The basics of DSDT from initramfs. In case this option is selected,
populate_rootfs() is called a bit earlier to have the initramfs content
available during ACPI initialization.

This is a very similar path to the one available at but with some update in the
documentation, default set to No and the change of populate_rootfs() the
"Jeff Mahony way" (which avoids reading the initramfs twice).

Signed-off-by: Eric Piel <[EMAIL PROTECTED]>

 Documentation/dsdt-initrd.txt |  100 +++++++++++++++++++++++++++++++++++++++++
 drivers/acpi/Kconfig          |   17 +++++++
 drivers/acpi/osl.c            |   73 ++++++++++++++++++++++++++++--
 init/initramfs.c              |    8 +++-
 init/main.c                   |    7 +++
 5 files changed, 200 insertions(+), 5 deletions(-)
 create mode 100644 Documentation/dsdt-initrd.txt

diff --git a/Documentation/dsdt-initrd.txt b/Documentation/dsdt-initrd.txt
new file mode 100644
index 0000000..b2e9460
--- /dev/null
+++ b/Documentation/dsdt-initrd.txt
@@ -0,0 +1,100 @@
+ACPI Custom DSDT read from initramfs
+2003 by Markus Gaugusch < dsdt at gaugusch dot at >
+Special thanks go to Thomas Renninger from SuSE, who updated the patch for
+2.6.0 and later modified it to read inside initramfs
+2004 - 2008 maintained by Eric Piel < eric dot piel at tremplin-utc dot net >
+This option is intended for people who would like to hack their DSDT and don't
+want to recompile their kernel after every change. It can also be useful to
+distros which offers pre-compiled kernels and want to allow their users to use
+a modified DSDT. In the Kernel config, enable the initial RAM filesystem
+support (in General Setup) and enable ACPI_CUSTOM_DSDT_INITRD at the ACPI
+options (General Setup|ACPI Support|Read Custom DSDT from initramfs).
+A custom DSDT (Differentiated System Description Table) is useful when your
+computer uses ACPI but problems occur due to broken implementation. Typically,
+your computer works but there are some troubles with the hardware detection or
+the power management. You can check that troubles come from errors in the DSDT 
+activating the ACPI debug option and reading the logs. This table is provided
+by the BIOS, therefore it might be a good idea to check for BIOS update on your
+vendor website before going any further. Errors are often caused by vendors
+testing their hardware only with Windows or because there is code which is
+executed only on a specific OS with a specific version and Linux hasn't been
+considered during the development.
+Before you run away from customising your DSDT, you should note that already
+corrected tables are available for a fair amount of computers on this web-page:
+ . Be careful though, to work correctly a DSDT has to
+match closely the hardware, including the amount of RAM, the frequency of the
+processor and the PCI cards present! If you are part of the unluckies who
+cannot find their hardware in this database, you can modify your DSDT by
+yourself. This process is less painful than it sounds. Download the Intel ASL
+compiler/decompiler at 
+As root, you then have to dump your DSDT and decompile it. By using the
+compiler messages as well as the kernel ACPI debug messages and the reference
+book (available at the Intel website and also at, it is
+quite easy to obtain a fully working table.
+Once your new DSDT is ready you'll have to add it to an initramfs so that the
+kernel can read the table at the very beginning of the boot. As the file has to
+be accessed very early during the boot process the initramfs has to be an
+initramfs. The file is contained into the initramfs under the name /DSDT.aml .
+To obtain such an initramfs, you might have to modify your initramfs script or
+you can add it later to the initramfs with the script appended to this
+document. The command will look like:
+initramfs-add-dsdt initramfs.img my-dsdt.aml
+In case you don't use any initramfs, the possibilities you have are to either
+start using one (try mkinitrd or yaird), or use the "Include Custom DSDT"
+configure option to directly include your DSDT inside the kernel.
+The message "Looking for DSDT in initramfs..." will tell you if the DSDT was
+found or not. If you need to update your DSDT, generate a new initramfs and
+perform the steps above. Don't forget that with Lilo, you'll have to re-run it.
+====================== Here starts initramfs-add-dsdt 
+# Adds a DSDT file to the initrd (if it's an initramfs)
+# first argument is the name of archive
+# second argument is the name of the file to add
+# The file will be copied as /DSDT.aml
+# 20060126: fix "Premature end of file" with some old cpio (Roland Robic)
+# 20060205: this time it should really work
+# check the arguments
+if [ $# -ne 2 ]; then
+       program_name=$(basename $0)
+       echo "\
+$program_name: too few arguments
+Usage: $program_name initrd-name.img DSDT-to-add.aml
+Adds a DSDT file to an initrd (in initramfs format)
+  initrd-name.img: filename of the initrd in initramfs format
+  DSDT-to-add.aml: filename of the DSDT file to add
+  " 1>&2
+    exit 1
+# we should check it's an initramfs
+tempcpio=$(mktemp -d)
+# cleanup on exit, hangup, interrupt, quit, termination
+trap 'rm -rf $tempcpio' 0 1 2 3 15
+# extract the archive
+gunzip -c "$1" > "$tempcpio"/initramfs.cpio || exit 1
+# copy the DSDT file at the root of the directory so that we can call it 
+cp -f "$2" "$tempcpio"/DSDT.aml
+# add the file
+cd "$tempcpio"
+(echo DSDT.aml | cpio --quiet -H newc -o -A -O "$tempcpio"/initramfs.cpio) || 
exit 1
+cd "$OLDPWD"
+# re-compress the archive
+gzip -c "$tempcpio"/initramfs.cpio > "$1"
diff --git a/drivers/acpi/Kconfig b/drivers/acpi/Kconfig
index ccf6ea9..8607ed3 100644
--- a/drivers/acpi/Kconfig
+++ b/drivers/acpi/Kconfig
@@ -274,6 +274,23 @@ config ACPI_CUSTOM_DSDT_FILE
          Enter the full path name to the file which includes the AmlCode
+       bool "Read Custom DSDT from initramfs"
+       depends on BLK_DEV_INITRD
+       default n
+       help
+         The DSDT (Differentiated System Description Table) often needs to be
+         overridden because of broken BIOS implementations. If this feature is
+         activated you will be able to provide a customized DSDT by adding it
+         to your initramfs. If your mkinitrd tool does not support this feature
+         a script is provided in the documentation. For more details see
+         <file:Documentation/dsdt-initrd.txt> or 
+         If there is no table found, it will fall-back to the custom DSDT
+         in-kernel (if activated) or to the DSDT from the BIOS.
+         Even if you do not need a new one at the moment, you may want to use a
+         better DSDT later. It is safe to say Y here.
        int "Disable ACPI for systems before Jan 1st this year" if X86_32
        default 0
diff --git a/drivers/acpi/osl.c b/drivers/acpi/osl.c
index e53fb51..4568a59 100644
--- a/drivers/acpi/osl.c
+++ b/drivers/acpi/osl.c
@@ -312,6 +312,66 @@ acpi_os_predefined_override(const struct 
acpi_predefined_names *init_val,
        return AE_OK;
+struct acpi_table_header *acpi_find_dsdt_initrd(void)
+       struct file *firmware_file;
+       mm_segment_t oldfs;
+       unsigned long len, len2;
+       struct acpi_table_header *dsdt_buffer, *ret = NULL;
+       struct kstat stat;
+       char *ramfs_dsdt_name = "/DSDT.aml";
+       printk(KERN_INFO PREFIX "Looking for DSDT in initramfs... ");
+       /*
+        * Never do this at home, only the user-space is allowed to open a file.
+        * The clean way would be to use the firmware loader. But this code 
must be run
+        * before there is any userspace available. So we need a static/init 
+        * infrastructure, which doesn't exist yet...
+        */
+       if (vfs_stat(ramfs_dsdt_name, &stat) < 0) {
+               printk("not found.\n");
+               return ret;
+       }
+       len = stat.size;
+       /* check especially against empty files */
+       if (len <= 4) {
+               printk("error, file is too small: only %lu bytes.\n", len);
+               return ret;
+       }
+       firmware_file = filp_open(ramfs_dsdt_name, O_RDONLY, 0);
+       if (IS_ERR(firmware_file)) {
+               printk("error, could not open file %s.\n", ramfs_dsdt_name);
+               return ret;
+       }
+       dsdt_buffer = ACPI_ALLOCATE(len);
+       if (!dsdt_buffer) {
+               printk("error when allocating %lu bytes of memory.\n", len);
+               goto err;
+       }
+       oldfs = get_fs();
+       set_fs(KERNEL_DS);
+       len2 = vfs_read(firmware_file, (char __user *)dsdt_buffer, len, 
+       set_fs(oldfs);
+       if (len2 < len) {
+               printk("error trying to read %lu bytes from %s.\n", len, 
+               ACPI_FREE(dsdt_buffer);
+               goto err;
+       }
+       printk("successfully read %lu bytes from %s.\n", len, ramfs_dsdt_name);
+       ret = dsdt_buffer;
+       filp_close(firmware_file, NULL);
+       return ret;
 acpi_os_table_override(struct acpi_table_header * existing_table,
                       struct acpi_table_header ** new_table)
@@ -319,13 +379,18 @@ acpi_os_table_override(struct acpi_table_header * 
        if (!existing_table || !new_table)
                return AE_BAD_PARAMETER;
+       *new_table = NULL;
        if (strncmp(existing_table->signature, "DSDT", 4) == 0)
                *new_table = (struct acpi_table_header *)AmlCode;
-       else
-               *new_table = NULL;
-       *new_table = NULL;
+       if (strncmp(existing_table->signature, "DSDT", 4) == 0) {
+               struct acpi_table_header *initrd_table = 
+               if (initrd_table)
+                       *new_table = initrd_table;
+       }
        return AE_OK;
diff --git a/init/initramfs.c b/init/initramfs.c
index 1db02a0..f2662e2 100644
--- a/init/initramfs.c
+++ b/init/initramfs.c
@@ -541,7 +541,7 @@ skip:
-static int __init populate_rootfs(void)
+int __init populate_rootfs(void)
        char *err = unpack_to_rootfs(__initramfs_start,
                         __initramfs_end - __initramfs_start, 0);
@@ -582,4 +582,10 @@ static int __init populate_rootfs(void)
        return 0;
+ * if this option is enabled, populate_rootfs() is called _earlier_ in the
+ * boot sequence. This insures that the ACPI initialisation can find the file.
+ */
diff --git a/init/main.c b/init/main.c
index f287ca5..7eb1b89 100644
--- a/init/main.c
+++ b/init/main.c
@@ -102,6 +102,12 @@ static inline void mark_rodata_ro(void) { }
 extern void tc_init(void);
+extern int populate_rootfs(void);
+static inline void populate_rootfs(void) {}
 enum system_states system_state;
@@ -643,6 +649,7 @@ asmlinkage void __init start_kernel(void)
+       populate_rootfs(); /* For DSDT override from initramfs */
        acpi_early_init(); /* before LAPIC and SMP init */
        /* Do the rest non-__init'ed, we're now alive */
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