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