1 .. SPDX-License-Identifier: GPL-2.0
2
3 =============
4 SSDT Overlays
5 =============
6
7 In order to support ACPI open-ended hardware configurations (e.g. development
8 boards) we need a way to augment the ACPI configuration provided by the firmware
9 image. A common example is connecting sensors on I2C / SPI buses on development
10 boards.
11
12 Although this can be accomplished by creating a kernel platform driver or
13 recompiling the firmware image with updated ACPI tables, neither is practical:
14 the former proliferates board specific kernel code while the latter requires
15 access to firmware tools which are often not publicly available.
16
17 Because ACPI supports external references in AML code a more practical
18 way to augment firmware ACPI configuration is by dynamically loading
19 user defined SSDT tables that contain the board specific information.
20
21 For example, to enumerate a Bosch BMA222E accelerometer on the I2C bus of the
22 Minnowboard MAX development board exposed via the LSE connector [1], the
23 following ASL code can be used::
24
25 DefinitionBlock ("minnowmax.aml", "SSDT", 1, "Vendor", "Accel", 0x00000003)
26 {
27 External (\_SB.I2C6, DeviceObj)
28
29 Scope (\_SB.I2C6)
30 {
31 Device (STAC)
32 {
33 Name (_HID, "BMA222E")
34 Name (RBUF, ResourceTemplate ()
35 {
36 I2cSerialBus (0x0018, ControllerInitiated, 0x00061A80,
37 AddressingMode7Bit, "\\_SB.I2C6", 0x00,
38 ResourceConsumer, ,)
39 GpioInt (Edge, ActiveHigh, Exclusive, PullDown, 0x0000,
40 "\\_SB.GPO2", 0x00, ResourceConsumer, , )
41 { // Pin list
42 0
43 }
44 })
45
46 Method (_CRS, 0, Serialized)
47 {
48 Return (RBUF)
49 }
50 }
51 }
52 }
53
54 which can then be compiled to AML binary format::
55
56 $ iasl minnowmax.asl
57
58 Intel ACPI Component Architecture
59 ASL Optimizing Compiler version 20140214-64 [Mar 29 2014]
60 Copyright (c) 2000 - 2014 Intel Corporation
61
62 ASL Input: minnomax.asl - 30 lines, 614 bytes, 7 keywords
63 AML Output: minnowmax.aml - 165 bytes, 6 named objects, 1 executable opcodes
64
65 [1] https://www.elinux.org/Minnowboard:MinnowMax#Low_Speed_Expansion_.28Top.29
66
67 The resulting AML code can then be loaded by the kernel using one of the methods
68 below.
69
70 Loading ACPI SSDTs from initrd
71 ==============================
72
73 This option allows loading of user defined SSDTs from initrd and it is useful
74 when the system does not support EFI or when there is not enough EFI storage.
75
76 It works in a similar way with initrd based ACPI tables override/upgrade: SSDT
77 AML code must be placed in the first, uncompressed, initrd under the
78 "kernel/firmware/acpi" path. Multiple files can be used and this will translate
79 in loading multiple tables. Only SSDT and OEM tables are allowed. See
80 initrd_table_override.txt for more details.
81
82 Here is an example::
83
84 # Add the raw ACPI tables to an uncompressed cpio archive.
85 # They must be put into a /kernel/firmware/acpi directory inside the
86 # cpio archive.
87 # The uncompressed cpio archive must be the first.
88 # Other, typically compressed cpio archives, must be
89 # concatenated on top of the uncompressed one.
90 mkdir -p kernel/firmware/acpi
91 cp ssdt.aml kernel/firmware/acpi
92
93 # Create the uncompressed cpio archive and concatenate the original initrd
94 # on top:
95 find kernel | cpio -H newc --create > /boot/instrumented_initrd
96 cat /boot/initrd >>/boot/instrumented_initrd
97
98 Loading ACPI SSDTs from EFI variables
99 =====================================
100
101 This is the preferred method, when EFI is supported on the platform, because it
102 allows a persistent, OS independent way of storing the user defined SSDTs. There
103 is also work underway to implement EFI support for loading user defined SSDTs
104 and using this method will make it easier to convert to the EFI loading
105 mechanism when that will arrive. To enable it, the
106 CONFIG_EFI_CUSTOM_SSDT_OVERLAYS should be chosen to y.
107
108 In order to load SSDTs from an EFI variable the ``"efivar_ssdt=..."`` kernel
109 command line parameter can be used (the name has a limitation of 16 characters).
110 The argument for the option is the variable name to use. If there are multiple
111 variables with the same name but with different vendor GUIDs, all of them will
112 be loaded.
113
114 In order to store the AML code in an EFI variable the efivarfs filesystem can be
115 used. It is enabled and mounted by default in /sys/firmware/efi/efivars in all
116 recent distribution.
117
118 Creating a new file in /sys/firmware/efi/efivars will automatically create a new
119 EFI variable. Updating a file in /sys/firmware/efi/efivars will update the EFI
120 variable. Please note that the file name needs to be specially formatted as
121 "Name-GUID" and that the first 4 bytes in the file (little-endian format)
122 represent the attributes of the EFI variable (see EFI_VARIABLE_MASK in
123 include/linux/efi.h). Writing to the file must also be done with one write
124 operation.
125
126 For example, you can use the following bash script to create/update an EFI
127 variable with the content from a given file::
128
129 #!/bin/sh -e
130
131 while [ -n "$1" ]; do
132 case "$1" in
133 "-f") filename="$2"; shift;;
134 "-g") guid="$2"; shift;;
135 *) name="$1";;
136 esac
137 shift
138 done
139
140 usage()
141 {
142 echo "Syntax: ${0##*/} -f filename [ -g guid ] name"
143 exit 1
144 }
145
146 [ -n "$name" -a -f "$filename" ] || usage
147
148 EFIVARFS="/sys/firmware/efi/efivars"
149
150 [ -d "$EFIVARFS" ] || exit 2
151
152 if stat -tf $EFIVARFS | grep -q -v de5e81e4; then
153 mount -t efivarfs none $EFIVARFS
154 fi
155
156 # try to pick up an existing GUID
157 [ -n "$guid" ] || guid=$(find "$EFIVARFS" -name "$name-*" | head -n1 | cut -f2- -d-)
158
159 # use a randomly generated GUID
160 [ -n "$guid" ] || guid="$(cat /proc/sys/kernel/random/uuid)"
161
162 # efivarfs expects all of the data in one write
163 tmp=$(mktemp)
164 /bin/echo -ne "\007\000\000\000" | cat - $filename > $tmp
165 dd if=$tmp of="$EFIVARFS/$name-$guid" bs=$(stat -c %s $tmp)
166 rm $tmp
167
168 Loading ACPI SSDTs from configfs
169 ================================
170
171 This option allows loading of user defined SSDTs from user space via the configfs
172 interface. The CONFIG_ACPI_CONFIGFS option must be select and configfs must be
173 mounted. In the following examples, we assume that configfs has been mounted in
174 /sys/kernel/config.
175
176 New tables can be loading by creating new directories in /sys/kernel/config/acpi/table
177 and writing the SSDT AML code in the aml attribute::
178
179 cd /sys/kernel/config/acpi/table
180 mkdir my_ssdt
181 cat ~/ssdt.aml > my_ssdt/aml
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