Documentation: ACPI: Document _DSC object usage for enum power state

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

Signed-off-by: Sakari Ailus <[email protected]>
Reviewed-by: Tomasz Figa <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: Sakari Ailus

Documentation/firmware-guide/acpi/index.rst

@@ -26,5 +26,6 @@ ACPI Support
    acpi-lid
    lpit
    video_extension
+   non-d0-probe
    extcon-intel-int3496
    intel-pmc-mux

Documentation/firmware-guide/acpi/non-d0-probe.rst

@@ -0,0 +1,78 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+========================================
+Probing devices in other D states than 0
+========================================
+
+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 _DSC (Device State for Configuration) object that evaluates to an integer
+may be used to tell Linux the highest allowed D state for a device during
+probe. The support for _DSC requires support from the kernel bus type if the
+bus driver normally sets the device in D0 state for probe.
+
+The downside of using _DSC 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.
+
+I²C
+---
+
+If an I²C driver indicates its support for this by setting the
+I2C_DRV_ACPI_WAIVE_D0_PROBE flag in struct i2c_driver.flags field and the
+_DSC 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.
+
+D states
+--------
+
+The D states and thus also the allowed values for _DSC 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
+
+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 _DSC 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 (_DSC, 0, NotSerialized)
+		{
+			Return (0x4)
+                }
+	}