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

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Date:   Tue, 3 Oct 2017 16:51:09 +0300
From:   "sakari.ailus@....fi" <sakari.ailus@....fi>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     "Mani, Rajmohan" <rajmohan.mani@...el.com>,
        "Mohandass, Divagar" <divagar.mohandass@...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, 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.

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