| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ======================================== |
| Probing devices in other D states than 0 |
| ======================================== |
| |
| Introduction |
| ============ |
| |
| In some cases it may be preferred to leave certain devices powered off for the |
| entire system bootup if powering on these devices has adverse side effects, |
| beyond just powering on the said device. |
| |
| How it works |
| ============ |
| |
| The _DSC (Device State for Configuration) object that evaluates to an integer |
| may be used to tell Linux the highest allowed D state for a device during |
| probe. The support for _DSC requires support from the kernel bus type if the |
| bus driver normally sets the device in D0 state for probe. |
| |
| The downside of using _DSC is that as the device is not powered on, even if |
| there's a problem with the device, the driver likely probes just fine but the |
| first user will find out the device doesn't work, instead of a failure at probe |
| time. This feature should thus be used sparingly. |
| |
| I²C |
| --- |
| |
| If an I²C driver indicates its support for this by setting the |
| I2C_DRV_ACPI_WAIVE_D0_PROBE flag in struct i2c_driver.flags field and the |
| _DSC object evaluates to integer higher than the D state of the device, |
| the device will not be powered on (put in D0 state) for probe. |
| |
| D states |
| -------- |
| |
| The D states and thus also the allowed values for _DSC are listed below. Refer |
| to [1] for more information on device power states. |
| |
| .. code-block:: text |
| |
| Number State Description |
| 0 D0 Device fully powered on |
| 1 D1 |
| 2 D2 |
| 3 D3hot |
| 4 D3cold Off |
| |
| References |
| ========== |
| |
| [1] https://uefi.org/specifications/ACPI/6.4/02_Definition_of_Terms/Definition_of_Terms.html#device-power-state-definitions |
| |
| Example |
| ======= |
| |
| An ASL example describing an ACPI device using _DSC object to tell Operating |
| System the device should remain powered off during probe looks like this. Some |
| objects not relevant from the example point of view have been omitted. |
| |
| .. code-block:: text |
| |
| Device (CAM0) |
| { |
| Name (_HID, "SONY319A") |
| Name (_UID, Zero) |
| Name (_CRS, ResourceTemplate () |
| { |
| I2cSerialBus(0x0020, ControllerInitiated, 0x00061A80, |
| AddressingMode7Bit, "\\_SB.PCI0.I2C0", |
| 0x00, ResourceConsumer) |
| }) |
| Method (_DSC, 0, NotSerialized) |
| { |
| Return (0x4) |
| } |
| } |
