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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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        pgaikwad@nvidia.com, sboyd@kernel.org, linux-clk@vger.kernel.org,
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        mperttunen@nvidia.com, spatra@nvidia.com, robh+dt@kernel.org,
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 <20190716083701.225f0fd9@dimatab>
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 <0ee06d1a-310d-59f7-0aa6-b688b33447f5@nvidia.com>
From:   Dmitry Osipenko <digetx@gmail.com>
Message-ID: <cedfafd0-4114-0821-0c4b-efc17c213449@gmail.com>
Date:   Wed, 17 Jul 2019 01:00:38 +0300
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17.07.2019 0:35, Sowjanya Komatineni пишет:
> 
> On 7/16/19 2:21 PM, Dmitry Osipenko wrote:
>> 17.07.2019 0:12, Sowjanya Komatineni пишет:
>>> 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
>> 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 min_millivolts
>> and frequency entries starting from 204MHZ defined per-table.
> Yes at Vmin CPU Fmax is ~800Mhz. So anything below that will work at
> Vmin voltage and PLLP max is 408Mhz.

Thank you for the clarification. It would be good to have that commented
in the code as well.