2 In order to support ACPI open-ended hardware configurations (e.g. development
3 boards) we need a way to augment the ACPI configuration provided by the firmware
4 image. A common example is connecting sensors on I2C / SPI buses on development
7 Although this can be accomplished by creating a kernel platform driver or
8 recompiling the firmware image with updated ACPI tables, neither is practical:
9 the former proliferates board specific kernel code while the latter requires
10 access to firmware tools which are often not publicly available.
12 Because ACPI supports external references in AML code a more practical
13 way to augment firmware ACPI configuration is by dynamically loading
14 user defined SSDT tables that contain the board specific information.
16 For example, to enumerate a Bosch BMA222E accelerometer on the I2C bus of the
17 Minnowboard MAX development board exposed via the LSE connector [1], the
18 following ASL code can be used:
20 DefinitionBlock ("minnowmax.aml", "SSDT", 1, "Vendor", "Accel", 0x00000003)
22 External (\_SB.I2C6, DeviceObj)
29 Name (_HID, "BMA222E")
31 Method (_CRS, 0, Serialized)
33 Name (RBUF, ResourceTemplate ()
35 I2cSerialBus (0x0018, ControllerInitiated, 0x00061A80,
36 AddressingMode7Bit, "\\_SB.I2C6", 0x00,
38 GpioInt (Edge, ActiveHigh, Exclusive, PullDown, 0x0000,
39 "\\_SB.GPO2", 0x00, ResourceConsumer, , )
50 which can then be compiled to AML binary format:
54 Intel ACPI Component Architecture
55 ASL Optimizing Compiler version 20140214-64 [Mar 29 2014]
56 Copyright (c) 2000 - 2014 Intel Corporation
58 ASL Input: minnomax.asl - 30 lines, 614 bytes, 7 keywords
59 AML Output: minnowmax.aml - 165 bytes, 6 named objects, 1 executable opcodes
61 [1] http://wiki.minnowboard.org/MinnowBoard_MAX#Low_Speed_Expansion_Connector_.28Top.29
63 The resulting AML code can then be loaded by the kernel using one of the methods
66 == Loading ACPI SSDTs from initrd ==
68 This option allows loading of user defined SSDTs from initrd and it is useful
69 when the system does not support EFI or when there is not enough EFI storage.
71 It works in a similar way with initrd based ACPI tables override/upgrade: SSDT
72 aml code must be placed in the first, uncompressed, initrd under the
73 "kernel/firmware/acpi" path. Multiple files can be used and this will translate
74 in loading multiple tables. Only SSDT and OEM tables are allowed. See
75 initrd_table_override.txt for more details.
79 # Add the raw ACPI tables to an uncompressed cpio archive.
80 # They must be put into a /kernel/firmware/acpi directory inside the
82 # The uncompressed cpio archive must be the first.
83 # Other, typically compressed cpio archives, must be
84 # concatenated on top of the uncompressed one.
85 mkdir -p kernel/firmware/acpi
86 cp ssdt.aml kernel/firmware/acpi
88 # Create the uncompressed cpio archive and concatenate the original initrd
90 find kernel | cpio -H newc --create > /boot/instrumented_initrd
91 cat /boot/initrd >>/boot/instrumented_initrd