[PATCH v9 3/7] Documentation: ACPI: Document _PRE object usage for enum power state

[Sakari Ailus]

Document the use of the _PRE object for setting desirable power state
during probe.

Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
---
 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