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Date:   Wed, 17 Jul 2019 06:54:31 +0300
From:   Dmitry Osipenko <digetx@gmail.com>
To:     Sowjanya Komatineni <skomatineni@nvidia.com>
Cc:     Peter De Schrijver <pdeschrijver@nvidia.com>,
        Joseph Lo <josephl@nvidia.com>, <thierry.reding@gmail.com>,
        <jonathanh@nvidia.com>, <tglx@linutronix.de>,
        <jason@lakedaemon.net>, <marc.zyngier@arm.com>,
        <linus.walleij@linaro.org>, <stefan@agner.ch>,
        <mark.rutland@arm.com>, <pgaikwad@nvidia.com>, <sboyd@kernel.org>,
        <linux-clk@vger.kernel.org>, <linux-gpio@vger.kernel.org>,
        <jckuo@nvidia.com>, <talho@nvidia.com>,
        <linux-tegra@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <mperttunen@nvidia.com>, <spatra@nvidia.com>, <robh+dt@kernel.org>,
        <devicetree@vger.kernel.org>
Subject: Re: [PATCH V5 11/18] clk: tegra210: Add support for Tegra210 clocks
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=D0=92 Tue, 16 Jul 2019 19:18:19 -0700
Sowjanya Komatineni <skomatineni@nvidia.com> =D0=BF=D0=B8=D1=88=D0=B5=D1=82:

> On 7/16/19 3:06 PM, Sowjanya Komatineni wrote:
> >
> > On 7/16/19 3:00 PM, Dmitry Osipenko wrote: =20
> >> 17.07.2019 0:35, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=D1=82: =
=20
> >>> On 7/16/19 2:21 PM, Dmitry Osipenko wrote: =20
> >>>> 17.07.2019 0:12, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=D1=82:=
 =20
> >>>>> On 7/16/19 1:47 PM, Dmitry Osipenko wrote: =20
> >>>>>> 16.07.2019 22:26, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=D1=
=82: =20
> >>>>>>> On 7/16/19 11:43 AM, Dmitry Osipenko wrote: =20
> >>>>>>>> 16.07.2019 21:30, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=
=D1=82: =20
> >>>>>>>>> On 7/16/19 11:25 AM, Dmitry Osipenko wrote: =20
> >>>>>>>>>> 16.07.2019 21:19, Sowjanya Komatineni =D0=BF=D0=B8=D1=88=D0=B5=
=D1=82: =20
> >>>>>>>>>>> On 7/16/19 9:50 AM, Sowjanya Komatineni wrote: =20
> >>>>>>>>>>>> On 7/16/19 8:00 AM, Dmitry Osipenko wrote: =20
> >>>>>>>>>>>>> 16.07.2019 11:06, Peter De Schrijver =D0=BF=D0=B8=D1=88=D0=
=B5=D1=82: =20
> >>>>>>>>>>>>>> On Tue, Jul 16, 2019 at 03:24:26PM +0800, Joseph Lo
> >>>>>>>>>>>>>> wrote: =20
> >>>>>>>>>>>>>>>> OK, Will add to CPUFreq driver... =20
> >>>>>>>>>>>>>>>>> The other thing that also need attention is that
> >>>>>>>>>>>>>>>>> T124 CPUFreq
> >>>>>>>>>>>>>>>>> driver
> >>>>>>>>>>>>>>>>> implicitly relies on DFLL driver to be probed first,
> >>>>>>>>>>>>>>>>> which is
> >>>>>>>>>>>>>>>>> icky.
> >>>>>>>>>>>>>>>>> =20
> >>>>>>>>>>>>>>>> Should I add check for successful dfll clk register
> >>>>>>>>>>>>>>>> explicitly in
> >>>>>>>>>>>>>>>> CPUFreq driver probe and defer till dfll clk
> >>>>>>>>>>>>>>>> registers? =20
> >>>>>>>>>>>>> Probably you should use the "device links". See [1][2]
> >>>>>>>>>>>>> for the
> >>>>>>>>>>>>> example.
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> [1]
> >>>>>>>>>>>>> https://elixir.bootlin.com/linux/v5.2.1/source/drivers/gpu/=
drm/tegra/dc.c#L2383=20
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> [2]
> >>>>>>>>>>>>> https://www.kernel.org/doc/html/latest/driver-api/device_li=
nk.html=20
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Return EPROBE_DEFER instead of EINVAL if
> >>>>>>>>>>>>> device_link_add() fails.
> >>>>>>>>>>>>> And
> >>>>>>>>>>>>> use of_find_device_by_node() to get the DFLL's device,
> >>>>>>>>>>>>> see [3].
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> [3]
> >>>>>>>>>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-=
next.git/tree/drivers/devfreq/tegra20-devfreq.c#n100=20
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> =20
> >>>>>>>>>>>> Will go thru and add... =20
> >>>>>>>>>> Looks like I initially confused this case with getting
> >>>>>>>>>> orphaned clock.
> >>>>>>>>>> I'm now seeing that the DFLL driver registers the clock
> >>>>>>>>>> and then clk_get(dfll) should be returning EPROBE_DEFER
> >>>>>>>>>> until DFLL driver is
> >>>>>>>>>> probed, hence everything should be fine as-is and there is
> >>>>>>>>>> no real
> >>>>>>>>>> need
> >>>>>>>>>> for the 'device link'. Sorry for the confusion!
> >>>>>>>>>> =20
> >>>>>>>>>>>>>>> Sorry, I didn't follow the mail thread. Just
> >>>>>>>>>>>>>>> regarding the DFLL
> >>>>>>>>>>>>>>> part.
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> As you know it, the DFLL clock is one of the CPU clock
> >>>>>>>>>>>>>>> sources and
> >>>>>>>>>>>>>>> integrated with DVFS control logic with the
> >>>>>>>>>>>>>>> regulator. We will not
> >>>>>>>>>>>>>>> switch
> >>>>>>>>>>>>>>> CPU to other clock sources once we switched to DFLL.
> >>>>>>>>>>>>>>> Because the
> >>>>>>>>>>>>>>> CPU has
> >>>>>>>>>>>>>>> been regulated by the DFLL HW with the DVFS table
> >>>>>>>>>>>>>>> (CVB or OPP
> >>>>>>>>>>>>>>> table
> >>>>>>>>>>>>>>> you see
> >>>>>>>>>>>>>>> in the driver.). We shouldn't reparent it to other
> >>>>>>>>>>>>>>> sources with
> >>>>>>>>>>>>>>> unknew
> >>>>>>>>>>>>>>> freq/volt pair. That's not guaranteed to work. We
> >>>>>>>>>>>>>>> allow switching to
> >>>>>>>>>>>>>>> open-loop mode but different sources. =20
> >>>>>>>>>>>>> Okay, then the CPUFreq driver will have to enforce DFLL=20
> >>>>>>>>>>>>> freq to
> >>>>>>>>>>>>> PLLP's
> >>>>>>>>>>>>> rate before switching to PLLP in order to have a proper
> >>>>>>>>>>>>> CPU voltage. =20
> >>>>>>>>>>>> PLLP freq is safe to work for any CPU voltage. So no
> >>>>>>>>>>>> need to enforce
> >>>>>>>>>>>> DFLL freq to PLLP rate before changing CCLK_G source to
> >>>>>>>>>>>> PLLP during
> >>>>>>>>>>>> suspend
> >>>>>>>>>>>> =20
> >>>>>>>>>>> Sorry, please ignore my above comment. During suspend,
> >>>>>>>>>>> need to change
> >>>>>>>>>>> CCLK_G source to PLLP when dfll is in closed loop mode
> >>>>>>>>>>> first and
> >>>>>>>>>>> then
> >>>>>>>>>>> dfll need to be set to open loop. =20
> >>>>>>>>>> Okay.
> >>>>>>>>>> =20
> >>>>>>>>>>>>>>> And I don't exactly understand why we need to switch
> >>>>>>>>>>>>>>> to PLLP in
> >>>>>>>>>>>>>>> CPU
> >>>>>>>>>>>>>>> idle
> >>>>>>>>>>>>>>> driver. Just keep it on CL-DVFS mode all the time.
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> In SC7 entry, the dfll suspend function moves it the=20
> >>>>>>>>>>>>>>> open-loop
> >>>>>>>>>>>>>>> mode. That's
> >>>>>>>>>>>>>>> all. The sc7-entryfirmware will handle the rest of the
> >>>>>>>>>>>>>>> sequence to
> >>>>>>>>>>>>>>> turn off
> >>>>>>>>>>>>>>> the CPU power.
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> In SC7 resume, the warmboot code will handle the=20
> >>>>>>>>>>>>>>> sequence to
> >>>>>>>>>>>>>>> turn on
> >>>>>>>>>>>>>>> regulator and power up the CPU cluster. And leave it
> >>>>>>>>>>>>>>> on PLL_P.
> >>>>>>>>>>>>>>> After
> >>>>>>>>>>>>>>> resuming to the kernel, we re-init DFLL, restore the
> >>>>>>>>>>>>>>> CPU clock
> >>>>>>>>>>>>>>> policy (CPU
> >>>>>>>>>>>>>>> runs on DFLL open-loop mode) and then moving to
> >>>>>>>>>>>>>>> close-loop mode. =20
> >>>>>>>>>>>>> The DFLL is re-inited after switching CCLK to DFLL
> >>>>>>>>>>>>> parent during of
> >>>>>>>>>>>>> the
> >>>>>>>>>>>>> early clocks-state restoring by CaR driver. Hence
> >>>>>>>>>>>>> instead of having
> >>>>>>>>>>>>> odd
> >>>>>>>>>>>>> hacks in the CaR driver, it is much nicer to have a
> >>>>>>>>>>>>> proper suspend-resume sequencing of the device drivers.
> >>>>>>>>>>>>> In this case CPUFreq
> >>>>>>>>>>>>> driver is the driver that enables DFLL and switches CPU
> >>>>>>>>>>>>> to that
> >>>>>>>>>>>>> clock
> >>>>>>>>>>>>> source, which means that this driver is also should be
> >>>>>>>>>>>>> responsible for
> >>>>>>>>>>>>> management of the DFLL's state during of suspend/resume
> >>>>>>>>>>>>> process. If
> >>>>>>>>>>>>> CPUFreq driver disables DFLL during suspend and
> >>>>>>>>>>>>> re-enables it during
> >>>>>>>>>>>>> resume, then looks like the CaR driver hacks around
> >>>>>>>>>>>>> DFLL are not
> >>>>>>>>>>>>> needed.
> >>>>>>>>>>>>> =20
> >>>>>>>>>>>>>>> The DFLL part looks good to me. BTW, change the patch
> >>>>>>>>>>>>>>> subject to
> >>>>>>>>>>>>>>> "Add
> >>>>>>>>>>>>>>> suspend-resume support" seems more appropriate to me.
> >>>>>>>>>>>>>>> =20
> >>>>>>>>>>>>>> To clarify this, the sequences for DFLL use are as
> >>>>>>>>>>>>>> follows (assuming
> >>>>>>>>>>>>>> all
> >>>>>>>>>>>>>> required DFLL hw configuration has been done)
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> Switch to DFLL:
> >>>>>>>>>>>>>> 0) Save current parent and frequency
> >>>>>>>>>>>>>> 1) Program DFLL to open loop mode
> >>>>>>>>>>>>>> 2) Enable DFLL
> >>>>>>>>>>>>>> 3) Change cclk_g parent to DFLL
> >>>>>>>>>>>>>> For OVR regulator:
> >>>>>>>>>>>>>> 4) Change PWM output pin from tristate to output
> >>>>>>>>>>>>>> 5) Enable DFLL PWM output
> >>>>>>>>>>>>>> For I2C regulator:
> >>>>>>>>>>>>>> 4) Enable DFLL I2C output
> >>>>>>>>>>>>>> 6) Program DFLL to closed loop mode
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> Switch away from DFLL:
> >>>>>>>>>>>>>> 0) Change cclk_g parent to PLLP so the CPU frequency
> >>>>>>>>>>>>>> is ok for
> >>>>>>>>>>>>>> any
> >>>>>>>>>>>>>> vdd_cpu voltage
> >>>>>>>>>>>>>> 1) Program DFLL to open loop mode
> >>>>>>>>>>>>>> =20
> >>>>>>>>>>> I see during switch away from DFLL (suspend), cclk_g
> >>>>>>>>>>> parent is not
> >>>>>>>>>>> changed to PLLP before changing dfll to open loop mode.
> >>>>>>>>>>>
> >>>>>>>>>>> Will add this ... =20
> >>>>>>>>>> The CPUFreq driver switches parent to PLLP during the
> >>>>>>>>>> probe, similar
> >>>>>>>>>> should be done on suspend.
> >>>>>>>>>>
> >>>>>>>>>> I'm also wondering if it's always safe to switch to PLLP
> >>>>>>>>>> in the probe.
> >>>>>>>>>> If CPU is running on a lower freq than PLLP, then some
> >>>>>>>>>> other more
> >>>>>>>>>> appropriate intermediate parent should be selected.
> >>>>>>>>>> =20
> >>>>>>>>> CPU parents are PLL_X, PLL_P, and dfll. PLL_X always runs
> >>>>>>>>> at higher
> >>>>>>>>> rate
> >>>>>>>>> so switching to PLL_P during CPUFreq probe prior to dfll
> >>>>>>>>> clock enable
> >>>>>>>>> should be safe. =20
> >>>>>>>> AFAIK, PLLX could run at ~200MHz. There is also a divided=20
> >>>>>>>> output of
> >>>>>>>> PLLP
> >>>>>>>> which CCLKG supports, the PLLP_OUT4.
> >>>>>>>>
> >>>>>>>> Probably, realistically, CPU is always running off a fast
> >>>>>>>> PLLX during
> >>>>>>>> boot, but I'm wondering what may happen on KEXEC. I guess
> >>>>>>>> ideally CPUFreq driver should also have a 'shutdown'
> >>>>>>>> callback to teardown DFLL
> >>>>>>>> on a reboot, but likely that there are other clock-related
> >>>>>>>> problems as
> >>>>>>>> well that may break KEXEC and thus it is not very important
> >>>>>>>> at the moment.
> >>>>>>>>
> >>>>>>>> [snip] =20
> >>>>>>> During bootup CPUG sources from PLL_X. By PLL_P source above
> >>>>>>> I meant
> >>>>>>> PLL_P_OUT4.
> >>>>>>>
> >>>>>>> As per clock policies, PLL_X is always used for high freq
> >>>>>>> like  =20
> >>>>>>> >800Mhz =20
> >>>>>>> and for low frequency it will be sourced from PLLP. =20
> >>>>>> Alright, then please don't forget to pre-initialize PLLP_OUT4=20
> >>>>>> rate to a
> >>>>>> reasonable value using tegra_clk_init_table or
> >>>>>> assigned-clocks. =20
> >>>>> PLLP_OUT4 rate update is not needed as it is safe to run at
> >>>>> 408Mhz because it is below fmax @ Vmin =20
> >>>> So even 204MHz CVB entries are having the same voltage as 408MHz,
> >>>> correct? It's not instantly obvious to me from the DFLL driver's
> >>>> code where the fmax @ Vmin is defined, I see that there is the=20
> >>>> min_millivolts
> >>>> and frequency entries starting from 204MHZ defined per-table. =20
> >>> Yes at Vmin CPU Fmax is ~800Mhz. So anything below that will work
> >>> at Vmin voltage and PLLP max is 408Mhz. =20
> >> Thank you for the clarification. It would be good to have that
> >> commented in the code as well. =20
> > OK, Will add... =20
>=20
> Regarding, adding suspend/resume to CPUFreq, CPUFreq suspend happens=20
> very early even before disabling non-boot CPUs and also need to
> export clock driver APIs to CPUFreq.
>=20
> Was thinking of below way of implementing this...
>=20
>=20
> Clock DFLL driver Suspend:
>=20
>  =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 - Save CPU clock policy registers,=
 and Perform dfll suspend=20
> which sets in open loop mode
>=20
> CPU Freq driver Suspend: does nothing
>=20
>=20
> Clock DFLL driver Resume:
>=20
>  =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 - Re-init DFLL, Set in Open-Loop m=
ode, restore CPU Clock
> policy registers which actually sets source to DFLL along with other
> CPU Policy register restore.
>=20
> CPU Freq driver Resume:
>=20
>  =C2=A0=C2=A0=C2=A0 =C2=A0=C2=A0=C2=A0 - do clk_prepare_enable which acut=
ally sets DFLL in Closed
> loop mode


It doesn't matter much when CPUFreq driver suspends, it's only
important that it suspends before CaR.

I'm not sure why do you need anything else from DFLL driver other than
what is already exposed via generic CCF API. It looks to me
that switching CPU's parent clock away from DFLL and then disabling
DFLL's clock is enough for suspend, accordingly to what Peter wrote. And
resuming is the same as what's done on CPUFreq's driver probe. The CCLK
policy should be saved and restored by the CaR driver, you don't need to
care about it. The cpufreq-dt driver sets
CPUFREQ_NEED_INITIAL_FREQ_CHECK, hence you don't need to care about
restoring the original CPU freq on resume, IIUC.