linux-kernel - Re: [PATCH v7 6/6] Documentation: ACPI: Document allow-low-power-probe

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Date:   Wed, 2 Sep 2020 18:34:30 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        linux-i2c <linux-i2c@...r.kernel.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Mani, Rajmohan" <rajmohan.mani@...el.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Bingbu Cao <bingbu.cao@...el.com>,
        Chiranjeevi Rapolu <chiranjeevi.rapolu@...el.com>,
        Hyungwoo Yang <hyungwoo.yang@...el.com>,
        linux-media@...r.kernel.org
Subject: Re: [PATCH v7 6/6] Documentation: ACPI: Document allow-low-power-probe
 _DSD property

On Wed, Sep 2, 2020 at 6:31 PM Sakari Ailus
<sakari.ailus@...ux.intel.com> wrote:
>
> Hi Rafael,
>
> Thank you for the review.
>
> On Wed, Sep 02, 2020 at 05:57:01PM +0200, Rafael J. Wysocki wrote:
> > On Tue, Sep 1, 2020 at 11:03 PM Sakari Ailus
> > <sakari.ailus@...ux.intel.com> wrote:
> > >
> > > Document the probe-low-power _DSD property and how it is used with I²C
> > > drivers.
> >
> > I would reorder the series to make this go right after the [1/6] or
> > maybe even fold it into that patch.
> >
> > The point is that the changes in [1/6] clearly depend on the property
> > defined here.
>
> Ack.
>
> >
> > > Signed-off-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> > > ---
> > >  .../acpi/dsd/allow-low-power-probe.rst        | 28 +++++++++++++++++++
> > >  Documentation/firmware-guide/acpi/index.rst   |  1 +
> > >  2 files changed, 29 insertions(+)
> > >  create mode 100644 Documentation/firmware-guide/acpi/dsd/allow-low-power-probe.rst
> > >
> > > diff --git a/Documentation/firmware-guide/acpi/dsd/allow-low-power-probe.rst b/Documentation/firmware-guide/acpi/dsd/allow-low-power-probe.rst
> > > new file mode 100644
> > > index 0000000000000..6fcc89162b898
> > > --- /dev/null
> > > +++ b/Documentation/firmware-guide/acpi/dsd/allow-low-power-probe.rst
> > > @@ -0,0 +1,28 @@
> > > +.. SPDX-License-Identifier: GPL-2.0
> > > +
> > > +======================================
> > > +Probing I²C devices in low power state
> > > +======================================
> > > +
> > > +Introduction
> > > +============
> > > +
> > > +In some cases it may be preferred to leave certain devices powered off for the
> > > +entire system bootup if powering on these devices has adverse side effects,
> > > +beyond just powering on the said device. Linux recognizes the _DSD property
> > > +"allow-low-power-probe" that can be used for this purpose.
> >
> > It would be good to refer to the document defining the generic _DSD
> > properties mechanism and the GUID used for that from here.
>
> I'll add a reference to the device properties UUID for _DSD spec.
>
> >
> > The meaning of  "_DSD property" may not be entirely clear to the
> > reader as it stands.
> >
> > And maybe call the property "i2c-allow-low-power-probe" or similar, to
> > indicate that it is specific to i2c.
>
> The bus determines that already, and it's only defined for I²C here. Should
> we need this in the future for e.g. I3C, there would be no need to think of
> renaming it, just changing the documentation and implementation.
>
> I don't have a strong opinion on that though.

The point is that the property is not going to be recognized beyond i2c.

> > > +
> > > +How it works
> > > +============
> > > +
> > > +The property "allow-low-power-probe" boolean property may be used to tell Linux
> >
> > "boolean device property" ?
>
> Oops. The other "property" was supposed to be removed by now. But yes, I
> agree with the suggestion.
>
> >
> > > +that the I²C framework should instruct the kernel ACPI framework to leave the
> > > +device in the low power state. If the driver indicates its support for this by
> > > +setting the I2C_DRV_FL_ALLOW_LOW_POWER_PROBE flag in struct i2c_driver.flags
> > > +field and the "allow-low-power-probe" property is present, the device will not
> > > +be powered on for probe.
> > > +
> > > +The downside is that as the device is not powered on, even if there's a problem
> > > +with the device, the driver likely probes just fine but the first user will
> > > +find out the device doesn't work, instead of a failure at probe time. This
> > > +feature should thus be used sparingly.
> >
> > It would be good to give an ASL example with this property provided.
>
> I'll add that for v8.
>
> >
> > > diff --git a/Documentation/firmware-guide/acpi/index.rst b/Documentation/firmware-guide/acpi/index.rst
> > > index ad3b5afdae77e..0070fcde9e669 100644
> > > --- a/Documentation/firmware-guide/acpi/index.rst
> > > +++ b/Documentation/firmware-guide/acpi/index.rst
> > > @@ -11,6 +11,7 @@ ACPI Support
> > >     dsd/graph
> > >     dsd/data-node-references
> > >     dsd/leds
> > > +   dsd/allow-low-power-probe
> > >     enumeration
> > >     osi
> > >     method-customizing
> > > --
>
> --
> Kind regards,
>
> Sakari Ailus