linux-kernel - Re: [PATCH v3 04/12] opp: Add dev_pm_opp_sync

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Date:   Mon, 18 Jan 2021 21:35:04 +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>,
        Peter Geis <pgwipeout@...il.com>,
        Nicolas Chauvet <kwizart@...il.com>,
        "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>, Nishanth Menon <nm@...com>,
        Yangtao Li <tiny.windzz@...il.com>,
        Matt Merhar <mattmerhar@...tonmail.com>,
        linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 04/12] opp: Add dev_pm_opp_sync_regulators()

18.01.2021 11:20, Viresh Kumar пишет:
>> +int dev_pm_opp_sync_regulators(struct device *dev)
>> +{
>> +	struct opp_table *opp_table;
>> +	struct regulator *reg;
>> +	int i, ret = 0;
>> +
>> +	/* Device may not have OPP table */
>> +	opp_table = _find_opp_table(dev);
>> +	if (IS_ERR(opp_table))
>> +		return 0;
>> +
>> +	/* Regulator may not be required for the device */
>> +	if (!opp_table->regulators)
>> +		goto put_table;
>> +
>> +	mutex_lock(&opp_table->lock);
> What exactly do you need this lock for ?

It is needed for protecting simultaneous invocations of
dev_pm_opp_sync_regulators() and dev_pm_opp_set_voltage().

The sync_regulators() should be invoked only after completion of the
set_voltage() in a case of Tegra power domain driver since potentially
both could be running in parallel. For example device driver may be
changing performance state in a work thread, while PM domain state is
syncing.

But maybe it will be better to move the protection to the PM driver
since we're focused on sync_regulators() and set_voltage() here, but
there are other OPP API functions which use regulators. I'll need to
take a closer look at it.