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

[Mohandass, Divagar]

Hi Tomasz,

>-----Original Message-----
>From: sakari.ailus@xxxxxx [mailto:sakari.ailus@xxxxxx]
>Sent: Tuesday, October 03, 2017 7:21 PM
>To: Tomasz Figa <tfiga@xxxxxxxxxxxx>
>Cc: Mani, Rajmohan <rajmohan.mani@xxxxxxxxx>; Mohandass, Divagar
><divagar.mohandass@xxxxxxxxx>; robh+dt@xxxxxxxxxx;
>mark.rutland@xxxxxxx; wsa@xxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx;
>linux-i2c@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>mika.westerberg@xxxxxxxxxxxxxxx; linux-pm@xxxxxxxxxxxxxxx
>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@xxxxxx 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 Iæ
>> > > 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@xxxxxx