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

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Date:   Wed, 23 Dec 2020 23:37:21 +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 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