[PATCH 1/3] clk: add flag for clocks that need to be enabled on rate changes

[Heiko Stübner]

Am Sonntag, 11. Oktober 2015, 12:41:09 schrieb Heiko Stübner:
> Hi Stephen,
> 
> Am Donnerstag, 8. Oktober 2015, 14:58:40 schrieb Stephen Boyd:
> > On 10/02, Heiko Stübner wrote:
> > > Hi,
> > > 
> > > any comment on these 3 patches?
> > 
> > Dong has a similar problem, but those patches conflate this with
> > enabling parent clocks during clk_disable_unused() which makes no
> > sense to me. So I'm ok with the requirement that we turn clocks
> > on to change rates, but I wonder if in this case we need to turn
> > on the clock that's changing rates itself, or if we just need to
> > turn on the parent and/or future parent of the clock during the
> > rate switch. Care to elaborate on that?
> 
> As you can see in the follow-up patches, the fractional dividers on Rockchip
> SoCs are quite strange in that they even need to have their _downstream_
> mux point to them to actually accept rate changes.
> 
> The register value always reflects the value set by the system, but hardware
> really only accepts it if the clock is enabled and even the downstream mux
> selects the fractional divider as parent (they call it a auto-gating
> feature).
> 
> So in the worst (and current) case, you end up with the register showing the
> right value, but the hardware can use completely different dividers from
> the previous setting.
> 
> That strange behaviour got quite deeply investigated between Rockchip and
> Google engineers who stumbled upon this in the first place, so I'm
> reasonably sure this is the right solution for that clock type :-) .

Xing Zheng now also independently stumbled upon this issue with his rk3036 
work. And came to the same conclusion that the gate must be enabled as well as 
the downstream mux be set to the fractional divider for it to actually accept 
a new setting.