linux-kernel - Re: [PATCH v2 28/48] soc/tegra: Introduce core power domain driver

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Message-ID: <2298c089-827c-40a5-3ebe-3a91f3918ad5@gmail.com>
Date:   Wed, 23 Dec 2020 23:59:04 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Mark Brown <broonie@...nel.org>,
        Liam Girdwood <lgirdwood@...il.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Peter Geis <pgwipeout@...il.com>,
        Nicolas Chauvet <kwizart@...il.com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Viresh Kumar <vireshk@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-media@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-clk@...r.kernel.org
Subject: Re: [PATCH v2 28/48] soc/tegra: Introduce core power domain driver

23.12.2020 23:37, Dmitry Osipenko пишет:
> 23.12.2020 08:57, Viresh Kumar пишет:
>> On 22-12-20, 22:39, Dmitry Osipenko wrote:
>>> 22.12.2020 22:21, Dmitry Osipenko пишет:
>>>>>> +	if (IS_ERR(opp)) {
>>>>>> +		dev_err(&genpd->dev, "failed to find OPP for level %u: %pe\n",
>>>>>> +			level, opp);
>>>>>> +		return PTR_ERR(opp);
>>>>>> +	}
>>>>>> +
>>>>>> +	err = dev_pm_opp_set_voltage(&genpd->dev, opp);
>>>>> IIUC, you implemented this callback because you want to use the voltage triplet
>>>>> present in the OPP table ?
>>>>>
>>>>> And so you are setting the regulator ("power") later in this patch ?
>>>> yes
>>>>
>>>>> I am not in favor of implementing this routine, as it just adds a wrapper above
>>>>> the regulator API. What you should be doing rather is get the regulator by
>>>>> yourself here (instead of depending on the OPP core). And then you can do
>>>>> dev_pm_opp_get_voltage() here and set the voltage yourself. You may want to
>>>>> implement a version supporting triplet here though for the same.
>>>>>
>>>>> And you won't require the sync version of the API as well then.
>>>>>
>>>> That's what I initially did for this driver. I don't mind to revert back
>>>> to the initial variant in v3, it appeared to me that it will be nicer
>>>> and cleaner to have OPP API managing everything here.
>>>
>>> I forgot one important detail (why the initial variant wasn't good)..
>>> OPP entries that have unsupportable voltages should be filtered out and
>>> OPP core performs the filtering only if regulator is assigned to the OPP
>>> table.
>>>
>>> If regulator is assigned to the OPP table, then we need to use OPP API
>>> for driving the regulator, hence that's why I added
>>> dev_pm_opp_sync_regulators() and dev_pm_opp_set_voltage().
>>>
>>> Perhaps it should be possible to add dev_pm_opp_get_regulator() that
>>
>> What's wrong with getting the regulator in the driver as well ? Apart from the
>> OPP core ?
> 
> The voltage syncing should be done for each consumer regulator
> individually [1].
> 
> Secondly, regulator core doesn't work well today if the same regulator
> is requested more than one time for the same device.
> 
>>> will return the OPP table regulator in order to allow driver to use the
>>> regulator directly. But I'm not sure whether this is a much better
>>> option than the opp_sync_regulators() and opp_set_voltage() APIs.
>>
>> set_voltage() is still fine as there is some data that the OPP core has, but
>> sync_regulator() has nothing to do with OPP core.
>>
>> And this may lead to more wrapper helpers in the OPP core, which I am afraid of.
>> And so even if it is not the best, I would like the OPP core to provide the data
>> and not get into this. Ofcourse there is an exception to this, opp_set_rate.
>>
> 
> The regulator_sync_voltage() should be invoked only if voltage was
> changed previously [1].
> 
> The OPP core already has the info about whether voltage was changed and
> it provides the necessary locking for both set_voltage() and
> sync_regulator(). Perhaps I'll need to duplicate that functionality in
> the PD driver, instead of making it all generic and re-usable by other
> drivers.
> 
> [1]
> https://elixir.bootlin.com/linux/v5.10.2/source/drivers/regulator/core.c#L4107
> 

I just realized that the locking is missing in the v2 patches, something
to fix in the next revision :)

Still I'm in favor of extending the OPP API with the new common
functions. But if you have a strong opinion about that, then I'll try to
work around it in the PD driver in v3.