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Subject: Re: [PATCH V5 11/18] clk: tegra210: Add support for Tegra210 clocks
To:     Sowjanya Komatineni <skomatineni@nvidia.com>,
        Peter De Schrijver <pdeschrijver@nvidia.com>,
        Joseph Lo <josephl@nvidia.com>
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From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <27641e30-fdd1-e53a-206d-71e1f23343fd@gmail.com>
Date:   Tue, 16 Jul 2019 23:47:46 +0300
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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.