs to be a hook in the ops to allow the clock or
> > clock chip to make a more global decision.
> 
> I think this is a common pattern. Another solution to this would be that
> the platform implements a clock whose only purpose is to build a bridge
> between the driver and the clock tree. There may be more constrains, for
> example in some cases you may need a clock which also runs in different
> sleep states whereas sometimes you may need a clock which is turned of
> when in sleep mode. I agree that this must be handled somewhere, but
> the clock framework is not the place to implement this stuff.
> 
> > >>
> > >> I think you should hold the prepare mutex around calls to
> > >> clk_round_rate, which will allow some code simplification similar to
> > >> what Russell suggested in another thread.  If you hold the mutex here,
> > >> as well as in clk_set_rate, and you call the round_rate op before the
> > >> set_rate op in clk_set_rate, op implementers can compute the rate in
> > >> their round_rate op, and save the register values needed to get that
> > >> rate in private temporary storage.  The set_rate op can then assume
> > >> that those register values are correct, because the lock is still
> > >> held, and just write them.  That moves all the computation to one
> > >> place, and it only needs to run once.
> > >
> > > This won't work in the case of cascaded dividers. These have to call
> > > clk_round_rate in their set_rate op for each possible divider value
> > > to get the best result. They can't do this when both set_rate and
> > > round_rate acquire the lock.
> > 
> > Then they call __clk_round_rate if they already have the lock?
> 
> I think this is an implementation detail. As our idea of how a clock
> framework should work still is quite different we can discuss this later ;)
> 
> > 
> > > [...]
> > >
> > >> > +struct clk *clk_register(struct clk_hw_ops *ops, struct clk_hw *hw,
> > >> > +               const char *name)
> > >> > +{
> > >> > +       struct clk *clk;
> > >> > +
> > >> > +       clk = kzalloc(sizeof(*clk), GFP_KERNEL);
> > >> > +       if (!clk)
> > >> > +               return NULL;
> > >> > +
> > >> > +       clk->name = name;
> > >> > +       clk->ops = ops;
> > >> > +       clk->hw = hw;
> > >> > +       hw->clk = clk;
> > >> > +
> > >> > +       /* Query the hardware for parent and initial rate */
> > >> > +
> > >> > +       if (clk->ops->get_parent)
> > >> > +               /* We don't to lock against prepare/enable here, as
> > >> > +                * the clock is not yet accessible from anywhere */
> > >> > +               clk->parent = clk->ops->get_parent(clk->hw);
> > >> > +
> > >> > +       if (clk->ops->recalc_rate)
> > >> > +               clk->rate = clk->ops->recalc_rate(clk->hw);
> > >> > +
> > >> > +
> > >> > +       return clk;
> > >> > +}
> > >> > +EXPORT_SYMBOL_GPL(clk_register);
> > >>
> > >> If you are requiring clk's parents (or possible parents?) to be
> > >> registered before clk, you could put the clk_lookup struct inside the
> > >> clk struct and call clkdev_add from clk_register, saving some
> > >> boilerplate in the platforms.
> > >
> > > There can be multiple struct clk_lookup for each clock.
> > 
> > Sure, and that could be handled by clk_register_alias.  Most of the
> > clocks have a single clk_lookup.
> 
> In my idea only few of the clocks have a clk_lookup (you mentioned a
> clock between the i2c divider and i2c gate elsewhere, this would never
> be passed to a device).
> 
> Sascha
> 
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