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 certain devices powered off for the
11 entire system bootup if powering on these devices has adverse side effects,
12 beyond just powering on the said device.
13
14 How it works
15 ============
16
17 The _DSC (Device State for Configuration) object that evaluates to an integer
18 may be used to tell Linux the highest allowed D state for a device during
19 probe. The support for _DSC requires support from the kernel bus type if the
20 bus driver normally sets the device in D0 state for probe.
21
22 The downside of using _DSC is that as the device is not powered on, even if
23 there's a problem with the device, the driver likely probes just fine but the
24 first user will find out the device doesn't work, instead of a failure at probe
25 time. This feature should thus be used sparingly.
26
27 I²C
28 ---
29
30 If an I²C driver indicates its support for this by setting the
31 I2C_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,
33 the device will not be powered on (put in D0 state) for probe.
34
35 D states
36 --------
37
38 The D states and thus also the allowed values for _DSC are listed below. Refer
39 to [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
50 References
51 ==========
52
53 [1] https://uefi.org/specifications/ACPI/6.4/02_Definition_of_Terms/Definition_of_Terms.html#device-power-state-definitions
54
55 Example
56 =======
57
58 An ASL example describing an ACPI device using _DSC object to tell Operating
59 System the device should remain powered off during probe looks like this. Some
60 objects 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 }
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.