Message-ID: <0706576a-ce61-1cf3-bed1-05f54a1e2489@nvidia.com>
Date:   Tue, 16 Jul 2019 11:30:47 -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 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.
>>>>> 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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