Message-ID: <2b701832-5548-7c83-7c17-05cc2f1470c8@nvidia.com>
Date:   Tue, 16 Jul 2019 11:19:15 -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 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...
>>>> 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.
>>>> 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 ...

>>> For OVR regulator:
>>> 2) Change PWM output pin from output to tristate: vdd_cpu will go back
>>>     to hardwired boot voltage.
>>> 3) Disable DFLL PWM output
>>>
>>> For I2C regulator:
>>> 2) Program vdd_cpu regulator voltage to the boot voltage
>>> 3) Disable DFLL I2C output
>>>
>>> 4) Reprogram parent saved in step 0 of 'Switch to DFLL' to the saved
>>>     frequency
>>> 5) Change cclk_g parent to saved parent
>>> 6) Disable DFLL
>
> This is the same sequence currently implemented. But dfll 
> suspend/resume calls are thru Tegra210 clock driver.
>
> Dmitry wants to have dfll suspend/resume along with CCLK_G restore to 
> happen from CPUFreq driver pm_ops rather than tegra210 clock driver or 
> tegra dfll driver.
>
> Will move it to CPUFreq driver...
>
Thanks!

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