[PATCH v10 3/7] Documentation: ACPI: Document _DSE object usage for enum power state

[Sakari Ailus]

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

Signed-off-by: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
Reviewed-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
---
 Documentation/firmware-guide/acpi/index.rst   |  1 +
 .../firmware-guide/acpi/low-power-probe.rst   | 69 +++++++++++++++++++
 2 files changed, 70 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..b96804d959a6c
--- /dev/null
+++ b/Documentation/firmware-guide/acpi/low-power-probe.rst
@@ -0,0 +1,69 @@
+.. 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 _DSE (Device State for Enumeration) object that evaluates to integer 0 may
+be used to tell Linux the highest allowed D state for a device 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
+_DSE object evaluates to integer higher than the D state of the device, the
+device will not be powered on (put in D0 state) for probe.
+
+The D states and thus also the allowed values for _DSE are listed below. Refer
+to [1] for more information on device power states.
+
+.. code-block:: text
+
+	Number	State	Description
+	0	D0	Device fully powered on
+	1	D1
+	2	D2
+	3	D3hot
+	4	D3cold	Off
+
+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.
+
+References
+==========
+
+[1] https://uefi.org/specifications/ACPI/6.4/02_Definition_of_Terms/Definition_of_Terms.html#device-power-state-definitions
+
+Example
+=======
+
+An ASL example describing an ACPI device using _DSE object to tell Operating
+System the device should remain powered off during probe 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 (_DSE, 0, NotSerialized)
+		{
+			Return (0x4)
+                }
+	}
-- 
2.20.1