Date:   Tue, 16 Jul 2019 14:12:36 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Joseph Lo <josephl@...dia.com>
CC:     <thierry.reding@...il.com>, <jonathanh@...dia.com>,
        <tglx@...utronix.de>, <jason@...edaemon.net>,
        <marc.zyngier@....com>, <linus.walleij@...aro.org>,
        <stefan@...er.ch>, <mark.rutland@....com>, <pgaikwad@...dia.com>,
        <sboyd@...nel.org>, <linux-clk@...r.kernel.org>,
        <linux-gpio@...r.kernel.org>, <jckuo@...dia.com>,
        <talho@...dia.com>, <linux-tegra@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <mperttunen@...dia.com>,
        <spatra@...dia.com>, <robh+dt@...nel.org>,
        <devicetree@...r.kernel.org>
Subject: Re: [PATCH V5 11/18] clk: tegra210: Add support for Tegra210 clocks


On 7/16/19 1:47 PM, Dmitry Osipenko wrote:
> 16.07.2019 22:26, Sowjanya Komatineni пишет:
>> On 7/16/19 11:43 AM, Dmitry Osipenko wrote:
>>> 16.07.2019 21:30, Sowjanya Komatineni пишет:
>>>> On 7/16/19 11:25 AM, Dmitry Osipenko wrote:
>>>>> 16.07.2019 21:19, Sowjanya Komatineni пишет:
>>>>>> On 7/16/19 9:50 AM, Sowjanya Komatineni wrote:
>>>>>>> On 7/16/19 8:00 AM, Dmitry Osipenko wrote:
>>>>>>>> 16.07.2019 11:06, Peter De Schrijver пишет:
>>>>>>>>> On Tue, Jul 16, 2019 at 03:24:26PM +0800, Joseph Lo wrote:
>>>>>>>>>>> OK, Will add to CPUFreq driver...
>>>>>>>>>>>> The other thing that also need attention is that T124 CPUFreq
>>>>>>>>>>>> driver
>>>>>>>>>>>> implicitly relies on DFLL driver to be probed first, which is
>>>>>>>>>>>> icky.
>>>>>>>>>>>>
>>>>>>>>>>> Should I add check for successful dfll clk register explicitly in
>>>>>>>>>>> CPUFreq driver probe and defer till dfll clk registers?
>>>>>>>> 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
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> [2]
>>>>>>>> https://www.kernel.org/doc/html/latest/driver-api/device_link.html
>>>>>>>>
>>>>>>>> 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
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>> Will go thru and add...
>>>>> 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!
>>>>>
>>>>>>>>>> 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.
>>>>>>>> Okay, then the CPUFreq driver will have to enforce DFLL freq to
>>>>>>>> PLLP's
>>>>>>>> rate before switching to PLLP in order to have a proper CPU voltage.
>>>>>>> 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
>>>>>>>
>>>>>> 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.
>>>>> Okay.
>>>>>
>>>>>>>>>> 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 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 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.
>>>>>>>> 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.
>>>>>>>>
>>>>>>>>>> The DFLL part looks good to me. BTW, change the patch subject to
>>>>>>>>>> "Add
>>>>>>>>>> suspend-resume support" seems more appropriate to me.
>>>>>>>>>>
>>>>>>>>> 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
>>>>>>>>>
>>>>>> 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 ...
>>>>> 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.
>>>>>
>>>> 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.
>>> AFAIK, PLLX could run at ~200MHz. There is also a divided 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]
>> 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 >800Mhz
>> and for low frequency it will be sourced from PLLP.
> Alright, then please don't forget to pre-initialize PLLP_OUT4 rate to a
> reasonable value using tegra_clk_init_table or assigned-clocks.

PLLP_OUT4 rate update is not needed as it is safe to run at 408Mhz 
because it is below fmax @ Vmin

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