linux-kernel - Re: [PATCH v4 6/6] Documentation: ACPI: Document probe-low-power

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Date:   Tue, 21 Jan 2020 17:09:44 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Sakari Ailus <sakari.ailus@...ux.intel.com>
Cc:     linux-i2c <linux-i2c@...r.kernel.org>,
        Wolfram Sang <wsa@...-dreams.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        ACPI Devel Maling List <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 Mailing List <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Mani, Rajmohan" <rajmohan.mani@...el.com>,
        Tomasz Figa <tfiga@...omium.org>
Subject: Re: [PATCH v4 6/6] Documentation: ACPI: Document probe-low-power _DSD property

On Tue, Jan 21, 2020 at 2:41 PM Sakari Ailus
<sakari.ailus@...ux.intel.com> wrote:
>
> Document the probe-low-power _DSD property and how it is used with I²C
> drivers.
>
> Signed-off-by: Sakari Ailus <sakari.ailus@...ux.intel.com>
> ---
>  .../acpi/dsd/probe-low-power.rst              | 28 +++++++++++++++++++
>  Documentation/firmware-guide/acpi/index.rst   |  1 +
>  2 files changed, 29 insertions(+)
>  create mode 100644 Documentation/firmware-guide/acpi/dsd/probe-low-power.rst
>
> diff --git a/Documentation/firmware-guide/acpi/dsd/probe-low-power.rst b/Documentation/firmware-guide/acpi/dsd/probe-low-power.rst
> new file mode 100644
> index 0000000000000..e0343ffefe071
> --- /dev/null
> +++ b/Documentation/firmware-guide/acpi/dsd/probe-low-power.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. The _DSD property
> +"probe-low-power" has been defined for this purpose.

Well, if you say "has been defined", you need to provide a pointer to
the definition document (a specification of some sort or similar).

> +
> +How it works
> +============
> +
> +The property "probe-low-power" boolean property may be used to tell Linux
> +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 in its struct i2c_driver.probe_low_power field and the
> +"probe-low-power" 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.
> diff --git a/Documentation/firmware-guide/acpi/index.rst b/Documentation/firmware-guide/acpi/index.rst
> index ad3b5afdae77e..4e4ac675c0ff8 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/probe-low-power
>     enumeration
>     osi
>     method-customizing
> --
> 2.20.1
>