linux-kernel - RE: [PATCH v6 3/3] eeprom: at24: enable runtime pm support

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Message-ID: <7B8CE47BD58441468D2BB13285B50E6031DF530B@BGSMSX107.gar.corp.intel.com>
Date:   Mon, 9 Oct 2017 03:14:24 +0000
From:   "Mohandass, Divagar" <divagar.mohandass@...el.com>
To:     "'sakari.ailus@....fi'" <sakari.ailus@....fi>,
        Tomasz Figa <tfiga@...omium.org>
CC:     "Mani, Rajmohan" <rajmohan.mani@...el.com>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "wsa@...-dreams.de" <wsa@...-dreams.de>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>
Subject: RE: [PATCH v6 3/3] eeprom: at24: enable runtime pm support

Hi Tomasz,

>-----Original Message-----
>From: sakari.ailus@....fi [mailto:sakari.ailus@....fi]
>Sent: Tuesday, October 03, 2017 7:21 PM
>To: Tomasz Figa <tfiga@...omium.org>
>Cc: Mani, Rajmohan <rajmohan.mani@...el.com>; Mohandass, Divagar
><divagar.mohandass@...el.com>; robh+dt@...nel.org;
>mark.rutland@....com; wsa@...-dreams.de; devicetree@...r.kernel.org;
>linux-i2c@...r.kernel.org; linux-kernel@...r.kernel.org;
>mika.westerberg@...ux.intel.com; linux-pm@...r.kernel.org
>Subject: Re: [PATCH v6 3/3] eeprom: at24: enable runtime pm support
>
>Hi Tomasz,
>
>On Wed, Sep 20, 2017 at 12:32:22PM +0300, sakari.ailus@....fi wrote:
>> > >> >> @@ -743,11 +770,17 @@ static int at24_probe(struct i2c_client
>> > >> >> *client, const struct i2c_device_id *id)
>> > >> >>
>> > >> >>       i2c_set_clientdata(client, at24);
>> > >> >>
>> > >> >> +     /* enable runtime pm */
>> > >> >> +     pm_runtime_get_noresume(&client->dev);
>> > >> >> +     pm_runtime_set_active(&client->dev);
>> > >> >> +     pm_runtime_enable(&client->dev);
>> > >>
>> > >> Do we need this get_noresume/set_active dance? I remember it was
>> > >> for some reason needed for PCI devices, but I don't see why for
>> > >> I2C anything else than just pm_runtime_enable() would be necessary.
>> > >
>> > > You specifically do not need (all) this for PCI devices, but AFAIU
>> > > for I涎
>> > > devices you do. The runtime PM status of a device is disabled by
>> > > default and the use count is zero, but on ACPI based systems the
>> > > device is still powered on.
>> >
>> > Okay, so _get_noresume() and _set_active() would do the thing for
>> > ACPI indeed, but not sure about other platforms. Perhaps _enable(),
>> > _get_sync() would be more general?
>>
>> What I ended up doing in e.g. the smiapp driver was to explicitly
>> power the device on first and then enable runtime PM. (See
>> drivers/media/i2c/smiapp/smiapp-core.c .) This approach works even if
>> CONFIG_PM is disabled, both on DT and ACPI.
>
>pm_runtime_get_noresume() + pm_runtime_put() can be replaced by a single
>pm_runtime_idle() call (where pm_runtime_put() was). pm_runtime_enable()
>is required to enable runtime PM for a device.

Verified the change suggested by Sakari. 
Let me know I can send a updated patch version with this change.

>
>pm_runtime_allow() may be omitted but then to make the runtime PM
>framework to make the power state transitions this needs to be configured
>from the user space --- which I don't think is intended.
>
>Cc linux-pm, too.
>
>--
>Regards,
>
>Sakari Ailus
>e-mail: sakari.ailus@....fi