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    Subject[PATCH v9 3/7] Documentation: ACPI: Document _PRE object usage for enum power state
    Date
    Document the use of the _PRE object for setting desirable power state
    during probe.

    Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
    ---
    Documentation/firmware-guide/acpi/index.rst | 1 +
    .../firmware-guide/acpi/low-power-probe.rst | 51 +++++++++++++++++++
    2 files changed, 52 insertions(+)
    create mode 100644 Documentation/firmware-guide/acpi/low-power-probe.rst

    diff --git a/Documentation/firmware-guide/acpi/index.rst b/Documentation/firmware-guide/acpi/index.rst
    index f72b5f1769fb2..d02712acccbc0 100644
    --- a/Documentation/firmware-guide/acpi/index.rst
    +++ b/Documentation/firmware-guide/acpi/index.rst
    @@ -25,5 +25,6 @@ ACPI Support
    acpi-lid
    lpit
    video_extension
    + low-power-probe
    extcon-intel-int3496
    intel-pmc-mux
    diff --git a/Documentation/firmware-guide/acpi/low-power-probe.rst b/Documentation/firmware-guide/acpi/low-power-probe.rst
    new file mode 100644
    index 0000000000000..3437f8c8f233a
    --- /dev/null
    +++ b/Documentation/firmware-guide/acpi/low-power-probe.rst
    @@ -0,0 +1,51 @@
    +.. 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.
    +
    +How it works
    +============
    +
    +The _PRE object that evaluates to integer 0 may be used to tell Linux that the
    +device should remain in the low power state during probe. 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 _PRE object evaluates to integer
    +0, 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.
    +
    +Example
    +=======
    +
    +An ASL example describing an ACPI device using this property looks like
    +this. Some objects not relevant from the example point of view have been
    +omitted.
    +
    +.. code-block:: text
    +
    + Device (CAM0)
    + {
    + Name (_HID, "SONY319A")
    + Name (_UID, Zero)
    + Name (_CRS, ResourceTemplate ()
    + {
    + I2cSerialBus(0x0020, ControllerInitiated, 0x00061A80,
    + AddressingMode7Bit, "\\_SB.PCI0.I2C0",
    + 0x00, ResourceConsumer)
    + })
    + Name (_PRE, 0, NotSerialized)
    + {
    + Return (0x0)
    + }
    + }
    --
    2.20.1
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