On Wed, Nov 23, 2011 at 12:33:47PM -0800, Turquette, Mike wrote:
> On Tue, Nov 22, 2011 at 6:03 PM, Saravana Kannan <skan...@codeaurora.org>
> wrote:
> > On 11/21/2011 05:40 PM, Mike Turquette wrote:
[...]
> >> +is modified slightly for brevity:
> >> +
> >> +struct clk {
> >> + const char *name;
> >> + const struct clk_hw_ops *ops;
> >
> > No strong opinion, but can we call this clk_ops for brevity?
>
> I prefer clk_hw_ops, as it clearly delineates that these operations
> know hardware-specific details.
>
I tend to agree with Saravana for brevity. Looking at clk-basic.c,
I do not think it's necessary to encode 'hw' in naming of clk_hw_fixed,
clk_hw_gate and any clocks wrapping 'struct clk' in there. For
example, naming like clk_dummy, clk_imx seems brevity and clear,
and we do not need clk_hw_dummy and clk_hw_imx necessarily.
[...]
> >> +
> >> +clk_set_rate - Attempts to change the clk rate to the one specified.
> >> +Depending on a variety of common flags it may fail to maintain system
> >> +stability or result in a variety of other clk rates changing.
> >
> > I'm not sure if this is intentional or if it's a mistake in phrasing it.
> > IMHO, the rates of other clocks that are actually made available to device
> > drivers should not be changed. This call might trigger rate changes inside
> > the tree to accommodate the request from various children, but should not
> > affect the rate of the leaf nodes.
> >
> > Can you please clarify the intention and/or the wording?
>
> Setting the flag CLK_GATE_SET_RATE tells clk_set_rate not to change
> the rate if the clk is enabled. This policy is not enforced
> abritrarily: you don't have to set the flag on your clk. I'll update
> the doc to make explicit mention of this flag.
>
I guess the concern is not about the flag but the result of clk_set_rate
that might change a variety of other clocks, while Saravana said it
should not. And I agree with Saravana.
> >> +clk_set_rate deserves a special mention because it is more complex than
> >> +the other operations. There are three key concepts to the common
> >> +clk_set_rate implementation:
> >> +
> >> +1) recursively traversing up the clk tree and changing clk rates, one
> >> +parent at a time, if each clk allows it
> >> +2) failing to change rate if the clk is enabled and must only change
> >> +rates while disabled
> >
> > Is this really true? Based on a quick glance at the other patches, it
> > doesn't look it disallows set rate if the clock is enabled. Which is
> > correct, because clock rates can be change while they are enabled (I'm
> > referring to leaf clocks) as long as the hardware supports doing it
> > correctly. So, this needs rewording? I'm guessing you are trying to say
> > that you can't change the parent rate if it has multiple enabled child
> > clocks running off of it and one of them wants to cause a parent rate
> > change? I'm not sure if even that should be enforced in the common code --
> > doesn't look like you do either.
>
> Same as my answer above. There is a flag which allows you to control
> this behavior.
>
On the contrary, I have clocks on mxs platform which can be set rate
only when they are enabled, while there are users call clk_set_rate
when the clock is not enabled yet. Do we need another flag
CLK_ON_SET_RATE for this type of clocks?
I'm unsure that clk API users will all agree with the use of the flags.
>From the code, the clock framework will make the call fail if users
attempt to clk_set_rate an enabled clock with flag CLK_GATE_SET_RATE.
And clk API users might argue that they do not (need to) know this
clock details, and it's clock itself (clock framework or/and clock
driver) who should handle this type of details.
> >
> >> +2) using clk rate change notifiers to allow devices to handle dynamic
> >
> > Must be 3)
>
> Haha good catch.
>
> >>
> >> +rate changes for clks which do support changing rates while enabled
> >
> > Again, I guess this applies to the other clock. Not the one for which
> > clk_set_rate() is being called.
>
> This applies to ANY clk which has the flag set and is called by
> __clk_set_rate (which may be called many times in a recursive path).
>
> >> +clk_set_rate(C, 26MHz);
> >> + __clk_set_rate(C, 26MHz);
> >> + clk->round_rate(C, 26MHz, *parent_rate);
> >> + [ round_rate returns 50MHz ]
> >> + [&parent_rate is 52MHz ]
> >> + clk->set_rate(C, 50Mhz);
> >> + [ clk C is set to 50MHz, which sets divider to 2 ]
> >> + __clk_set_rate(clk->parent, parent_rate);
> >> + clk->round_rate(B, 52MHz, *parent_rate);
> >> + [ round_rate returns 100MHz ]
> >> + [&parent_rate is 104MHz ]
> >> + clk->set_rate(B, 100MHz);
> >> + [ clk B stays at 100MHz, divider stays at 2 ]
> >> + __clk_set_rate(clk->parent, parent_rate);
> >> + [ round_rate returns 104MHz ]
> >> + [&parent_rate is ignored ]
> >> + clk->set_rate(A, 104MHz);
> >> + [ clk A is set to 104MHz]
> >> +
> >
> > I will come back to this topic later on. I like the idea of propagating the
> > needs to the parent, but not sure if the current implementation will work
> > for all types of clock trees/set rates even though the HW might actually
> > allow it to be done right.
>
> I'll need more to go on than "it may not work".
clk A
|
|
|
clk B -----------\
| |
| |
| |
clk C clk D
You have stated "Another caveat is that child clks might run at weird
intermediate frequencies during a complex upwards propagation". I'm
not sure that intermediate frequency will be safe/reasonable for all
the clocks.
Another thing is what we mentioned above, the set_rate propagating
should not change other leaf clocks' frequency. For example, there is
timer_clk (clk D) as another child of clk B. When the set_rate of
clk C gets propagated to change frequency for clk B, it will in turn
change the frequency for timer_clk. I'm sure that some talk need to
happen between clk framework and timer driver before frequency of clk B
gets actually changed. Is this something will be handled by rate
change notifications which is to be added?
> As proof of concept I
> converted OMAP4's CPUfreq driver to use this strategy. Quick
> explanation:
>
> OMAP4's CPUfreq driver currently adjusts the rate of a PLL via
> clk_set_rate. However the PLL isn't *really* the clk closest to the
> ARM IP, there are other divider clocks after the PLL, which typically
> divide by 1. So practically speaking the PLL is the one that matters
> with respect to getting the Cortex-A9 rates to change.
>
> To test the recursive clk_set_rate I wrote a new .round_rate for the
> clks below the PLL and set the CLK_PARENT_SET_RATE flag on those clks.
> Then I changed the CPUfreq driver to call clk_set_rate on the lowest
> clk in that path (instead of the PLL) which better reflects reality.
> The code worked perfectly, propagating the request up to the PLL where
> the rate was finally set.
>
> This was a simple test since that PLL is dedicated to the Cortex-A9s,
> but the code does work. If there is a sequencing issue please let me
> know and I'll see how things can be re-arranged or made more flexible
> for your platform.
>
I'm currently looking at imx6 and mxs (imx28). On imx6, I do not see
the need for set_rate propagation at all. On mxs, I do see the need,
but it's a simple propagation, with only one level up and 1-1
parent-child relationship. I'm not sure if it's a good idea to support
the full set_rate propagation from the beginning, since it makes the
implementation difficult dramatically.
--
Regards,
Shawn
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