1.. SPDX-License-Identifier: GPL-2.0
2
3========================================
4Probing devices in other D states than 0
5========================================
6
7Introduction
8============
9
10In some cases it may be preferred to leave certain devices powered off for the
11entire system bootup if powering on these devices has adverse side effects,
12beyond just powering on the said device.
13
14How it works
15============
16
17The _DSC (Device State for Configuration) object that evaluates to an integer
18may be used to tell Linux the highest allowed D state for a device during
19probe. The support for _DSC requires support from the kernel bus type if the
20bus driver normally sets the device in D0 state for probe.
21
22The downside of using _DSC is that as the device is not powered on, even if
23there's a problem with the device, the driver likely probes just fine but the
24first user will find out the device doesn't work, instead of a failure at probe
25time. This feature should thus be used sparingly.
26
27I²C
28---
29
30If an I²C driver indicates its support for this by setting the
31I2C_DRV_ACPI_WAIVE_D0_PROBE flag in struct i2c_driver.flags field and the
32_DSC object evaluates to integer higher than the D state of the device,
33the device will not be powered on (put in D0 state) for probe.
34
35D states
36--------
37
38The D states and thus also the allowed values for _DSC are listed below. Refer
39to [1] for more information on device power states.
40
41.. code-block:: text
42
43	Number	State	Description
44	0	D0	Device fully powered on
45	1	D1
46	2	D2
47	3	D3hot
48	4	D3cold	Off
49
50References
51==========
52
53[1] https://uefi.org/specifications/ACPI/6.4/02_Definition_of_Terms/Definition_of_Terms.html#device-power-state-definitions
54
55Example
56=======
57
58An ASL example describing an ACPI device using _DSC object to tell Operating
59System the device should remain powered off during probe looks like this. Some
60objects not relevant from the example point of view have been omitted.
61
62.. code-block:: text
63
64	Device (CAM0)
65	{
66		Name (_HID, "SONY319A")
67		Name (_UID, Zero)
68		Name (_CRS, ResourceTemplate ()
69		{
70			I2cSerialBus(0x0020, ControllerInitiated, 0x00061A80,
71				     AddressingMode7Bit, "\\_SB.PCI0.I2C0",
72				     0x00, ResourceConsumer)
73		})
74		Method (_DSC, 0, NotSerialized)
75		{
76			Return (0x4)
77		}
78	}
79