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

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Date:   Thu, 26 Dec 2019 10:06:25 +0100
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Stephen Boyd <sboyd@...nel.org>,
        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


On Tue 24 Dec 2019 at 04:36, Stephen Boyd <sboyd@...nel.org> wrote:

> 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.
>

Martin,

It looks like a green light to me ;) Just add a detailed comment on the
mali top clock explaining things and it should be alright.

>>    
>> >
>> > 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?
>

Exactly

>> 
>> > 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!