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1.. SPDX-License-Identifier: GPL-2.02 3========================================4Probing devices in other D states than 05========================================6 7Introduction8============9 10In some cases it may be preferred to leave certain devices powered off for the11entire system bootup if powering on these devices has adverse side effects,12beyond just powering on the said device.13 14How it works15============16 17The _DSC (Device State for Configuration) object that evaluates to an integer18may be used to tell Linux the highest allowed D state for a device during19probe. The support for _DSC requires support from the kernel bus type if the20bus 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 if23there's a problem with the device, the driver likely probes just fine but the24first user will find out the device doesn't work, instead of a failure at probe25time. This feature should thus be used sparingly.26 27I²C28---29 30If an I²C driver indicates its support for this by setting the31I2C_DRV_ACPI_WAIVE_D0_PROBE flag in struct i2c_driver.flags field and the32_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 states36--------37 38The D states and thus also the allowed values for _DSC are listed below. Refer39to [1] for more information on device power states.40 41.. code-block:: text42 43 Number State Description44 0 D0 Device fully powered on45 1 D146 2 D247 3 D3hot48 4 D3cold Off49 50References51==========52 53[1] https://uefi.org/specifications/ACPI/6.4/02_Definition_of_Terms/Definition_of_Terms.html#device-power-state-definitions54 55Example56=======57 58An ASL example describing an ACPI device using _DSC object to tell Operating59System the device should remain powered off during probe looks like this. Some60objects not relevant from the example point of view have been omitted.61 62.. code-block:: text63 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