linux-kernel - Re: [PATCH v5 1/6] i2c: Allow driver to manage the device's power state during probe

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Date:   Tue, 11 Aug 2020 11:57:44 +0300
From:   Sakari Ailus <sakari.ailus@...ux.intel.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     linux-i2c@...r.kernel.org, Wolfram Sang <wsa@...-dreams.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-acpi@...r.kernel.org, Bingbu Cao <bingbu.cao@...el.com>,
        linux-media@...r.kernel.org,
        Chiranjeevi Rapolu <chiranjeevi.rapolu@...el.com>,
        Hyungwoo Yang <hyungwoo.yang@...el.com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        rajmohan.mani@...el.com, Tomasz Figa <tfiga@...omium.org>,
        "Qiu, Tian Shu" <tian.shu.qiu@...el.com>,
        devicetree@...r.kernel.org
Subject: Re: [PATCH v5 1/6] i2c: Allow driver to manage the device's power
 state during probe

Hi Sudeep,

Thanks for the review.

On Mon, Aug 10, 2020 at 03:41:48PM +0100, Sudeep Holla wrote:
> On Mon, Aug 10, 2020 at 05:27:42PM +0300, Sakari Ailus wrote:
> > Enable drivers to tell ACPI that there's no need to power on a device for
> > probe. Drivers should still perform this by themselves if there's a need
> > to. In some cases powering on the device during probe is undesirable, and
> > this change enables a driver to choose what fits best for it.
> >
> > Add a field called "flags" into struct i2c_driver for driver flags, and a
> > flag I2C_DRV_FL_ALLOW_LOW_POWER_PROBE to tell a driver supports probe in
> > low power state.
> >
> > Signed-off-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > ---
> >  drivers/i2c/i2c-core-base.c | 17 ++++++++++++++---
> >  include/linux/i2c.h         | 14 ++++++++++++++
> >  2 files changed, 28 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/i2c/i2c-core-base.c b/drivers/i2c/i2c-core-base.c
> > index 34a9609f256da..cde9cf49a07e6 100644
> > --- a/drivers/i2c/i2c-core-base.c
> > +++ b/drivers/i2c/i2c-core-base.c
> > @@ -436,6 +436,14 @@ static int i2c_smbus_host_notify_to_irq(const struct i2c_client *client)
> >  	return irq > 0 ? irq : -ENXIO;
> >  }
> >
> > +static bool allow_low_power_probe(struct device *dev)
> > +{
> > +	struct i2c_driver *driver = to_i2c_driver(dev->driver);
> > +
> > +	return driver->flags & I2C_DRV_FL_ALLOW_LOW_POWER_PROBE &&
> > +		device_property_present(dev, "allow-low-power-probe");
> 
> I assume this change makes even the DT property "allow-low-power-probe"
> work in the same way. Should we have proper DT binding for that ?
> 
> This comment applies for any property using device_property_* but has
> no explicit DT binding ? Just asking the question to know the strategy
> followed. Sorry if this is redundant question, feel free to point me
> to the past discussions.

It's not a redundant question, no.

I²C drivers on OF are responsible for controlling device's power state
already (using runtime PM or without) so I think the drivers could use the
property directly on OF systems (and document the property in DT bindings
first) if there's a need to. IOW this code isn't needed on OF.

Note that the power_on or power_off arguments are not used by
genpd_dev_pm_attach() or genpd_dev_pm_detach() so this patch only affects
ACPI. I think I should check the device is an ACPI device above, for
clarity.

Cc also DT list. The entire set is here:

<URL:https://lore.kernel.org/linux-acpi/20200810142747.12400-1-sakari.ailus@linux.intel.com/>

-- 
Kind regards,

Sakari Ailus