1 .. SPDX-License-Identifier: GPL-2.0
2
3 ========================================
4 Probing devices in other D states than 0
5 ========================================
6
7 Introduction
8 ============
9
10 In some cases it may be preferred to leave cer
11 entire system bootup if powering on these devi
12 beyond just powering on the said device.
13
14 How it works
15 ============
16
17 The _DSC (Device State for Configuration) obje
18 may be used to tell Linux the highest allowed
19 probe. The support for _DSC requires support f
20 bus driver normally sets the device in D0 stat
21
22 The downside of using _DSC is that as the devi
23 there's a problem with the device, the driver
24 first user will find out the device doesn't wo
25 time. This feature should thus be used sparing
26
27 I²C
28 ---
29
30 If an I²C driver indicates its support for th
31 I2C_DRV_ACPI_WAIVE_D0_PROBE flag in struct i2c
32 _DSC object evaluates to integer higher than t
33 the device will not be powered on (put in D0 s
34
35 D states
36 --------
37
38 The D states and thus also the allowed values
39 to [1] for more information on device power st
40
41 .. code-block:: text
42
43 Number State Description
44 0 D0 Device fully powered o
45 1 D1
46 2 D2
47 3 D3hot
48 4 D3cold Off
49
50 References
51 ==========
52
53 [1] https://uefi.org/specifications/ACPI/6.4/0
54
55 Example
56 =======
57
58 An ASL example describing an ACPI device using
59 System the device should remain powered off du
60 objects not relevant from the example point of
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, C
71 Addressin
72 0x00, Res
73 })
74 Method (_DSC, 0, NotSerialized
75 {
76 Return (0x4)
77 }
78 }
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