linux-kernel - Re: [PATCH 0/1] clk: Meson8/8b/8m2: fix the mali clock flags

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Message-Id: <20191224033636.1BB3F206B7@mail.kernel.org>
Date:   Mon, 23 Dec 2019 19:36:35 -0800
From:   Stephen Boyd <sboyd@...nel.org>
To:     Jerome Brunet <jbrunet@...libre.com>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        linux-amlogic@...ts.infradead.org, narmstrong@...libre.com
Cc:     mturquette@...libre.com, linux-clk@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/1] clk: Meson8/8b/8m2: fix the mali clock flags

Quoting Jerome Brunet (2019-12-16 11:17:21)
> 
> On Mon 16 Dec 2019 at 18:50, Stephen Boyd <sboyd@...nel.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!