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From:   Nicolas Boichat <drinkcat@chromium.org>
Date:   Mon, 9 Mar 2020 09:53:35 +0800
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Subject: Re: [PATCH v4 7/7] RFC: drm/panfrost: devfreq: Add support for 2 regulators
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Looping back on this, after digging a bit deeper...

On Fri, Feb 14, 2020 at 9:38 AM Nick Fan (=E8=8C=83=E5=93=B2=E7=B6=AD) <Nic=
k.Fan@mediatek.com> wrote:
> [snip]
> > > Another thing that I'm not implementing is the dance that Mediatek
> > > does in their kbase driver when changing the clock (described in
> > > patch
> > > 2/7):
> > > ""
> > > The binding we use with out-of-tree Mali drivers includes more
> > > clocks, this is used for devfreq: the out-of-tree driver switches
> > > clk_mux to clk_sub_parent (26Mhz), adjusts clk_main_parent, then
> > > switches clk_mux back to clk_main_parent:
> > > (see
> > > https://chromium.googlesource.com/chromiumos/third_party/kernel/+/ch
> > > romeos-4.19/drivers/gpu/arm/midgard/platform/mediatek/mali_kbase_run
> > > time_pm.c#423)
> > > clocks =3D
> > >         <&topckgen CLK_TOP_MFGPLL_CK>,
> > >         <&topckgen CLK_TOP_MUX_MFG>,
> > >         <&clk26m>,
> > >         <&mfgcfg CLK_MFG_BG3D>;
> > > clock-names =3D
> > >         "clk_main_parent",
> > >         "clk_mux",
> > >         "clk_sub_parent",
> > >         "subsys_mfg_cg";
> > > ""
> > > Is there a clean/simple way to implement this in the clock
> > > framework/device tree? Or should we implement something in the
> > > panfrost driver?
> >
> > Putting parent clocks into 'clocks' for a device is a pretty common
> > abuse. The 'assigned-clocks' binding is what's used for parent clock
> > setup. Not sure that's going to help here though. Is this dance
> > because the parent clock frequency can't be changed cleanly?
>
> Nick/Weiyi, any idea why we do that dance in the first place? (maybe the =
PLL clock is unstable while it's being changed?)
>
> Clock source may become unstable during clock frequency changes, so it is=
 always safer to switch to a more reliable clock source.
> Otherwise, it may cause some problem in some corner case.
> I would suggest to keep it.

The Mediatek CPUfreq driver actually does a very similar dance:
https://github.com/torvalds/linux/blob/master/drivers/cpufreq/mediatek-cpuf=
req.c#L249

What they have in the device tree is the main clock, and the
"intermediate" clock that is required during switching:
clocks =3D <&mcucfg CLK_MCU_MP0_SEL>, <&topckgen CLK_TOP_ARMPLL_DIV_PLL1>;
clock-names =3D "cpu", "intermediate";

The topology looks like this:
 clk26m                              15       15        1    26000000
        0     0  50000
    armpll_ll                         1        1        0  1417000000
        0     0  50000
       mcu_mp0_sel                    0        0        0  1417000000
        0     0  50000

And device tree provides mcu_mp0_sel as "cpu", and the armpll_div_pll1
as "intermediate".

The driver looks up armpll_ll by calling get_parent, then:
 - set_parent(mcu_mp0_sel, armpll_div_pll1)
 - set_rate(armpll_ll, new_rate)
 - set_parent(mcu_mp0_sel, armpll_ll)

On MT8183's GPU, the topology is a little bit more complicated (but I
think there should be a way to merge mfg_bg3d an mfg_sel in the clock
core)
 clk26m                              15       15        1    26000000
        0     0  50000
    mfgpll                            1        1        0   419999817
        0     0  50000
       mfgpll_ck                      2        2        0   419999817
        0     0  50000
          mfg_sel                     3        3        0   419999817
        0     0  50000
             mfg_bg3d                 1        1        0   419999817
        0     0  50000

We're going to need a special panfrost devfreq driver for mt8183
anyway (to handle the 2 regulators), so it would be easy to take a
similar approach:
 - Add "intermediate" clock in the device tree (clk26m)
 - Find mfg_sel/mfgpll_ck using 1/2 clk_get_parent calls.
 - Switch mfg_sel to clk26m, set mfgpll_ck rate, switch mfg_sel back
to mfgpll_ck.

(BTW, I tried to look, and couldn't find examples or reparenting
during clock changes in drivers/clk, are there existing drivers doing
similar things? Or this would be new?).