Re: [PATCH 02/10] clk: mediatek: mt8186-topckgen: Drop flags for main/univpll fixed factors

[Chen-Yu Tsai]

On Mon, Oct 24, 2022 at 3:23 AM AngeloGioacchino Del Regno
<angelogioacchino.delregno@xxxxxxxxxxxxx> wrote:
>
> The mainpll and univpll clocks are used as clock sources for multiple
> peripherals of different kind, some of which are critical (like AXIs);
> a rate change on any of these two will produce a rate change on many
> devices and that's likely to produce system instability if not done
> correctly: this is the reason why we have "fixed factor" clocks, used
> by MUX clocks to provide different rates based on PLL output dividers.
>
> Though, there's one fundamental issue that must be resolved somehow:
>
> When performing GPU DVFS, we get a rate request that will try to change
> the frequency of MAINPLL due to the CLK_TOP_MFG mux having clk26m,
> mfgpll (the GPU dedicated PLL), mainpll_d3, mainpll_d5 (fixed factor
> dividers) as possible parents.
>
> In order to solve that, there are two ways:
>  1. Add new "fake" mainpll_d3_fixed, mainpll_d5_fixed clocks, clones
>     of mainpll_d3, mainpll_d5 clocks, for the only purpose of not
>     declaring CLK_SET_RATE_PARENT; or
>  2. Simply drop said flag from the original dividers.
>
> After some careful validation, I cannot see anything calling a rate
> change request during runtime for MAINPLL, nor for UNIVPLL (which would,
> again, mean that we're reclocking lots of peripherals at once!), so it
> is safe *and sane* to simply remove the CLK_SET_RATE_PARENT flag to all
> of the main/univpll fixed factor divider clocks.
>
> Besides, if for any (doubtful) reason main/univpll rate change will be
> required in the future, it's still possible to call that on the PLL main
> clocks, so we're still covered anyway.
>
> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>

AFAIK this matches what we do for some other SoC families, so

Reviewed-by: Chen-Yu Tsai <wenst@xxxxxxxxxxxx>