182 lines
6.6 KiB
ReStructuredText
182 lines
6.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=============
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SSDT Overlays
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=============
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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 (_HID, "BMA222E")
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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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Method (_CRS, 0, Serialized)
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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] https://www.elinux.org/Minnowboard:MinnowMax#Low_Speed_Expansion_.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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==============================
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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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Loading ACPI SSDTs from EFI variables
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=====================================
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This is the preferred method, when EFI is supported on the platform, because it
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allows a persistent, OS independent way of storing the user defined SSDTs. There
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is also work underway to implement EFI support for loading user defined SSDTs
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and using this method will make it easier to convert to the EFI loading
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mechanism when that will arrive. To enable it, the
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CONFIG_EFI_CUSTOM_SSDT_OVERLAYS should be chosen to y.
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In order to load SSDTs from an EFI variable the ``"efivar_ssdt=..."`` kernel
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command line parameter can be used (the name has a limitation of 16 characters).
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The argument for the option is the variable name to use. If there are multiple
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variables with the same name but with different vendor GUIDs, all of them will
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be loaded.
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In order to store the AML code in an EFI variable the efivarfs filesystem can be
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used. It is enabled and mounted by default in /sys/firmware/efi/efivars in all
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recent distribution.
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Creating a new file in /sys/firmware/efi/efivars will automatically create a new
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EFI variable. Updating a file in /sys/firmware/efi/efivars will update the EFI
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variable. Please note that the file name needs to be specially formatted as
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"Name-GUID" and that the first 4 bytes in the file (little-endian format)
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represent the attributes of the EFI variable (see EFI_VARIABLE_MASK in
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include/linux/efi.h). Writing to the file must also be done with one write
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operation.
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For example, you can use the following bash script to create/update an EFI
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variable with the content from a given file::
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#!/bin/sh -e
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while [ -n "$1" ]; do
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case "$1" in
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"-f") filename="$2"; shift;;
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"-g") guid="$2"; shift;;
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*) name="$1";;
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esac
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shift
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done
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usage()
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{
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echo "Syntax: ${0##*/} -f filename [ -g guid ] name"
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exit 1
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}
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[ -n "$name" -a -f "$filename" ] || usage
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EFIVARFS="/sys/firmware/efi/efivars"
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[ -d "$EFIVARFS" ] || exit 2
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if stat -tf $EFIVARFS | grep -q -v de5e81e4; then
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mount -t efivarfs none $EFIVARFS
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fi
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# try to pick up an existing GUID
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[ -n "$guid" ] || guid=$(find "$EFIVARFS" -name "$name-*" | head -n1 | cut -f2- -d-)
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# use a randomly generated GUID
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[ -n "$guid" ] || guid="$(cat /proc/sys/kernel/random/uuid)"
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# efivarfs expects all of the data in one write
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tmp=$(mktemp)
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/bin/echo -ne "\007\000\000\000" | cat - $filename > $tmp
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dd if=$tmp of="$EFIVARFS/$name-$guid" bs=$(stat -c %s $tmp)
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rm $tmp
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Loading ACPI SSDTs from configfs
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================================
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This option allows loading of user defined SSDTs from user space via the configfs
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interface. The CONFIG_ACPI_CONFIGFS option must be select and configfs must be
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mounted. In the following examples, we assume that configfs has been mounted in
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/sys/kernel/config.
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New tables can be loading by creating new directories in /sys/kernel/config/acpi/table
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and writing the SSDT AML code in the aml attribute::
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cd /sys/kernel/config/acpi/table
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mkdir my_ssdt
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cat ~/ssdt.aml > my_ssdt/aml
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