Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 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 5 boards. 6 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. 11 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. 15 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: 19 20 DefinitionBlock ("minnowmax.aml", "SSDT", 1, "Vendor", "Accel", 0x00000003) 21 { 22 External (\_SB.I2C6, DeviceObj) 23 24 Scope (\_SB.I2C6) 25 { 26 Device (STAC) 27 { 28 Name (_ADR, Zero) 29 Name (_HID, "BMA222E") 30 31 Method (_CRS, 0, Serialized) 32 { 33 Name (RBUF, ResourceTemplate () 34 { 35 I2cSerialBus (0x0018, ControllerInitiated, 0x00061A80, 36 AddressingMode7Bit, "\\_SB.I2C6", 0x00, 37 ResourceConsumer, ,) 38 GpioInt (Edge, ActiveHigh, Exclusive, PullDown, 0x0000, 39 "\\_SB.GPO2", 0x00, ResourceConsumer, , ) 40 { // Pin list 41 0 42 } 43 }) 44 Return (RBUF) 45 } 46 } 47 } 48 } 49 50 which can then be compiled to AML binary format: 51 52 $ iasl minnowmax.asl 53 54 Intel ACPI Component Architecture 55 ASL Optimizing Compiler version 20140214-64 [Mar 29 2014] 56 Copyright (c) 2000 - 2014 Intel Corporation 57 58 ASL Input: minnomax.asl - 30 lines, 614 bytes, 7 keywords 59 AML Output: minnowmax.aml - 165 bytes, 6 named objects, 1 executable opcodes 60 61 [1] http://wiki.minnowboard.org/MinnowBoard_MAX#Low_Speed_Expansion_Connector_.28Top.29 62 63 The resulting AML code can then be loaded by the kernel using one of the methods 64 below. 65 66 == Loading ACPI SSDTs from initrd == 67 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. 70 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. 76 77 Here is an example: 78 79 # Add the raw ACPI tables to an uncompressed cpio archive. 80 # They must be put into a /kernel/firmware/acpi directory inside the 81 # cpio archive. 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 87 88 # Create the uncompressed cpio archive and concatenate the original initrd 89 # on top: 90 find kernel | cpio -H newc --create > /boot/instrumented_initrd 91 cat /boot/initrd >>/boot/instrumented_initrd 92 93 == Loading ACPI SSDTs from EFI variables == 94 95 This is the preferred method, when EFI is supported on the platform, because it 96 allows a persistent, OS independent way of storing the user defined SSDTs. There 97 is also work underway to implement EFI support for loading user defined SSDTs 98 and using this method will make it easier to convert to the EFI loading 99 mechanism when that will arrive. 100 101 In order to load SSDTs from an EFI variable the efivar_ssdt kernel command line 102 parameter can be used. The argument for the option is the variable name to 103 use. If there are multiple variables with the same name but with different 104 vendor GUIDs, all of them will be loaded. 105 106 In order to store the AML code in an EFI variable the efivarfs filesystem can be 107 used. It is enabled and mounted by default in /sys/firmware/efi/efivars in all 108 recent distribution. 109 110 Creating a new file in /sys/firmware/efi/efivars will automatically create a new 111 EFI variable. Updating a file in /sys/firmware/efi/efivars will update the EFI 112 variable. Please note that the file name needs to be specially formatted as 113 "Name-GUID" and that the first 4 bytes in the file (little-endian format) 114 represent the attributes of the EFI variable (see EFI_VARIABLE_MASK in 115 include/linux/efi.h). Writing to the file must also be done with one write 116 operation. 117 118 For example, you can use the following bash script to create/update an EFI 119 variable with the content from a given file: 120 121 #!/bin/sh -e 122 123 while ! [ -z "$1" ]; do 124 case "$1" in 125 "-f") filename="$2"; shift;; 126 "-g") guid="$2"; shift;; 127 *) name="$1";; 128 esac 129 shift 130 done 131 132 usage() 133 { 134 echo "Syntax: ${0##*/} -f filename [ -g guid ] name" 135 exit 1 136 } 137 138 [ -n "$name" -a -f "$filename" ] || usage 139 140 EFIVARFS="/sys/firmware/efi/efivars" 141 142 [ -d "$EFIVARFS" ] || exit 2 143 144 if stat -tf $EFIVARFS | grep -q -v de5e81e4; then 145 mount -t efivarfs none $EFIVARFS 146 fi 147 148 # try to pick up an existing GUID 149 [ -n "$guid" ] || guid=$(find "$EFIVARFS" -name "$name-*" | head -n1 | cut -f2- -d-) 150 151 # use a randomly generated GUID 152 [ -n "$guid" ] || guid="$(cat /proc/sys/kernel/random/uuid)" 153 154 # efivarfs expects all of the data in one write 155 tmp=$(mktemp) 156 /bin/echo -ne "\007\000\000\000" | cat - $filename > $tmp 157 dd if=$tmp of="$EFIVARFS/$name-$guid" bs=$(stat -c %s $tmp) 158 rm $tmp 159 160 == Loading ACPI SSDTs from configfs == 161 162 This option allows loading of user defined SSDTs from userspace via the configfs 163 interface. The CONFIG_ACPI_CONFIGFS option must be select and configfs must be 164 mounted. In the following examples, we assume that configfs has been mounted in 165 /config. 166 167 New tables can be loading by creating new directories in /config/acpi/table/ and 168 writing the SSDT aml code in the aml attribute: 169 170 cd /config/acpi/table 171 mkdir my_ssdt 172 cat ~/ssdt.aml > my_ssdt/aml