Re: [PATCH 4/5] clk: Add flag to prevent frequency changes when walking subtrees

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Maxime,

>> The exclusive rate code could support this if it doesn't already do so.
>> If you call clk_set_rate_exclusive(child, <constant rate>) followed by
>> clk_set_rate(parent, <new rate>) the core code should try to keep the
>> child at the constant rate, or fail the clk_set_rate() call on the
>> parent. It should be possible to confirm this with some KUnit tests for
>> clk_set_rate_exclusive(). Similarly, if another child, child_B, of the
>> parent changes the parent rate, we should speculate the new rate of the
>> child_A that's protected and fail if we can't maintain the rate. We need
>> to start generating a list of clks that we operate a rate change on to
>> support this though, because right now we rely on the stack to track the
>> clks that we change the rate of.
>> 
>> Initially we thought that we could do this with clk notifiers. That may
>> work here, but I suspect it will be clunky to get working because clk
>> notifiers operate on struct clk.
>
> I think notifiers are great for customers, but not really adequate for
> the clock drivers tree. Indeed, you can only react to a (sub)tree
> configuration using notifiers, but you can't affect it to try something
> new that would be a better fit.
>
> Like, if we have a PLL A, with two child clocks that are dividers. B is
> initially (exclusively) set to freq X, and then you want to set C to 2X.
>
> The best thing to do is to set A to 2X, and double B's divider. It's
> simple enough, but we have no way to try to negociate that at the
> moment.

Indeed. Do you have something in mind to address this situation (eg. a
new clk provider or core API)?

Thanks,
Miquèl