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SubjectRe: [PATCH 0/1] clk: Meson8/8b/8m2: fix the mali clock flags
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Quoting Jerome Brunet (2019-12-16 11:17:21)
>
> On Mon 16 Dec 2019 at 18:50, Stephen Boyd <sboyd@kernel.org> wrote:
>
> > Quoting Jerome Brunet (2019-12-16 01:13:31)
> >>
> >> *updated last* which crucial to your use case.
> >>
> >> I just wonder if this crucial part something CCF guarantee and you can
> >> rely on it ... or if it might break in the future.
> >>
> >> Stephen, any thoughts on this ?
> >
> > We have problems with the order in which we call the set_rate clk_op.
> > Sometimes clk providers want us to call from leaf to root but instead we
> > call from root to leaf because of implementation reasons. Controlling
> > the order in which clk operations are done is an unsolved problem. But
> > yes, in the future I'd like to see us introduce the vaporware that is
> > coordinated clk rates that would allow clk providers to decide what this
> > order should be, instead of having to do this "root-to-leaf" update.
> > Doing so would help us with the clk dividers that have some parent
> > changing rate that causes the downstream device to be overclocked while
> > we change the parent before the divider.
> >
> > If there are more assumptions like this about how the CCF is implemented
> > then we'll have to be extra careful to not disturb the "normal" order of
> > operations when introducing something that allows clk providers to
> > modify it.
>
> I understand that CCR would, in theory, allow to define that sort of
> details. Still defining (and documenting) the default behavior would be
> nice.
>
> So the question is:
> * Can we rely set_rate() doing a root-to-leaf update until CCR comes
> around ?
> * If not, for use cases like the one described by Martin, I guess we
> are stuck with the notifier ? Or would you have something else to
> propose ?

I suppose we should just state that clk_set_rate() should do a
root-to-leaf update. It's not like anyone is interested in changing
this behavior. The notifier is not ideal. I've wanted to add a new
clk_op that would cover some amount of the notifier users by having a
'pre_set_rate' clk op that can mux the clk over to something safe or
setup a divider to something that is known to be safe and work. Then we
can avoid having to register for a notifier just to do something right
before the root-to-leaf update happens.

>
> >
> > Also, isn't CLK_SET_RATE_GATE broken in the case that clk_set_rate()
> > isn't called on that particular clk? I seem to recall that the flag only
> > matters when it's applied to the "leaf" or entry point into the CCF from
> > a consumer API.
>
> It did but not anymore
>
> > I've wanted to fix that but never gotten around to it.
>
> I fixed that already :P
> CLK_SET_RATE_GATE is a special case of clock protect. The clock is
> protecting itself so it is going down through the tree.
>

Ahaha ok. As you can see I'm trying to forget clock protect ;-)


>
> > The whole flag sort of irks me because I don't understand what consumers
> > are supposed to do when this flag is set on a clk. How do they discover
> > it?
>
> Actually (ATM) the consumer is not even aware of it. If a clock with
> CLK_SET_RATE_GATE is enabled, it will return the current rate to
> .round_rate() and .set_rate() ... as if it was fixed.

And then when the clk is disabled it will magically "unstick" and start
to accept the same rate request again?

>
> > They're supposed to "just know" and turn off the clk first and then
> > call clk_set_rate()?
>
> ATM, yes ... if CCF cannot switch to another "unlocked" subtree (the
> case here)
>
> > Why can't the framework do this all in the clk_set_rate() call?
>
> When there is multiple consumers the behavior would become a bit
> difficult to predict and drivers may have troubles anticipating that,
> maybe, the clock is locked.

Fun times!

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