linux-kernel - Re: [PATCH V5 11/18] clk: tegra210: Add support for Tegra210 clocks

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Message-ID: <1c85cb35-ce7c-1dd1-f637-0c91b2b36db3@nvidia.com>
Date:   Thu, 18 Jul 2019 13:36:35 -0700
From:   Sowjanya Komatineni <skomatineni@...dia.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Peter De Schrijver <pdeschrijver@...dia.com>
CC:     <sboyd@...nel.org>, Michael Turquette <mturquette@...libre.com>,
        Joseph Lo <josephl@...dia.com>, <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>,
        <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 7/18/19 1:26 PM, Dmitry Osipenko wrote:
> 18.07.2019 22:42, Peter De Schrijver пишет:
>> On Thu, Jul 18, 2019 at 02:44:56AM +0300, Dmitry Osipenko wrote:
>>>> dependencies I am referring are dfll_ref, dfll_soc, and DVFS peripheral
>>>> clocks which need to be restored prior to DFLL reinit.
>>> Okay, but that shouldn't be a problem if clock dependencies are set up
>>> properly.
>>>
>>>>>> reverse list order during restore might not work as all other clocks are
>>>>>> in proper order no with any ref clocks for plls getting restored prior
>>>>>> to their clients
>>>>> Why? The ref clocks should be registered first and be the roots for PLLs
>>>>> and the rest. If it's not currently the case, then this need to be
>>>>> fixed. You need to ensure that each clock is modeled properly. If some
>>>>> child clock really depends on multiple parents, then the parents need to
>>>>> in the correct order or CCF need to be taught about such
>>>>> multi-dependencies.
>>>>>
>>>>> If some required feature is missed, then you have to implement it
>>>>> properly and for all, that's how things are done in upstream. Sometimes
>>>>> it's quite a lot of extra work that everyone are benefiting from in
>>>>> the end.
>>>>>
>>>>> [snip]
>>>> Yes, we should register ref/parents before their clients.
>>>>
>>>> cclk_g clk is registered last after all pll and peripheral clocks are
>>>> registers during clock init.
>>>>
>>>> dfllCPU_out clk is registered later during dfll-fcpu driver probe and
>>>> gets added to the clock list.
>>>>
>>>> Probably the issue seems to be not linking dfll_ref and dfll_soc
>>>> dependencies for dfllCPU_out thru clock list.
>>>>
>>>> clk-dfll driver during dfll_init_clks gets ref_clk and soc_clk reference
>>>> thru DT.
>> The dfll does not have any parents. It has some clocks which are needed
>> for the logic part of the dfll to function, but there's no parent clock
>> as such unlike for peripheral clocks or PLLs where the parent is at
>> least used as a reference. The I2C controller of the DFLL shares the
>> lines with a normal I2C controller using some arbitration logic. That
>> logic only works if the clock for the normal I2C controller is enabled.
>> So you need probably 3 clocks enabled to initialize the dfll in that
>> case. I don't think it makes sense to add complicated logic to the clock
>> core to deal with this rather strange case. To me it makes more sense to
>> use pmops and open code the sequence there.
> It looks to me that dfllCPU is a PLL and dfll_ref is its reference
> parent, while dfll_soc clocks the logic that dynamically reconfigures
> dfllCPU in background. I see that PLLP is defined as a parent for
> dfll_ref and dfll_soc in the code. Hence seems dfll_ref should be set as
> a parent for dfllCPU, no?

dfll_soc will not be restored by the time dfllCPU resume happens after 
dfll_ref.

without dfll_soc, dfllCPU cannot be resumed either. So if we decide to 
use parent we should use dfll_soc.

> Either way is good to me, given that DFLL will be disabled during
> suspend. Resetting DFLL on DFLL's driver resume using PM ops should be
> good. And then it also will be better to error out if DFLL is active
> during suspend on the DFLL's driver suspend.

Doing in dfll-fcpu pm_ops is much better as it happens right after all 
clocks are restored and unlike other clock enables, dfll need dfll 
controller programming as well and is actually registered in dfll-fcpu 
driver.

With this, below is the sequence:

CPUFreq suspend switches CPU to PLLP and disables dfll

Will add dfll_suspend/resume in dfll-fcpu driver and in dfll suspend 
will check for dfll active and will error out suspend.

dfll resume does dfll reinit.

CPUFreq resume enables dfll and switches CPU to dfll.


Will go with doing in dfll-fcpu pm_ops rather than parenting dfllCPU_OUT...