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