Commit 475fb4e8 authored by Octavian Purdila's avatar Octavian Purdila Committed by Rafael J. Wysocki

efi / ACPI: load SSTDs from EFI variables

This patch allows SSDTs to be loaded from EFI variables. It works by
specifying the EFI variable name containing the SSDT to be loaded. All
variables with the same name (regardless of the vendor GUID) will be

Note that we can't use acpi_install_table and we must rely on the
dynamic ACPI table loading and bus re-scanning mechanisms. That is
because I2C/SPI controllers are initialized earlier then the EFI
subsystems and all I2C/SPI ACPI devices are enumerated when the
I2C/SPI controllers are initialized.
Signed-off-by: default avatarOctavian Purdila <>
Reviewed-by: default avatarMatt Fleming <>
Signed-off-by: default avatarRafael J. Wysocki <>
parent 7f24467f
......@@ -89,3 +89,70 @@ cp ssdt.aml kernel/firmware/acpi
# on top:
find kernel | cpio -H newc --create > /boot/instrumented_initrd
cat /boot/initrd >>/boot/instrumented_initrd
== Loading ACPI SSDTs from EFI variables ==
This is the preferred method, when EFI is supported on the platform, because it
allows a persistent, OS independent way of storing the user defined SSDTs. There
is also work underway to implement EFI support for loading user defined SSDTs
and using this method will make it easier to convert to the EFI loading
mechanism when that will arrive.
In order to load SSDTs from an EFI variable the efivar_ssdt kernel command line
parameter can be used. The argument for the option is the variable name to
use. If there are multiple variables with the same name but with different
vendor GUIDs, all of them will be loaded.
In order to store the AML code in an EFI variable the efivarfs filesystem can be
used. It is enabled and mounted by default in /sys/firmware/efi/efivars in all
recent distribution.
Creating a new file in /sys/firmware/efi/efivars will automatically create a new
EFI variable. Updating a file in /sys/firmware/efi/efivars will update the EFI
variable. Please note that the file name needs to be specially formatted as
"Name-GUID" and that the first 4 bytes in the file (little-endian format)
represent the attributes of the EFI variable (see EFI_VARIABLE_MASK in
include/linux/efi.h). Writing to the file must also be done with one write
For example, you can use the following bash script to create/update an EFI
variable with the content from a given file:
#!/bin/sh -e
while ! [ -z "$1" ]; do
case "$1" in
"-f") filename="$2"; shift;;
"-g") guid="$2"; shift;;
*) name="$1";;
echo "Syntax: ${0##*/} -f filename [ -g guid ] name"
exit 1
[ -n "$name" -a -f "$filename" ] || usage
[ -d "$EFIVARFS" ] || exit 2
if stat -tf $EFIVARFS | grep -q -v de5e81e4; then
mount -t efivarfs none $EFIVARFS
# try to pick up an existing GUID
[ -n "$guid" ] || guid=$(find "$EFIVARFS" -name "$name-*" | head -n1 | cut -f2- -d-)
# use a randomly generated GUID
[ -n "$guid" ] || guid="$(cat /proc/sys/kernel/random/uuid)"
# efivarfs expects all of the data in one write
/bin/echo -ne "\007\000\000\000" | cat - $filename > $tmp
dd if=$tmp of="$EFIVARFS/$name-$guid" bs=$(stat -c %s $tmp)
rm $tmp
......@@ -1185,6 +1185,13 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
Address Range Mirroring feature even if your box
doesn't support it.
efivar_ssdt= [EFI; X86] Name of an EFI variable that contains an SSDT
that is to be dynamically loaded by Linux. If there are
multiple variables with the same name but with different
vendor GUIDs, all of them will be loaded. See
Documentation/acpi/ssdt-overlays.txt for details.
eisa_irq_edge= [PARISC,HW]
See header of drivers/parisc/eisa.c.
......@@ -24,6 +24,9 @@
#include <linux/of_fdt.h>
#include <linux/io.h>
#include <linux/platform_device.h>
#include <linux/slab.h>
#include <linux/acpi.h>
#include <linux/ucs2_string.h>
#include <asm/early_ioremap.h>
......@@ -195,6 +198,96 @@ static void generic_ops_unregister(void)
static char efivar_ssdt[EFIVAR_SSDT_NAME_MAX] __initdata;
static int __init efivar_ssdt_setup(char *str)
if (strlen(str) < sizeof(efivar_ssdt))
memcpy(efivar_ssdt, str, strlen(str));
pr_warn("efivar_ssdt: name too long: %s\n", str);
return 0;
__setup("efivar_ssdt=", efivar_ssdt_setup);
static __init int efivar_ssdt_iter(efi_char16_t *name, efi_guid_t vendor,
unsigned long name_size, void *data)
struct efivar_entry *entry;
struct list_head *list = data;
char utf8_name[EFIVAR_SSDT_NAME_MAX];
int limit = min_t(unsigned long, EFIVAR_SSDT_NAME_MAX, name_size);
ucs2_as_utf8(utf8_name, name, limit - 1);
if (strncmp(utf8_name, efivar_ssdt, limit) != 0)
return 0;
entry = kmalloc(sizeof(*entry), GFP_KERNEL);
if (!entry)
return 0;
memcpy(entry->var.VariableName, name, name_size);
memcpy(&entry->var.VendorGuid, &vendor, sizeof(efi_guid_t));
efivar_entry_add(entry, list);
return 0;
static __init int efivar_ssdt_load(void)
struct efivar_entry *entry, *aux;
unsigned long size;
void *data;
int ret;
ret = efivar_init(efivar_ssdt_iter, &entries, true, &entries);
list_for_each_entry_safe(entry, aux, &entries, list) {
pr_info("loading SSDT from variable %s-%pUl\n", efivar_ssdt,
ret = efivar_entry_size(entry, &size);
if (ret) {
pr_err("failed to get var size\n");
goto free_entry;
data = kmalloc(size, GFP_KERNEL);
if (!data)
goto free_entry;
ret = efivar_entry_get(entry, NULL, &size, data);
if (ret) {
pr_err("failed to get var data\n");
goto free_data;
ret = acpi_load_table(data);
if (ret) {
pr_err("failed to load table: %d\n", ret);
goto free_data;
goto free_entry;
return ret;
static inline int efivar_ssdt_load(void) { return 0; }
* We register the efi subsystem with the firmware subsystem and the
* efivars subsystem with the efi subsystem, if the system was booted with
......@@ -218,6 +311,9 @@ static int __init efisubsys_init(void)
if (error)
goto err_put;
if (efi_enabled(EFI_RUNTIME_SERVICES))
error = sysfs_create_group(efi_kobj, &efi_subsys_attr_group);
if (error) {
pr_err("efi: Sysfs attribute export failed with error %d.\n",
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