Date:   Tue, 16 Jul 2019 14:35:44 -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 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.

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