* [PATCH v3 1/7] block: store GPT partition type UUID
2026-11-02 21:29 [PATCH v3 0/7] init: discoverable root partitions Vincent Mailhol
@ 2026-11-02 21:29 ` Vincent Mailhol
2026-11-02 21:29 ` [PATCH v3 2/7] block: add DPS root partition type UUID definitions Vincent Mailhol
` (5 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: Vincent Mailhol @ 2026-11-02 21:29 UTC (permalink / raw)
To: Jens Axboe, Davidlohr Bueso, Alexander Viro, Christian Brauner, Jan Kara
Cc: linux-kernel, linux-block, linux-efi, linux-fsdevel, Dave Hansen,
James Bottomley, Helge Deller, Matthew Wilcox, Vincent Mailhol
DPS [1] identifies OS partitions by GPT partition type UUID.
Keep that UUID in struct partition_meta_info and populate it while
scanning GPT partition tables so early root discovery can use it.
[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
block/partitions/efi.c | 1 +
include/linux/blkdev.h | 1 +
2 files changed, 2 insertions(+)
diff --git a/block/partitions/efi.c b/block/partitions/efi.c
index 9865d59093fa..4a3835ed9561 100644
--- a/block/partitions/efi.c
+++ b/block/partitions/efi.c
@@ -742,6 +742,7 @@ int efi_partition(struct parsed_partitions *state)
info = &state->parts[i + 1].info;
efi_guid_to_str(&ptes[i].unique_partition_guid, info->uuid);
+ efi_guid_to_str(&ptes[i].partition_type_guid, info->type_uuid);
/* Naively convert UTF16-LE to 7 bits. */
label_max = min(ARRAY_SIZE(info->volname) - 1,
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 0c2010230632..0e0f5d68425d 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -67,6 +67,7 @@ extern const struct class block_class;
struct partition_meta_info {
char uuid[PARTITION_META_INFO_UUIDLTH];
+ char type_uuid[PARTITION_META_INFO_UUIDLTH];
u8 volname[PARTITION_META_INFO_VOLNAMELTH];
};
--
2.56.0
^ permalink raw reply [flat|nested] 8+ messages in thread* [PATCH v3 2/7] block: add DPS root partition type UUID definitions
2026-11-02 21:29 [PATCH v3 0/7] init: discoverable root partitions Vincent Mailhol
2026-11-02 21:29 ` [PATCH v3 1/7] block: store GPT partition type UUID Vincent Mailhol
@ 2026-11-02 21:29 ` Vincent Mailhol
2026-11-02 21:29 ` [PATCH v3 3/7] init: factor out root device lookup into lookup_root_device() Vincent Mailhol
` (4 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: Vincent Mailhol @ 2026-11-02 21:29 UTC (permalink / raw)
To: Jens Axboe, Davidlohr Bueso, Alexander Viro, Christian Brauner, Jan Kara
Cc: linux-kernel, linux-block, linux-efi, linux-fsdevel, Dave Hansen,
James Bottomley, Helge Deller, Matthew Wilcox, Vincent Mailhol
DPS [1] assigns a native root partition type UUID to each supported
architecture. Add a hidden DPS_ROOT_PARTITION_TYPE_UUID string symbol
and populate it from the architecture selected in Kconfig. Only
architectures with EFI support in the kernel are added.
Gate the symbol on EFI and EFI_PARTITION because discovery needs block
devices, EFI runtime services and GPT partition parsing.
Derive the hidden DPS_ROOT_AUTO_DISCOVERY boolean from whether
DPS_ROOT_PARTITION_TYPE_UUID is nonempty.
[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
block/Kconfig | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/block/Kconfig b/block/Kconfig
index 70e4a66d941f..3b5d3ad42dde 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -211,6 +211,20 @@ config BLK_INLINE_ENCRYPTION_FALLBACK
source "block/partitions/Kconfig"
+config DPS_ROOT_PARTITION_TYPE_UUID
+ depends on EFI && EFI_PARTITION
+ string
+ default "69dad710-2ce4-4e3c-b16c-21a1d49abed3" if ARM
+ default "b921b045-1df0-41c3-af44-4c6f280d3fae" if ARM64
+ default "77055800-792c-4f94-b39a-98c91b762bb6" if LOONGARCH && 64BIT
+ default "72ec70a6-cf74-40e6-bd49-4bda08e8f224" if RISCV && 64BIT
+ default "60d5a7fe-8e7d-435c-b714-3dd8162144e1" if RISCV
+ default "4f68bce3-e8cd-4db1-96e7-fbcaf984b709" if X86_64
+ default "44479540-f297-41b2-9af7-d131d5f0458a" if X86
+
+config DPS_ROOT_AUTO_DISCOVERY
+ def_bool DPS_ROOT_PARTITION_TYPE_UUID != ""
+
config BLK_PM
def_bool PM
--
2.56.0
^ permalink raw reply [flat|nested] 8+ messages in thread* [PATCH v3 3/7] init: factor out root device lookup into lookup_root_device()
2026-11-02 21:29 [PATCH v3 0/7] init: discoverable root partitions Vincent Mailhol
2026-11-02 21:29 ` [PATCH v3 1/7] block: store GPT partition type UUID Vincent Mailhol
2026-11-02 21:29 ` [PATCH v3 2/7] block: add DPS root partition type UUID definitions Vincent Mailhol
@ 2026-11-02 21:29 ` Vincent Mailhol
2026-11-02 21:29 ` [PATCH v3 4/7] block: discover root by DPS partition type UUID Vincent Mailhol
` (3 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: Vincent Mailhol @ 2026-11-02 21:29 UTC (permalink / raw)
To: Jens Axboe, Davidlohr Bueso, Alexander Viro, Christian Brauner, Jan Kara
Cc: linux-kernel, linux-block, linux-efi, linux-fsdevel, Dave Hansen,
James Bottomley, Helge Deller, Matthew Wilcox, Vincent Mailhol
DPS root detection will also need to work if root_wait is set, meaning
that wait_for_root() needs to handle the DPS logic.
Move the early_lookup_bdev() calls and the rootwait error handling into
the new lookup_root_device() so later changes can extend the lookup
policy without duplicating the rootwait retry logic.
While at it, reword the error message to add the "VFS: " prefix.
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
init/do_mounts.c | 23 +++++++++++++++--------
1 file changed, 15 insertions(+), 8 deletions(-)
diff --git a/init/do_mounts.c b/init/do_mounts.c
index b088021b1546..b0dccad20971 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -402,6 +402,18 @@ void __init mount_root(char *root_device_name)
}
}
+static int __init lookup_root_device(const char *root_device_name, dev_t *dev)
+{
+ int err;
+
+ err = early_lookup_bdev(root_device_name, dev);
+ if (err == -EINVAL && root_wait) {
+ pr_err("VFS: root= is invalid; disabling rootwait.\n");
+ root_wait = 0;
+ }
+ return err;
+}
+
/* wait for any asynchronous scanning to complete */
static void __init wait_for_root(char *root_device_name)
{
@@ -415,7 +427,7 @@ static void __init wait_for_root(char *root_device_name)
end = ktime_add_ms(ktime_get_raw(), root_wait);
while (!driver_probe_done() ||
- early_lookup_bdev(root_device_name, &ROOT_DEV) < 0) {
+ lookup_root_device(root_device_name, &ROOT_DEV) < 0) {
msleep(5);
if (root_wait > 0 && ktime_after(ktime_get_raw(), end))
break;
@@ -440,14 +452,9 @@ static dev_t __init parse_root_device(char *root_device_name)
if (strcmp(root_device_name, "/dev/ram") == 0)
return Root_RAM0;
- error = early_lookup_bdev(root_device_name, &dev);
- if (error) {
- if (error == -EINVAL && root_wait) {
- pr_err("Disabling rootwait; root= is invalid.\n");
- root_wait = 0;
- }
+ error = lookup_root_device(root_device_name, &dev);
+ if (error)
return 0;
- }
return dev;
}
--
2.56.0
^ permalink raw reply [flat|nested] 8+ messages in thread* [PATCH v3 4/7] block: discover root by DPS partition type UUID
2026-11-02 21:29 [PATCH v3 0/7] init: discoverable root partitions Vincent Mailhol
` (2 preceding siblings ...)
2026-11-02 21:29 ` [PATCH v3 3/7] init: factor out root device lookup into lookup_root_device() Vincent Mailhol
@ 2026-11-02 21:29 ` Vincent Mailhol
2026-11-02 21:29 ` [PATCH v3 5/7] block: skip partitions with DPS no-auto GPT attribute Vincent Mailhol
` (2 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: Vincent Mailhol @ 2026-11-02 21:29 UTC (permalink / raw)
To: Jens Axboe, Davidlohr Bueso, Alexander Viro, Christian Brauner, Jan Kara
Cc: linux-kernel, linux-block, linux-efi, linux-fsdevel, Dave Hansen,
James Bottomley, Helge Deller, Matthew Wilcox, Vincent Mailhol
When root= and ROOT_DEV are not set, discover the root block device
using the architecture's DPS root partition type UUID. Preserve explicit
root selection and an existing ROOT_DEV supplied by the boot protocol.
DPS [1] restricts discovery to the disk containing the active EFI System
Partition. Read LoaderDevicePartUUID from the Boot Loader Interface to
identify that disk and cache the UUID for subsequent lookup attempts.
Add early_lookup_bdev_by_type_uuid() to select the first matching
partition. The EFI System Partition may become visible before the root
partition has been registered. Hold the disk's open_mutex during lookup
so partition scanning finishes before returning -ENODEV.
Add early_lookup_dps_root() and have lookup_root_device() call it when
root= is absent. early_lookup_bdev() is kept unchanged so other callers
cannot trigger root discovery inadvertently. Return -EINVAL if the EFI
variable cannot be read or does not contain a valid UUID, so
lookup_root_device() disables rootwait. Return -EAGAIN if the EFI
partition is not available yet so rootwait can retry. Return -ENODEV if
its disk has no matching root partition. In that case, have
lookup_root_device() disable rootwait, clear the device number and
return zero to stop retries. This is safe because partition registration
failures do not schedule a rescan, nor does rootwait.
[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
block/early-lookup.c | 117 +++++++++++++++++++++++++++++++++++++++++++++++--
include/linux/blkdev.h | 10 +++++
init/do_mounts.c | 25 +++++++++--
3 files changed, 145 insertions(+), 7 deletions(-)
diff --git a/block/early-lookup.c b/block/early-lookup.c
index 3fb57f7d2b12..e3da4bfb41e7 100644
--- a/block/early-lookup.c
+++ b/block/early-lookup.c
@@ -5,10 +5,14 @@
*/
#include <linux/blkdev.h>
#include <linux/ctype.h>
+#include <linux/efi.h>
+#include <linux/ucs2_string.h>
+#include <linux/uuid.h>
struct uuidcmp {
const char *uuid;
int len;
+ struct gendisk *disk;
};
/**
@@ -45,13 +49,11 @@ static int __init match_dev_by_uuid(struct device *dev, const void *data)
*/
static int __init devt_from_partuuid(const char *uuid_str, dev_t *devt)
{
- struct uuidcmp cmp;
+ struct uuidcmp cmp = { .uuid = uuid_str };
struct device *dev = NULL;
int offset = 0;
char *slash;
- cmp.uuid = uuid_str;
-
slash = strchr(uuid_str, '/');
/* Check for optional partition number offset attributes. */
if (slash) {
@@ -252,6 +254,115 @@ int __init early_lookup_bdev(const char *name, dev_t *devt)
return devt_from_devnum(name, devt);
}
+#ifdef CONFIG_DPS_ROOT_AUTO_DISCOVERY
+static int __init match_dev_by_type_uuid(struct device *dev, const void *data)
+{
+ struct block_device *bdev = dev_to_bdev(dev);
+ const struct uuidcmp *cmp = data;
+
+ return bdev->bd_disk == cmp->disk && bdev->bd_meta_info &&
+ !strcasecmp(cmp->uuid, bdev->bd_meta_info->type_uuid);
+}
+
+/*
+ * Search the ESP's disk for an eligible partition of the requested type.
+ * Return -EAGAIN until the ESP is available, or -ENODEV if no match exists.
+ */
+static int __init early_lookup_bdev_by_type_uuid(const char *type_uuid,
+ const char *esp_partuuid,
+ dev_t *devt)
+{
+ struct uuidcmp esp_cmp = {
+ .uuid = esp_partuuid,
+ .len = UUID_STRING_LEN,
+ };
+ struct uuidcmp type_cmp = {
+ .uuid = type_uuid,
+ };
+ struct device *esp_dev;
+ struct device *type_dev;
+
+ esp_dev = class_find_device(&block_class, NULL, &esp_cmp,
+ &match_dev_by_uuid);
+ if (!esp_dev)
+ return -EAGAIN;
+
+ type_cmp.disk = dev_to_disk(esp_dev);
+ scoped_guard(mutex, &type_cmp.disk->open_mutex)
+ type_dev = class_find_device(&block_class, NULL, &type_cmp,
+ &match_dev_by_type_uuid);
+ put_device(esp_dev);
+ if (!type_dev)
+ return -ENODEV;
+
+ *devt = type_dev->devt;
+ pr_info("VFS: Discovered root partition /dev/%pg with GPT type UUID %s\n",
+ dev_to_bdev(type_dev), type_uuid);
+ put_device(type_dev);
+ return 0;
+}
+
+static int __init efi_get_esp_partuuid(char *partuuid, size_t size)
+{
+ static efi_guid_t loader_guid __initdata = LINUX_EFI_LOADER_ENTRY_GUID;
+ efi_char16_t partuuid_ucs2[UUID_STRING_LEN + 1] = {};
+ unsigned long partuuid_ucs2_size = sizeof(partuuid_ucs2);
+ efi_status_t status;
+
+ if (!efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE))
+ return -EOPNOTSUPP;
+
+ status = efi.get_variable(L"LoaderDevicePartUUID", &loader_guid, NULL,
+ &partuuid_ucs2_size, partuuid_ucs2);
+ if (status != EFI_SUCCESS)
+ return efi_status_to_err(status);
+
+ if (partuuid_ucs2_size != sizeof(partuuid_ucs2) ||
+ partuuid_ucs2[UUID_STRING_LEN] ||
+ ucs2_as_utf8((u8 *)partuuid, partuuid_ucs2, size) != UUID_STRING_LEN ||
+ !uuid_is_valid(partuuid))
+ return -EINVAL;
+
+ return 0;
+}
+
+/**
+ * early_lookup_dps_root - discover the root device by DPS partition type UUID
+ * @dev: receives the discovered device number
+ *
+ * Return: 0 on success, -EINVAL if the ESP partition UUID cannot be read or
+ * is invalid, -EAGAIN if the EFI partition is not yet available, or -ENODEV
+ * if no matching root exists.
+ * With DPS disabled, return 0 and set @dev to zero.
+ */
+int __init early_lookup_dps_root(dev_t *dev)
+{
+ static const char root_type_uuids[][UUID_STRING_LEN + 1] __initconst = {
+ CONFIG_DPS_ROOT_PARTITION_TYPE_UUID,
+ };
+ static char esp_partuuid[UUID_STRING_LEN + 1] __initdata;
+ int err;
+
+ if (!esp_partuuid[0]) {
+ err = efi_get_esp_partuuid(esp_partuuid, sizeof(esp_partuuid));
+ if (err) {
+ pr_err("VFS: Unable to get EFI partition UUID: %pe, skipping root partition discovery\n",
+ ERR_PTR(err));
+ return -EINVAL;
+ }
+ }
+
+ for (int i = 0; i < ARRAY_SIZE(root_type_uuids); i++) {
+ err = early_lookup_bdev_by_type_uuid(root_type_uuids[i],
+ esp_partuuid, dev);
+ if (err != -ENODEV)
+ return err;
+ }
+
+ return -ENODEV;
+}
+#endif /* CONFIG_DPS_ROOT_AUTO_DISCOVERY */
+
static char __init *bdevt_str(dev_t devt, char *buf)
{
if (MAJOR(devt) <= 0xff && MINOR(devt) <= 0xff) {
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 0e0f5d68425d..5cfef05b552e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1748,6 +1748,16 @@ static inline int early_lookup_bdev(const char *pathname, dev_t *dev)
}
#endif /* CONFIG_BLOCK */
+#ifdef CONFIG_DPS_ROOT_AUTO_DISCOVERY
+int __init early_lookup_dps_root(dev_t *dev);
+#else
+static inline int early_lookup_dps_root(dev_t *dev)
+{
+ *dev = 0;
+ return 0;
+}
+#endif /* CONFIG_DPS_ROOT_AUTO_DISCOVERY */
+
int bdev_freeze(struct block_device *bdev);
int bdev_thaw(struct block_device *bdev);
int bdev_deny_freeze(struct block_device *bdev);
diff --git a/init/do_mounts.c b/init/do_mounts.c
index b0dccad20971..c1106e3cab3d 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -406,10 +406,24 @@ static int __init lookup_root_device(const char *root_device_name, dev_t *dev)
{
int err;
- err = early_lookup_bdev(root_device_name, dev);
+ if (root_device_name[0]) {
+ err = early_lookup_bdev(root_device_name, dev);
+ if (err == -EINVAL && root_wait) {
+ pr_err("VFS: root= is invalid; disabling rootwait.\n");
+ root_wait = 0;
+ }
+ return err;
+ }
+
+ err = early_lookup_dps_root(dev);
if (err == -EINVAL && root_wait) {
- pr_err("VFS: root= is invalid; disabling rootwait.\n");
+ pr_err("VFS: Root discovery is unavailable; disabling rootwait.\n");
root_wait = 0;
+ } else if (err == -ENODEV) {
+ pr_err("VFS: No discoverable root partition found; aborting.\n");
+ root_wait = 0;
+ *dev = 0;
+ return 0;
}
return err;
}
@@ -422,7 +436,10 @@ static void __init wait_for_root(char *root_device_name)
if (ROOT_DEV != 0)
return;
- pr_info("Waiting for root device %s...\n", root_device_name);
+ if (root_device_name[0])
+ pr_info("Waiting for root device %s...\n", root_device_name);
+ else if (IS_ENABLED(CONFIG_DPS_ROOT_AUTO_DISCOVERY))
+ pr_info("Waiting for discoverable root partition...\n");
end = ktime_add_ms(ktime_get_raw(), root_wait);
@@ -480,7 +497,7 @@ void __init prepare_namespace(void)
md_run_setup();
- if (saved_root_name[0])
+ if (saved_root_name[0] || !ROOT_DEV)
ROOT_DEV = parse_root_device(saved_root_name);
initrd_load();
--
2.56.0
^ permalink raw reply [flat|nested] 8+ messages in thread* [PATCH v3 5/7] block: skip partitions with DPS no-auto GPT attribute
2026-11-02 21:29 [PATCH v3 0/7] init: discoverable root partitions Vincent Mailhol
` (3 preceding siblings ...)
2026-11-02 21:29 ` [PATCH v3 4/7] block: discover root by DPS partition type UUID Vincent Mailhol
@ 2026-11-02 21:29 ` Vincent Mailhol
2026-11-02 21:29 ` [PATCH v3 6/7] block: add secondary DPS root partition type UUID Vincent Mailhol
2026-11-02 21:29 ` [PATCH v3 7/7] docs: document discoverable root partitions Vincent Mailhol
6 siblings, 0 replies; 8+ messages in thread
From: Vincent Mailhol @ 2026-11-02 21:29 UTC (permalink / raw)
To: Jens Axboe, Davidlohr Bueso, Alexander Viro, Christian Brauner, Jan Kara
Cc: linux-kernel, linux-block, linux-efi, linux-fsdevel, Dave Hansen,
James Bottomley, Helge Deller, Matthew Wilcox, Vincent Mailhol
DPS [1] defines GPT attribute bit 63 as no-auto. Partitions with this
bit set must not be used by automatic discovery.
Store the GPT attributes in partition_meta_info and check
GPT_ATTR_NO_AUTO_DISCOVERY when matching a root partition by type UUID.
[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
block/early-lookup.c | 1 +
block/partitions/efi.c | 1 +
include/linux/blkdev.h | 4 ++++
3 files changed, 6 insertions(+)
diff --git a/block/early-lookup.c b/block/early-lookup.c
index e3da4bfb41e7..c59d14889fee 100644
--- a/block/early-lookup.c
+++ b/block/early-lookup.c
@@ -261,6 +261,7 @@ static int __init match_dev_by_type_uuid(struct device *dev, const void *data)
const struct uuidcmp *cmp = data;
return bdev->bd_disk == cmp->disk && bdev->bd_meta_info &&
+ !(bdev->bd_meta_info->gpt_attr & GPT_ATTR_NO_AUTO_DISCOVERY) &&
!strcasecmp(cmp->uuid, bdev->bd_meta_info->type_uuid);
}
diff --git a/block/partitions/efi.c b/block/partitions/efi.c
index 4a3835ed9561..ed5e2f21ad85 100644
--- a/block/partitions/efi.c
+++ b/block/partitions/efi.c
@@ -741,6 +741,7 @@ int efi_partition(struct parsed_partitions *state)
state->parts[i + 1].flags = ADDPART_FLAG_RAID;
info = &state->parts[i + 1].info;
+ info->gpt_attr = le64_to_cpu(ptes[i].attributes);
efi_guid_to_str(&ptes[i].unique_partition_guid, info->uuid);
efi_guid_to_str(&ptes[i].partition_type_guid, info->type_uuid);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 5cfef05b552e..fd6d809189e8 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -5,6 +5,7 @@
#ifndef _LINUX_BLKDEV_H
#define _LINUX_BLKDEV_H
+#include <linux/bits.h>
#include <linux/types.h>
#include <linux/blk_types.h>
#include <linux/blk_plug.h>
@@ -65,10 +66,13 @@ extern const struct class block_class;
*/
#define PARTITION_META_INFO_UUIDLTH (UUID_STRING_LEN + 1)
+#define GPT_ATTR_NO_AUTO_DISCOVERY BIT_U64(63)
+
struct partition_meta_info {
char uuid[PARTITION_META_INFO_UUIDLTH];
char type_uuid[PARTITION_META_INFO_UUIDLTH];
u8 volname[PARTITION_META_INFO_VOLNAMELTH];
+ u64 gpt_attr;
};
/**
--
2.56.0
^ permalink raw reply [flat|nested] 8+ messages in thread* [PATCH v3 6/7] block: add secondary DPS root partition type UUID
2026-11-02 21:29 [PATCH v3 0/7] init: discoverable root partitions Vincent Mailhol
` (4 preceding siblings ...)
2026-11-02 21:29 ` [PATCH v3 5/7] block: skip partitions with DPS no-auto GPT attribute Vincent Mailhol
@ 2026-11-02 21:29 ` Vincent Mailhol
2026-11-02 21:29 ` [PATCH v3 7/7] docs: document discoverable root partitions Vincent Mailhol
6 siblings, 0 replies; 8+ messages in thread
From: Vincent Mailhol @ 2026-11-02 21:29 UTC (permalink / raw)
To: Jens Axboe, Davidlohr Bueso, Alexander Viro, Christian Brauner, Jan Kara
Cc: linux-kernel, linux-block, linux-efi, linux-fsdevel, Dave Hansen,
James Bottomley, Helge Deller, Matthew Wilcox, Vincent Mailhol
Some 64-bit architectures can run 32-bit user space. For those
configurations, also allow DPS root discovery to use the compatible
32-bit root partition type UUID as a fallback.
Add a secondary root partition type UUID Kconfig symbol and include it
in the DPS root lookup when it is defined.
Try the secondary UUID only when the native lookup returns -ENODEV.
The fallback reflects compatibility support built into the kernel. If
32-bit execution is disabled, for example with ia32_emulation=0, the
selected root's 32-bit init will fail to execute. Do not check
ia32_enabled(): rejecting the fallback would instead leave no
discoverable root partition, resulting in a kernel panic either way.
Suggested-by: Dave Hansen <dave.hansen@intel.com>
Link: https://lore.kernel.org/r/03be57ae-0e41-4b8a-adc5-bdd85ccce951@intel.com
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
block/Kconfig | 7 +++++++
block/early-lookup.c | 3 +++
2 files changed, 10 insertions(+)
diff --git a/block/Kconfig b/block/Kconfig
index 3b5d3ad42dde..f3a0730ffcde 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -222,6 +222,13 @@ config DPS_ROOT_PARTITION_TYPE_UUID
default "4f68bce3-e8cd-4db1-96e7-fbcaf984b709" if X86_64
default "44479540-f297-41b2-9af7-d131d5f0458a" if X86
+config DPS_ROOT_PARTITION_TYPE_UUID_SECONDARY
+ depends on DPS_ROOT_PARTITION_TYPE_UUID != "" && COMPAT
+ string
+ default "69dad710-2ce4-4e3c-b16c-21a1d49abed3" if ARM64
+ default "60d5a7fe-8e7d-435c-b714-3dd8162144e1" if RISCV
+ default "44479540-f297-41b2-9af7-d131d5f0458a" if X86_64 && IA32_EMULATION
+
config DPS_ROOT_AUTO_DISCOVERY
def_bool DPS_ROOT_PARTITION_TYPE_UUID != ""
diff --git a/block/early-lookup.c b/block/early-lookup.c
index c59d14889fee..2e7dace5efa3 100644
--- a/block/early-lookup.c
+++ b/block/early-lookup.c
@@ -340,6 +340,9 @@ int __init early_lookup_dps_root(dev_t *dev)
{
static const char root_type_uuids[][UUID_STRING_LEN + 1] __initconst = {
CONFIG_DPS_ROOT_PARTITION_TYPE_UUID,
+#ifdef CONFIG_DPS_ROOT_PARTITION_TYPE_UUID_SECONDARY
+ CONFIG_DPS_ROOT_PARTITION_TYPE_UUID_SECONDARY,
+#endif
};
static char esp_partuuid[UUID_STRING_LEN + 1] __initdata;
int err;
--
2.56.0
^ permalink raw reply [flat|nested] 8+ messages in thread* [PATCH v3 7/7] docs: document discoverable root partitions
2026-11-02 21:29 [PATCH v3 0/7] init: discoverable root partitions Vincent Mailhol
` (5 preceding siblings ...)
2026-11-02 21:29 ` [PATCH v3 6/7] block: add secondary DPS root partition type UUID Vincent Mailhol
@ 2026-11-02 21:29 ` Vincent Mailhol
6 siblings, 0 replies; 8+ messages in thread
From: Vincent Mailhol @ 2026-11-02 21:29 UTC (permalink / raw)
To: Jens Axboe, Davidlohr Bueso, Alexander Viro, Christian Brauner, Jan Kara
Cc: linux-kernel, linux-block, linux-efi, linux-fsdevel, Dave Hansen,
James Bottomley, Helge Deller, Matthew Wilcox, Vincent Mailhol,
Jonathan Corbet, Shuah Khan, Randy Dunlap, linux-doc
Document the automatic root block device discovery feature.
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-doc@vger.kernel.org
---
Documentation/admin-guide/discoverable-root.rst | 46 +++++++++++++++++++++++++
Documentation/admin-guide/index.rst | 1 +
Documentation/admin-guide/kernel-parameters.txt | 5 +++
3 files changed, 52 insertions(+)
diff --git a/Documentation/admin-guide/discoverable-root.rst b/Documentation/admin-guide/discoverable-root.rst
new file mode 100644
index 000000000000..29c93b3526a6
--- /dev/null
+++ b/Documentation/admin-guide/discoverable-root.rst
@@ -0,0 +1,46 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _discoverable_root:
+
+Discoverable root partitions
+============================
+
+On EFI systems, the kernel can discover the root block device from GPT partition
+type UUID metadata on the disk containing the active EFI System Partition.
+
+This follows the `Discoverable Partitions Specification`_ which defines a list
+of architecture-specific root partition type UUIDs.
+
+An initramfs providing an early userspace ``init``, a ``root=`` kernel
+command-line option, or a root device supplied through the boot protocol takes
+precedence over this automatic root partition discovery.
+
+Automatic discovery uses the Boot Loader Interface ``LoaderDevicePartUUID`` EFI
+variable to identify the partition whose disk is searched. This variable is
+provided by the boot loader or, when the kernel is booted directly by EFI
+firmware, by the Linux EFI stub.
+
+The kernel first searches for the native root partition type UUID. On
+architectures that can run compatible 32-bit user space, the matching 32-bit
+root partition type UUID may be used as a fallback if no partition with the
+native type UUID is found. If multiple partitions on that disk match the
+selected type UUID, the kernel selects the first match in block device
+enumeration order, but this may change. Systems should not expose multiple
+eligible root partitions.
+
+Partitions marked with the DPS ``no-auto`` GPT attribute are skipped. This
+allows a partition with an otherwise discoverable type UUID to opt out from
+automatic discovery.
+
+With ``rootwait`` or ``rootwait=<seconds>``, discovery retries while the EFI
+System Partition is unavailable, subject to any specified timeout. Once its disk
+has been scanned, discovery either selects an eligible root partition or stops
+if none was found. Waiting is also disabled if ``LoaderDevicePartUUID`` is not
+available or is malformed.
+
+The DPS read-only attribute is not enforced by kernel root discovery. The
+root filesystem is mounted read-only by default unless ``rw`` is specified,
+and user space remains responsible for later remount policy.
+
+.. _Discoverable Partitions Specification:
+ https://uapi-group.org/specifications/specs/discoverable_partitions_specification/
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst
index cd28dfe91b06..0d9c2796ae09 100644
--- a/Documentation/admin-guide/index.rst
+++ b/Documentation/admin-guide/index.rst
@@ -50,6 +50,7 @@ Booting the kernel
bootconfig
kernel-parameters
+ discoverable-root
efi-stub
initrd
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 0b3c140babd9..2e7b89a05944 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6746,6 +6746,11 @@ Kernel parameters
ramdisk, "nfs" and "cifs" for root on a network file
system, or "mtd" and "ubi" for mounting from raw flash.
+ If this option is omitted, the kernel may try to
+ discover the root block device from the GPT partition
+ type UUID metadata when additional requirements are met.
+ See Documentation/admin-guide/discoverable-root.rst.
+
rootdelay= [KNL] Delay (in seconds) to pause before attempting to
mount the root filesystem
--
2.56.0
^ permalink raw reply [flat|nested] 8+ messages in thread