92 lines
3.4 KiB
Plaintext
92 lines
3.4 KiB
Plaintext
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In order to support ACPI open-ended hardware configurations (e.g. development
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boards) we need a way to augment the ACPI configuration provided by the firmware
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image. A common example is connecting sensors on I2C / SPI buses on development
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boards.
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Although this can be accomplished by creating a kernel platform driver or
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recompiling the firmware image with updated ACPI tables, neither is practical:
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the former proliferates board specific kernel code while the latter requires
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access to firmware tools which are often not publicly available.
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Because ACPI supports external references in AML code a more practical
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way to augment firmware ACPI configuration is by dynamically loading
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user defined SSDT tables that contain the board specific information.
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For example, to enumerate a Bosch BMA222E accelerometer on the I2C bus of the
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Minnowboard MAX development board exposed via the LSE connector [1], the
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following ASL code can be used:
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DefinitionBlock ("minnowmax.aml", "SSDT", 1, "Vendor", "Accel", 0x00000003)
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{
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External (\_SB.I2C6, DeviceObj)
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Scope (\_SB.I2C6)
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{
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Device (STAC)
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{
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Name (_ADR, Zero)
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Name (_HID, "BMA222E")
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Method (_CRS, 0, Serialized)
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{
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Name (RBUF, ResourceTemplate ()
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{
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I2cSerialBus (0x0018, ControllerInitiated, 0x00061A80,
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AddressingMode7Bit, "\\_SB.I2C6", 0x00,
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ResourceConsumer, ,)
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GpioInt (Edge, ActiveHigh, Exclusive, PullDown, 0x0000,
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"\\_SB.GPO2", 0x00, ResourceConsumer, , )
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{ // Pin list
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0
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}
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})
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Return (RBUF)
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}
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}
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}
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}
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which can then be compiled to AML binary format:
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$ iasl minnowmax.asl
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Intel ACPI Component Architecture
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ASL Optimizing Compiler version 20140214-64 [Mar 29 2014]
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Copyright (c) 2000 - 2014 Intel Corporation
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ASL Input: minnomax.asl - 30 lines, 614 bytes, 7 keywords
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AML Output: minnowmax.aml - 165 bytes, 6 named objects, 1 executable opcodes
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[1] http://wiki.minnowboard.org/MinnowBoard_MAX#Low_Speed_Expansion_Connector_.28Top.29
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The resulting AML code can then be loaded by the kernel using one of the methods
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below.
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== Loading ACPI SSDTs from initrd ==
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This option allows loading of user defined SSDTs from initrd and it is useful
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when the system does not support EFI or when there is not enough EFI storage.
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It works in a similar way with initrd based ACPI tables override/upgrade: SSDT
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aml code must be placed in the first, uncompressed, initrd under the
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"kernel/firmware/acpi" path. Multiple files can be used and this will translate
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in loading multiple tables. Only SSDT and OEM tables are allowed. See
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initrd_table_override.txt for more details.
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Here is an example:
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# Add the raw ACPI tables to an uncompressed cpio archive.
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# They must be put into a /kernel/firmware/acpi directory inside the
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# cpio archive.
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# The uncompressed cpio archive must be the first.
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# Other, typically compressed cpio archives, must be
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# concatenated on top of the uncompressed one.
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mkdir -p kernel/firmware/acpi
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cp ssdt.aml kernel/firmware/acpi
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# Create the uncompressed cpio archive and concatenate the original initrd
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# on top:
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find kernel | cpio -H newc --create > /boot/instrumented_initrd
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cat /boot/initrd >>/boot/instrumented_initrd
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