[PATCH 00/19] init: discoverable root partitions, a.k.a. an omittable "root=" cmdline option

[PATCH 00/19] init: discoverable root partitions, a.k.a. an omittable "root=" cmdline option

[Vincent Mailhol]

DPS [1] defines GPT partition type UUIDs for OS partitions and
attributes that control whether such partitions should be
automatically discovered. The specification states that:

  The OS can discover and mount the necessary file systems with a
  non-existent or incomplete /etc/fstab file and without the root=
  kernel command line option.

DPS is already implemented in systemd-gpt-auto-generator [2], which,
when embedded in an initrd, indeed allows automatic detection of the
root filesystem through its partition type UUID.

This series adds this discovery feature directly into the kernel so
that people who are not using systemd or not using an initrd can still
benefit from it. The implementation follows the same model as
systemd-gpt-auto-generator:

  - GPT partition type UUIDs are used for automatic discovery policy
    only. No root=PARTTYPEUUID=xxx cmdline option or similar syntax is
    added.

  - The root= cmdline option takes precedence. This prevents unexpected
    behaviour.

  - Only the disk with the active EFI System Partition is scanned, as
    required by DPS. The disk is identified through the Boot Loader
    Interface LoaderDevicePartUUID EFI variable.

The DPS no-auto attribute is also implemented, giving another option for
the user to disable this auto discovery. However, the DPS read-only
attribute is intentionally not enforced. The kernel already mounts the
root filesystem read-only by default unless the command line requests
rw, and user space remains responsible for deciding whether a discovered
root should later be remounted read-write based on DPS metadata and
local policy. The other partition type UUIDs (home, swap, var...) are
also out of scope for the same reason: user space remains responsible
for mounting anything other than the root partition.

Patch 1 adds the ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID capability and
the hidden CONFIG_DPS_ROOT_AUTO_DISCOVERY Kconfig symbol used to signal
whether the feature is available. Patches 2 to 12 declare the
ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID capability for the supported
architectures and define their architecture-specific root partition type
UUID values in asm/dps_root.h.

Patches 13 to 16 make the GPT partition type UUID and the no-auto
attribute available during early block lookup.

Patch 17 is a small code refactor that prepares for patch 18, which
updates the root mount path so that, when root= is omitted, the kernel
reads LoaderDevicePartUUID and uses the early block lookup
infrastructure to discover the DPS root partition on that disk.

Finally, patch 19 documents this automatic root discovery feature.

Tested with GRUB, which implements the LoaderDevicePartUUID EFI variable
in its bli module [3]. With this, I was able to boot a kernel with a
completely empty cmdline and no initrd.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

[2] systemd-gpt-auto-generator
Link: https://www.freedesktop.org/software/systemd/man/latest/systemd-gpt-auto-generator.html

[3] GRUB -- §16.2 bli
Link: https://www.gnu.org/software/grub/manual/grub/html_node/bli_005fmodule.html

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
Vincent Mailhol (19):
      init: add DPS root partition type UUID capability
      alpha: define DPS root partition type UUID
      arc: define DPS root partition type UUID
      arm: define DPS root partition type UUID
      arm64: define DPS root partition type UUID
      loongarch: define DPS root partition type UUID
      mips: define DPS root partition type UUIDs
      parisc: define DPS root partition type UUID
      powerpc: define DPS root partition type UUIDs
      riscv: define DPS root partition type UUIDs
      s390: define DPS root partition type UUIDs
      x86: define DPS root partition type UUIDs
      block: store GPT partition type UUID
      block: add early_lookup_bdev_by_type_uuid()
      block: store GPT attributes as a raw value
      block: don't discover partition with DPS no-auto GPT attribute
      init: factor out root device lookup into lookup_root_device()
      init: discover root by DPS partition type UUID
      docs: document discoverable root partitions

 Documentation/admin-guide/discoverable-root.rst | 33 +++++++++
 Documentation/admin-guide/index.rst             |  1 +
 Documentation/admin-guide/kernel-parameters.txt |  5 ++
 arch/alpha/Kconfig                              |  1 +
 arch/alpha/include/asm/dps_root.h               |  8 +++
 arch/arc/Kconfig                                |  1 +
 arch/arc/include/asm/dps_root.h                 |  8 +++
 arch/arm/Kconfig                                |  1 +
 arch/arm/include/asm/dps_root.h                 |  8 +++
 arch/arm64/Kconfig                              |  1 +
 arch/arm64/include/asm/dps_root.h               |  8 +++
 arch/loongarch/Kconfig                          |  1 +
 arch/loongarch/include/asm/dps_root.h           |  8 +++
 arch/mips/Kconfig                               |  1 +
 arch/mips/include/asm/dps_root.h                | 20 ++++++
 arch/parisc/Kconfig                             |  1 +
 arch/parisc/include/asm/dps_root.h              |  8 +++
 arch/powerpc/Kconfig                            |  1 +
 arch/powerpc/include/asm/dps_root.h             | 16 +++++
 arch/riscv/Kconfig                              |  1 +
 arch/riscv/include/asm/dps_root.h               | 12 ++++
 arch/s390/Kconfig                               |  1 +
 arch/s390/include/asm/dps_root.h                | 12 ++++
 arch/x86/Kconfig                                |  1 +
 arch/x86/include/asm/dps_root.h                 | 12 ++++
 block/blk.h                                     |  1 +
 block/early-lookup.c                            | 68 +++++++++++++++++-
 block/partitions/core.c                         |  2 +
 block/partitions/efi.c                          |  3 +
 block/partitions/efi.h                          | 11 ++-
 include/linux/blk_types.h                       |  1 +
 include/linux/blkdev.h                          |  5 ++
 include/linux/root_dev.h                        |  6 ++
 init/Kconfig                                    |  6 ++
 init/do_mounts.c                                | 94 ++++++++++++++++++++++++-
 35 files changed, 355 insertions(+), 12 deletions(-)
---
base-commit: 36808d5e983985bbda87e01059cccc071fe3ec8d
change-id: 20260611-discoverable-root_partitions-bdacbada570d

Best regards,
-- 
Vincent Mailhol <mailhol@kernel.org>

[PATCH 01/19] init: add DPS root partition type UUID capability

[Vincent Mailhol]

DPS [1] assigns native root partition type UUIDs per architecture.

Add the ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID config option so that
architectures can opt in.

Architectures that support this feature should define
DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h as a string
representation of the architecture's DPS root partition type UUID.

Add the hidden DPS_ROOT_AUTO_DISCOVERY symbol for the combination of
BLOCK, EFI and ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID, and use it to
expose DPS_ROOT_PARTITION_TYPE_UUID from the common linux/root_dev.h
header only when all prerequisites are met.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 include/linux/root_dev.h | 6 ++++++
 init/Kconfig             | 6 ++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/root_dev.h b/include/linux/root_dev.h
index 847c9a06101b..6b52a10b0bca 100644
--- a/include/linux/root_dev.h
+++ b/include/linux/root_dev.h
@@ -5,6 +5,12 @@
 #include <linux/major.h>
 #include <linux/types.h>
 #include <linux/kdev_t.h>
+#include <linux/uuid.h>
+
+#ifdef CONFIG_DPS_ROOT_AUTO_DISCOVERY
+#include <asm/dps_root.h>
+static_assert(sizeof(DPS_ROOT_PARTITION_TYPE_UUID) == UUID_STRING_LEN + 1);
+#endif
 
 enum {
 	Root_NFS = MKDEV(UNNAMED_MAJOR, 255),
diff --git a/init/Kconfig b/init/Kconfig
index 147da6370bf0..982c6ad9da4d 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2287,6 +2287,12 @@ config ARCH_HAS_PREPARE_SYNC_CORE_CMD
 config ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
 	bool
 
+config ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
+	bool
+
+config DPS_ROOT_AUTO_DISCOVERY
+	def_bool BLOCK && EFI && ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
+
 # It may be useful for an architecture to override the definitions of the
 # SYSCALL_DEFINE() and __SYSCALL_DEFINEx() macros in <linux/syscalls.h>
 # and the COMPAT_ variants in <linux/compat.h>, in particular to use a

-- 
2.53.0

[PATCH 02/19] alpha: define DPS root partition type UUID

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for alpha and
select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Magnus Lindholm <linmag7@gmail.com>
Cc: linux-alpha@vger.kernel.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/alpha/Kconfig                | 1 +
 arch/alpha/include/asm/dps_root.h | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 7b7dafe7d9df..400cbb7525c8 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -5,6 +5,7 @@ config ALPHA
 	select ARCH_32BIT_USTAT_F_TINODE
 	select ARCH_HAS_CURRENT_STACK_POINTER
 	select ARCH_HAS_DMA_OPS if PCI
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select ARCH_MIGHT_HAVE_PC_SERIO
 	select ARCH_MODULE_NEEDS_WEAK_PER_CPU if SMP
diff --git a/arch/alpha/include/asm/dps_root.h b/arch/alpha/include/asm/dps_root.h
new file mode 100644
index 000000000000..7f70a83f72de
--- /dev/null
+++ b/arch/alpha/include/asm/dps_root.h
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_ALPHA_DPS_ROOT_H
+#define _ASM_ALPHA_DPS_ROOT_H
+
+#define DPS_ROOT_PARTITION_TYPE_UUID "6523f8ae-3eb1-4e2a-a05a-18b695ae656f"
+
+#endif /* _ASM_ALPHA_DPS_ROOT_H */

-- 
2.53.0

[PATCH 03/19] arc: define DPS root partition type UUID

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for ARC and select
ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: Vineet Gupta <vgupta@kernel.org>
Cc: linux-snps-arc@lists.infradead.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/arc/Kconfig                | 1 +
 arch/arc/include/asm/dps_root.h | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index 2ed7186c81c5..cc3a57a0111f 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -10,6 +10,7 @@ config ARC
 	select ARCH_HAS_CACHE_LINE_SIZE
 	select ARCH_HAS_DEBUG_VM_PGTABLE
 	select ARCH_HAS_DMA_PREP_COHERENT
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_SETUP_DMA_OPS
 	select ARCH_HAS_SYNC_DMA_FOR_CPU
diff --git a/arch/arc/include/asm/dps_root.h b/arch/arc/include/asm/dps_root.h
new file mode 100644
index 000000000000..c9db3ddf1a53
--- /dev/null
+++ b/arch/arc/include/asm/dps_root.h
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_ARC_DPS_ROOT_H
+#define _ASM_ARC_DPS_ROOT_H
+
+#define DPS_ROOT_PARTITION_TYPE_UUID "d27f46ed-2919-4cb8-bd25-9531f3c16534"
+
+#endif /* _ASM_ARC_DPS_ROOT_H */

-- 
2.53.0

[PATCH 04/19] arm: define DPS root partition type UUID

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for ARM and select
ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/arm/Kconfig                | 1 +
 arch/arm/include/asm/dps_root.h | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 73e6647bea46..deedb5d808fb 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -14,6 +14,7 @@ config ARM
 	select ARCH_HAS_DMA_ALLOC if MMU
 	select ARCH_HAS_DMA_OPS
 	select ARCH_HAS_DMA_WRITE_COMBINE if !ARM_DMA_MEM_BUFFERABLE
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_KEEPINITRD
diff --git a/arch/arm/include/asm/dps_root.h b/arch/arm/include/asm/dps_root.h
new file mode 100644
index 000000000000..e9f0f24bcac2
--- /dev/null
+++ b/arch/arm/include/asm/dps_root.h
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_ARM_DPS_ROOT_H
+#define _ASM_ARM_DPS_ROOT_H
+
+#define DPS_ROOT_PARTITION_TYPE_UUID "69dad710-2ce4-4e3c-b16c-21a1d49abed3"
+
+#endif /* _ASM_ARM_DPS_ROOT_H */

-- 
2.53.0

[PATCH 05/19] arm64: define DPS root partition type UUID

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for arm64 and select
ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/arm64/Kconfig                | 1 +
 arch/arm64/include/asm/dps_root.h | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index fe60738e5943..190f8dde63b2 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -26,6 +26,7 @@ config ARM64
 	select ARCH_HAS_DEBUG_VM_PGTABLE
 	select ARCH_HAS_DMA_OPS if XEN
 	select ARCH_HAS_DMA_PREP_COHERENT
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select ARCH_HAS_ACPI_TABLE_UPGRADE if ACPI
 	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_FORTIFY_SOURCE
diff --git a/arch/arm64/include/asm/dps_root.h b/arch/arm64/include/asm/dps_root.h
new file mode 100644
index 000000000000..7344f9a52343
--- /dev/null
+++ b/arch/arm64/include/asm/dps_root.h
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_ARM64_DPS_ROOT_H
+#define _ASM_ARM64_DPS_ROOT_H
+
+#define DPS_ROOT_PARTITION_TYPE_UUID "b921b045-1df0-41c3-af44-4c6f280d3fae"
+
+#endif /* _ASM_ARM64_DPS_ROOT_H */

-- 
2.53.0

[PATCH 06/19] loongarch: define DPS root partition type UUID

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for LoongArch64
and select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: WANG Xuerui <kernel@xen0n.name>
Cc: loongarch@lists.linux.dev
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/loongarch/Kconfig                | 1 +
 arch/loongarch/include/asm/dps_root.h | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index 606597da46b8..64c27740b8a1 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -17,6 +17,7 @@ config LOONGARCH
 	select ARCH_HAS_CPU_FINALIZE_INIT
 	select ARCH_HAS_CURRENT_STACK_POINTER
 	select ARCH_HAS_DEBUG_VM_PGTABLE
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_KCOV
diff --git a/arch/loongarch/include/asm/dps_root.h b/arch/loongarch/include/asm/dps_root.h
new file mode 100644
index 000000000000..6022b60428ad
--- /dev/null
+++ b/arch/loongarch/include/asm/dps_root.h
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_LOONGARCH_DPS_ROOT_H
+#define _ASM_LOONGARCH_DPS_ROOT_H
+
+#define DPS_ROOT_PARTITION_TYPE_UUID "77055800-792c-4f94-b39a-98c91b762bb6"
+
+#endif /* _ASM_LOONGARCH_DPS_ROOT_H */

-- 
2.53.0

[PATCH 07/19] mips: define DPS root partition type UUIDs

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for the MIPS
endian and 32/64-bit variants described by the specification and select
ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/mips/Kconfig                |  1 +
 arch/mips/include/asm/dps_root.h | 20 ++++++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 4364f3dba688..15dd7d336cfa 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -10,6 +10,7 @@ config MIPS
 	select ARCH_HAS_CURRENT_STACK_POINTER
 	select ARCH_HAS_DEBUG_VIRTUAL if !64BIT
 	select ARCH_HAS_DMA_OPS if MACH_JAZZ
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_KCOV
 	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE if !EVA
diff --git a/arch/mips/include/asm/dps_root.h b/arch/mips/include/asm/dps_root.h
new file mode 100644
index 000000000000..b07cab7399ad
--- /dev/null
+++ b/arch/mips/include/asm/dps_root.h
@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_MIPS_DPS_ROOT_H
+#define _ASM_MIPS_DPS_ROOT_H
+
+#ifdef CONFIG_CPU_LITTLE_ENDIAN
+#ifdef CONFIG_64BIT
+#define DPS_ROOT_PARTITION_TYPE_UUID "700bda43-7a34-4507-b179-eeb93d7a7ca3"
+#else
+#define DPS_ROOT_PARTITION_TYPE_UUID "37c58c8a-d913-4156-a25f-48b1b64e07f0"
+#endif
+#else
+#ifdef CONFIG_64BIT
+#define DPS_ROOT_PARTITION_TYPE_UUID "d113af76-80ef-41b4-bdb6-0cff4d3d4a25"
+#else
+#define DPS_ROOT_PARTITION_TYPE_UUID "e9434544-6e2c-47cc-bae2-12d6deafb44c"
+#endif
+#endif
+
+#endif /* _ASM_MIPS_DPS_ROOT_H */

-- 
2.53.0

[PATCH 08/19] parisc: define DPS root partition type UUID

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for parisc and
select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-parisc@vger.kernel.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/parisc/Kconfig                | 1 +
 arch/parisc/include/asm/dps_root.h | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
index d3afac2f0d9b..94f0a758176a 100644
--- a/arch/parisc/Kconfig
+++ b/arch/parisc/Kconfig
@@ -24,6 +24,7 @@ config PARISC
 	select ARCH_STACKWALK
 	select ARCH_HAS_CACHE_LINE_SIZE
 	select ARCH_HAS_DEBUG_VM_PGTABLE
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select HAVE_RELIABLE_STACKTRACE
 	select RTC_CLASS
 	select RTC_DRV_GENERIC
diff --git a/arch/parisc/include/asm/dps_root.h b/arch/parisc/include/asm/dps_root.h
new file mode 100644
index 000000000000..bf475cd5b01c
--- /dev/null
+++ b/arch/parisc/include/asm/dps_root.h
@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_PARISC_DPS_ROOT_H
+#define _ASM_PARISC_DPS_ROOT_H
+
+#define DPS_ROOT_PARTITION_TYPE_UUID "1aacdb3b-5444-4138-bd9e-e5c2239b2346"
+
+#endif /* _ASM_PARISC_DPS_ROOT_H */

-- 
2.53.0

[PATCH 09/19] powerpc: define DPS root partition type UUIDs

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for the powerpc
endian and 32/64-bit variants described by the specification and select
ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/powerpc/Kconfig                |  1 +
 arch/powerpc/include/asm/dps_root.h | 16 ++++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index e93df95b79e7..e611280a6057 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -134,6 +134,7 @@ config PPC
 	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select ARCH_HAS_DMA_MAP_DIRECT 		if PPC_PSERIES
 	select ARCH_HAS_DMA_OPS			if PPC64
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_GCOV_PROFILE_ALL
 	select ARCH_HAS_GIGANTIC_PAGE		if ARCH_SUPPORTS_HUGETLBFS
diff --git a/arch/powerpc/include/asm/dps_root.h b/arch/powerpc/include/asm/dps_root.h
new file mode 100644
index 000000000000..6b3592a73d3d
--- /dev/null
+++ b/arch/powerpc/include/asm/dps_root.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_POWERPC_DPS_ROOT_H
+#define _ASM_POWERPC_DPS_ROOT_H
+
+#ifdef CONFIG_PPC64
+#ifdef CONFIG_CPU_LITTLE_ENDIAN
+#define DPS_ROOT_PARTITION_TYPE_UUID "c31c45e6-3f39-412e-80fb-4809c4980599"
+#else
+#define DPS_ROOT_PARTITION_TYPE_UUID "912ade1d-a839-4913-8964-a10eee08fbd2"
+#endif
+#else
+#define DPS_ROOT_PARTITION_TYPE_UUID "1de3f1ef-fa98-47b5-8dcd-4a860a654d78"
+#endif
+
+#endif /* _ASM_POWERPC_DPS_ROOT_H */

-- 
2.53.0

[PATCH 10/19] riscv: define DPS root partition type UUIDs

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for the RISC-V
32/64-bit variants described by the specification and select
ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: Paul Walmsley <pjw@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: linux-riscv@lists.infradead.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/riscv/Kconfig                |  1 +
 arch/riscv/include/asm/dps_root.h | 12 ++++++++++++
 2 files changed, 13 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 1a2fadccd4c4..526be67e858a 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -28,6 +28,7 @@ config RISCV
 	select ARCH_HAS_DEBUG_VIRTUAL if MMU
 	select ARCH_HAS_DEBUG_VM_PGTABLE
 	select ARCH_HAS_DEBUG_WX
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select ARCH_HAS_ELF_CORE_EFLAGS if BINFMT_ELF && ELF_CORE
 	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_FORTIFY_SOURCE
diff --git a/arch/riscv/include/asm/dps_root.h b/arch/riscv/include/asm/dps_root.h
new file mode 100644
index 000000000000..86f8e1dbd6ad
--- /dev/null
+++ b/arch/riscv/include/asm/dps_root.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_RISCV_DPS_ROOT_H
+#define _ASM_RISCV_DPS_ROOT_H
+
+#ifdef CONFIG_64BIT
+#define DPS_ROOT_PARTITION_TYPE_UUID "72ec70a6-cf74-40e6-bd49-4bda08e8f224"
+#else
+#define DPS_ROOT_PARTITION_TYPE_UUID "60d5a7fe-8e7d-435c-b714-3dd8162144e1"
+#endif
+
+#endif /* _ASM_RISCV_DPS_ROOT_H */

-- 
2.53.0

[PATCH 11/19] s390: define DPS root partition type UUIDs

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for the s390
(32-bit) and s390x (64-bit) variants described by the specification and
select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: linux-s390@vger.kernel.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/s390/Kconfig                |  1 +
 arch/s390/include/asm/dps_root.h | 12 ++++++++++++
 2 files changed, 13 insertions(+)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 9921a3772bf7..af2cf67fffd7 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -91,6 +91,7 @@ config S390
 	select ARCH_HAS_DEBUG_WX
 	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select ARCH_HAS_DMA_OPS if PCI
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FORCE_DMA_UNENCRYPTED
 	select ARCH_HAS_FORTIFY_SOURCE
diff --git a/arch/s390/include/asm/dps_root.h b/arch/s390/include/asm/dps_root.h
new file mode 100644
index 000000000000..e72e44a66097
--- /dev/null
+++ b/arch/s390/include/asm/dps_root.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_S390_DPS_ROOT_H
+#define _ASM_S390_DPS_ROOT_H
+
+#ifdef CONFIG_64BIT
+#define DPS_ROOT_PARTITION_TYPE_UUID "5eead9a9-fe09-4a1e-a1d7-520d00531306"
+#else
+#define DPS_ROOT_PARTITION_TYPE_UUID "08a7acea-624c-4a20-91e8-6e0fa67d23f9"
+#endif
+
+#endif /* _ASM_S390_DPS_ROOT_H */

-- 
2.53.0

[PATCH 12/19] x86: define DPS root partition type UUIDs

[Vincent Mailhol]

DPS [1] assigns GPT partition type UUIDs to operating system partitions.
Root partitions use architecture-specific type UUIDs so the OS can
discover the intended root filesystem without relying on a root= cmdline
option.

Define DPS_ROOT_PARTITION_TYPE_UUID in asm/dps_root.h for the x86
32/64-bit variants described by the specification and select
ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID.

[1] The Discoverable Partitions Specification (DPS)
Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/

Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 arch/x86/Kconfig                |  1 +
 arch/x86/include/asm/dps_root.h | 12 ++++++++++++
 2 files changed, 13 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index b875d2f27e48..6b6e7b56f972 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -82,6 +82,7 @@ config X86
 	select ARCH_HAS_DEBUG_VM_PGTABLE	if !X86_PAE
 	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select ARCH_HAS_DMA_OPS			if GART_IOMMU || XEN
+	select ARCH_HAS_DPS_ROOT_PARTITION_TYPE_UUID
 	select ARCH_HAS_EARLY_DEBUG		if KGDB
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_EXECMEM_ROX		if X86_64 && STRICT_MODULE_RWX
diff --git a/arch/x86/include/asm/dps_root.h b/arch/x86/include/asm/dps_root.h
new file mode 100644
index 000000000000..7c6ba5519d88
--- /dev/null
+++ b/arch/x86/include/asm/dps_root.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef _ASM_X86_DPS_ROOT_H
+#define _ASM_X86_DPS_ROOT_H
+
+#ifdef CONFIG_X86_64
+#define DPS_ROOT_PARTITION_TYPE_UUID "4f68bce3-e8cd-4db1-96e7-fbcaf984b709"
+#else
+#define DPS_ROOT_PARTITION_TYPE_UUID "44479540-f297-41b2-9af7-d131d5f0458a"
+#endif
+
+#endif /* _ASM_X86_DPS_ROOT_H */

-- 
2.53.0

[PATCH 13/19] block: store GPT partition type UUID

[Vincent Mailhol]

DPS [1] identifies OS partitions by GPT partition type UUID.

Keep that UUID in partition_meta_info and populate it while scanning EFI
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 890128cdea1c..8ce85d21a1f4 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];
 };
 

-- 
2.53.0

[PATCH 14/19] block: add early_lookup_bdev_by_type_uuid()

[Vincent Mailhol]

Add early_lookup_bdev_by_type_uuid() to find the root block device by
its GPT type UUID on the disk containing the active EFI System
Partition.

DPS [1] requires OS partition discovery to be limited to the disk
containing the active EFI System Partition. Reuse the existing block
class lookup and UUID matching callback to identify that disk, then do a
second lookup constrained to it. If the disk contains several partitions
with a matching type UUID, use the first match, following the DPS
discovery rule.

Extend struct uuidcmp with the new disk field so that it can be used in
the new match_dev_by_type_uuid() callback function.

Update devt_from_partuuid() to initialize cmp with a designated
initializer so that the new cmp.disk field is zero-initialized. This is
not strictly needed, but keeps the code cleaner.

[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   | 67 +++++++++++++++++++++++++++++++++++++++++++++++---
 include/linux/blkdev.h |  4 +++
 2 files changed, 68 insertions(+), 3 deletions(-)

diff --git a/block/early-lookup.c b/block/early-lookup.c
index 3fb57f7d2b12..cd10785e70ac 100644
--- a/block/early-lookup.c
+++ b/block/early-lookup.c
@@ -5,10 +5,12 @@
  */
 #include <linux/blkdev.h>
 #include <linux/ctype.h>
+#include <linux/uuid.h>
 
 struct uuidcmp {
 	const char *uuid;
 	int len;
+	struct gendisk *disk;
 };
 
 /**
@@ -45,13 +47,11 @@ 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 +252,67 @@ int __init early_lookup_bdev(const char *name, dev_t *devt)
 	return devt_from_devnum(name, devt);
 }
 
+#ifdef CONFIG_DPS_ROOT_AUTO_DISCOVERY
+/**
+ * match_dev_by_type_uuid - callback for finding a partition using its type UUID
+ * @dev:	device passed in by the caller
+ * @data:	opaque pointer to the desired struct uuidcmp to match
+ *
+ * Returns: 1 if the device matches, and 0 otherwise.
+ */
+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);
+}
+
+/**
+ * early_lookup_bdev_by_type_uuid - look up a partition by its type UUID
+ * @type_uuid:		partition type UUID to search for
+ * @efi_partuuid:	partition UUID identifying the active EFI partition
+ * @devt:		matching dev_t result
+ *
+ * This helper follows the Discoverable Partitions Specification rules. It uses
+ * @efi_partuuid to find the disk containing the active EFI System Partition,
+ * then searches only partitions on that disk for the partition type UUID
+ * specified by @type_uuid.
+ *
+ * Returns: 0 on success or a negative error code on failure.
+ */
+int __init early_lookup_bdev_by_type_uuid(const char *type_uuid,
+					  const char *efi_partuuid, dev_t *devt)
+{
+	struct uuidcmp efi_cmp = {
+		.uuid = efi_partuuid,
+		.len = UUID_STRING_LEN,
+	};
+	struct uuidcmp type_cmp = {
+		.uuid = type_uuid,
+	};
+	struct device *efi_dev;
+	struct device *type_dev;
+
+	efi_dev = class_find_device(&block_class, NULL, &efi_cmp,
+				    &match_dev_by_uuid);
+	if (!efi_dev)
+		return -ENODEV;
+
+	type_cmp.disk = dev_to_disk(efi_dev);
+	type_dev = class_find_device(&block_class, NULL, &type_cmp,
+				     &match_dev_by_type_uuid);
+	put_device(efi_dev);
+	if (!type_dev)
+		return -ENODEV;
+
+	*devt = type_dev->devt;
+	put_device(type_dev);
+	return 0;
+}
+#endif
+
 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 8ce85d21a1f4..c2b7d07c92e7 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1800,6 +1800,10 @@ void sync_bdevs(bool wait);
 void bdev_statx(const struct path *path, struct kstat *stat, u32 request_mask);
 void printk_all_partitions(void);
 int __init early_lookup_bdev(const char *pathname, dev_t *dev);
+#ifdef CONFIG_DPS_ROOT_AUTO_DISCOVERY
+int __init early_lookup_bdev_by_type_uuid(const char *type_uuid,
+					  const char *efi_partuuid, dev_t *dev);
+#endif
 #else
 static inline void invalidate_bdev(struct block_device *bdev)
 {

-- 
2.53.0

[PATCH 15/19] block: store GPT attributes as a raw value

[Vincent Mailhol]

struct _gpt_entry_attributes currently models the GPT partition entry
attributes field with bitfields. This is broken on machines using the
__BIG_ENDIAN_BITFIELD ABI because GPT always stores the attributes on
disk as a 64-bit little-endian.

No current code consumes individual fields from that structure. So just
remove struct _gpt_entry_attributes entirely and replace it by an __le64
value in struct _gpt_entry.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 block/partitions/efi.h | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/block/partitions/efi.h b/block/partitions/efi.h
index 84b9f36b9e47..1f56f93b2804 100644
--- a/block/partitions/efi.h
+++ b/block/partitions/efi.h
@@ -75,18 +75,12 @@ typedef struct _gpt_header {
 	 */
 } __packed gpt_header;
 
-typedef struct _gpt_entry_attributes {
-	u64 required_to_function:1;
-	u64 reserved:47;
-        u64 type_guid_specific:16;
-} __packed gpt_entry_attributes;
-
 typedef struct _gpt_entry {
 	efi_guid_t partition_type_guid;
 	efi_guid_t unique_partition_guid;
 	__le64 starting_lba;
 	__le64 ending_lba;
-	gpt_entry_attributes attributes;
+	__le64 attributes;
 	__le16 partition_name[72/sizeof(__le16)];
 } __packed gpt_entry;
 

-- 
2.53.0

[PATCH 16/19] block: don't discover partition with DPS no-auto GPT attribute

[Vincent Mailhol]

DPS [1] defines GPT attribute bit 63 as no-auto. Partitions with this
bit set must not be used by automatic discovery.

Add the new GPT_ATTRIBUTE_NO_AUTO flag to designate the GPT attribute
bit 63.

Add the new ADDPART_FLAG_NO_AUTO flag and set it when
GPT_ATTRIBUTE_NO_AUTO is set during the partition scan. Then, propagate
it to the new BD_NO_AUTO_DISCOVERY flag.

Finally, add a condition to match_dev_by_type_uuid() to exclude any
partition with that flag from the automatic discovery.

[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/blk.h               | 1 +
 block/early-lookup.c      | 1 +
 block/partitions/core.c   | 2 ++
 block/partitions/efi.c    | 2 ++
 block/partitions/efi.h    | 3 +++
 include/linux/blk_types.h | 1 +
 6 files changed, 10 insertions(+)

diff --git a/block/blk.h b/block/blk.h
index b998a7761faf..14e0f349ff14 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -604,6 +604,7 @@ void blk_free_ext_minor(unsigned int minor);
 #define ADDPART_FLAG_RAID	1
 #define ADDPART_FLAG_WHOLEDISK	2
 #define ADDPART_FLAG_READONLY	4
+#define ADDPART_FLAG_NO_AUTO	8
 int bdev_add_partition(struct gendisk *disk, int partno, sector_t start,
 		sector_t length);
 int bdev_del_partition(struct gendisk *disk, int partno);
diff --git a/block/early-lookup.c b/block/early-lookup.c
index cd10785e70ac..8db0abec141e 100644
--- a/block/early-lookup.c
+++ b/block/early-lookup.c
@@ -266,6 +266,7 @@ 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_test_flag(bdev, BD_NO_AUTO_DISCOVERY) &&
 	       !strcasecmp(cmp->uuid, bdev->bd_meta_info->type_uuid);
 }
 
diff --git a/block/partitions/core.c b/block/partitions/core.c
index 5d5332ce586b..4529ea1d308e 100644
--- a/block/partitions/core.c
+++ b/block/partitions/core.c
@@ -382,6 +382,8 @@ static struct block_device *add_partition(struct gendisk *disk, int partno,
 
 	if (flags & ADDPART_FLAG_READONLY)
 		bdev_set_flag(bdev, BD_READ_ONLY);
+	if (flags & ADDPART_FLAG_NO_AUTO)
+		bdev_set_flag(bdev, BD_NO_AUTO_DISCOVERY);
 
 	/* everything is up and running, commence */
 	err = xa_insert(&disk->part_tbl, partno, bdev, GFP_KERNEL);
diff --git a/block/partitions/efi.c b/block/partitions/efi.c
index 4a3835ed9561..50c21625e256 100644
--- a/block/partitions/efi.c
+++ b/block/partitions/efi.c
@@ -739,6 +739,8 @@ int efi_partition(struct parsed_partitions *state)
 		/* If this is a RAID volume, tell md */
 		if (!efi_guidcmp(ptes[i].partition_type_guid, PARTITION_LINUX_RAID_GUID))
 			state->parts[i + 1].flags = ADDPART_FLAG_RAID;
+		if (le64_to_cpu(ptes[i].attributes) & GPT_ATTRIBUTE_NO_AUTO)
+			state->parts[i + 1].flags |= ADDPART_FLAG_NO_AUTO;
 
 		info = &state->parts[i + 1].info;
 		efi_guid_to_str(&ptes[i].unique_partition_guid, info->uuid);
diff --git a/block/partitions/efi.h b/block/partitions/efi.h
index 1f56f93b2804..fb50edb66e84 100644
--- a/block/partitions/efi.h
+++ b/block/partitions/efi.h
@@ -11,6 +11,7 @@
 #ifndef FS_PART_EFI_H_INCLUDED
 #define FS_PART_EFI_H_INCLUDED
 
+#include <linux/bits.h>
 #include <linux/types.h>
 #include <linux/fs.h>
 #include <linux/kernel.h>
@@ -30,6 +31,8 @@
 #define GPT_HEADER_REVISION_V1 0x00010000
 #define GPT_PRIMARY_PARTITION_TABLE_LBA 1
 
+#define GPT_ATTRIBUTE_NO_AUTO BIT_U64(63)
+
 #define PARTITION_SYSTEM_GUID \
     EFI_GUID( 0xC12A7328, 0xF81F, 0x11d2, \
               0xBA, 0x4B, 0x00, 0xA0, 0xC9, 0x3E, 0xC9, 0x3B) 
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 8808ee76e73c..c6cdc99b0490 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -54,6 +54,7 @@ struct block_device {
 #ifdef CONFIG_FAIL_MAKE_REQUEST
 #define BD_MAKE_IT_FAIL		(1u<<12)
 #endif
+#define BD_NO_AUTO_DISCOVERY	(1u<<13)
 	dev_t			bd_dev;
 	struct address_space	*bd_mapping;	/* page cache */
 

-- 
2.53.0

[PATCH 17/19] init: factor out root device lookup into lookup_root_device()

[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 early_lookup_bdev() out of wait_for_root() into the new
lookup_root_device() so later changes can extend the lookup policy
without duplicating the retry logic.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 init/do_mounts.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 95e0b3a0f711..5fb5aeb88da9 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -402,6 +402,11 @@ void __init mount_root(char *root_device_name)
 	}
 }
 
+static int __init lookup_root_device(char *root_device_name)
+{
+	return early_lookup_bdev(root_device_name, &ROOT_DEV);
+}
+
 /* wait for any asynchronous scanning to complete */
 static void __init wait_for_root(char *root_device_name)
 {
@@ -415,7 +420,7 @@ 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) < 0) {
 		msleep(5);
 		if (root_wait > 0 && ktime_after(ktime_get_raw(), end))
 			break;

-- 
2.53.0

[PATCH 18/19] init: discover root by DPS partition type UUID

[Vincent Mailhol]

When the root= cmdline option is absent, try to discover the root block
device using the architecture's DPS root partition type UUID.

DPS limits root discovery to the disk containing the active EFI System
Partition. Read LoaderDevicePartUUID from the Boot Loader Interface and
pass it to early_lookup_bdev_by_type_uuid() so the block lookup only
considers partitions on that disk.

Print a dedicated wait message while waiting for a discoverable root
partition and emit an informational message when discovery succeeds.

If LoaderDevicePartUUID cannot be read or does not contain a valid UUID,
clear root_wait so the kernel does not keep retrying a discovery path
that cannot succeed.

Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 init/do_mounts.c | 89 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 87 insertions(+), 2 deletions(-)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index 5fb5aeb88da9..20c176945b32 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -10,6 +10,7 @@
 #include <linux/delay.h>
 #include <linux/mount.h>
 #include <linux/device.h>
+#include <linux/efi.h>
 #include <linux/init.h>
 #include <linux/fs.h>
 #include <linux/initrd.h>
@@ -19,6 +20,8 @@
 #include <linux/ramfs.h>
 #include <linux/shmem_fs.h>
 #include <linux/ktime.h>
+#include <linux/ucs2_string.h>
+#include <linux/uuid.h>
 
 #include <linux/nfs_fs.h>
 #include <linux/nfs_fs_sb.h>
@@ -402,9 +405,86 @@ void __init mount_root(char *root_device_name)
 	}
 }
 
+#ifdef CONFIG_DPS_ROOT_AUTO_DISCOVERY
+static char efi_partuuid[EFI_VARIABLE_GUID_LEN + 1] __initdata;
+
+static int __init efi_loader_get_device_part_uuid(char *efi_uuid, size_t size)
+{
+	efi_char16_t efi_uuid_ucs2[EFI_VARIABLE_GUID_LEN + 1] = {};
+	unsigned long efi_uuid_ucs2_size = sizeof(efi_uuid_ucs2);
+	efi_status_t status;
+
+	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE))
+		return -EOPNOTSUPP;
+
+	status = efi.get_variable(L"LoaderDevicePartUUID",
+				  &LINUX_EFI_LOADER_ENTRY_GUID, NULL,
+				  &efi_uuid_ucs2_size, efi_uuid_ucs2);
+	if (status != EFI_SUCCESS)
+		return efi_status_to_err(status);
+
+	if (ucs2_as_utf8((u8 *)efi_uuid, efi_uuid_ucs2, size) != UUID_STRING_LEN ||
+	    !uuid_is_valid(efi_uuid))
+		return -EINVAL;
+
+	return 0;
+}
+
+static int __init lookup_dps_root(dev_t *dev)
+{
+	static const char dps_root_partition_type_uuid[] __initconst =
+		DPS_ROOT_PARTITION_TYPE_UUID;
+	int err;
+
+	err = early_lookup_bdev_by_type_uuid(dps_root_partition_type_uuid,
+					     efi_partuuid, dev);
+	if (!err)
+		pr_info("VFS: Discovered root partition with GPT type UUID %s\n",
+			dps_root_partition_type_uuid);
+
+	return err;
+}
+
+static dev_t __init try_dps_root_discovery(void)
+{
+	dev_t dev;
+	int err;
+
+	err = efi_loader_get_device_part_uuid(efi_partuuid,
+					      sizeof(efi_partuuid));
+	if (err) {
+		pr_err("VFS: Unable to get LoaderDevicePartUUID EFI variable: %pe, skipping root partition discovery\n",
+			ERR_PTR(err));
+		if (root_wait) {
+			pr_err("Disabling rootwait\n");
+			root_wait = 0;
+		}
+		return 0;
+	}
+
+	if (!lookup_dps_root(&dev))
+		return dev;
+
+	return 0;
+}
+#else
+static int __init lookup_dps_root(dev_t *dev)
+{
+	return 0;
+}
+
+static dev_t __init try_dps_root_discovery(void)
+{
+	return 0;
+}
+#endif
+
 static int __init lookup_root_device(char *root_device_name)
 {
-	return early_lookup_bdev(root_device_name, &ROOT_DEV);
+	if (root_device_name[0])
+		return early_lookup_bdev(root_device_name, &ROOT_DEV);
+	else
+		return lookup_dps_root(&ROOT_DEV);
 }
 
 /* wait for any asynchronous scanning to complete */
@@ -415,7 +495,10 @@ 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);
 
@@ -480,6 +563,8 @@ void __init prepare_namespace(void)
 
 	if (saved_root_name[0])
 		ROOT_DEV = parse_root_device(saved_root_name);
+	else
+		ROOT_DEV = try_dps_root_discovery();
 
 	initrd_load();
 

-- 
2.53.0

[PATCH 19/19] docs: document discoverable root partitions

[Vincent Mailhol]

Document the automatic root block device discovery feature.

Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
---
 Documentation/admin-guide/discoverable-root.rst | 33 +++++++++++++++++++++++++
 Documentation/admin-guide/index.rst             |  1 +
 Documentation/admin-guide/kernel-parameters.txt |  5 ++++
 3 files changed, 39 insertions(+)

diff --git a/Documentation/admin-guide/discoverable-root.rst b/Documentation/admin-guide/discoverable-root.rst
new file mode 100644
index 000000000000..9645bf39e405
--- /dev/null
+++ b/Documentation/admin-guide/discoverable-root.rst
@@ -0,0 +1,33 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _discoverable_root:
+
+Discoverable root partitions
+============================
+
+On EFI systems using a supported architecture, 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.
+
+Specifying ``root=`` on the kernel command line takes precedence and entirely
+disables this automatic root partition discovery.
+
+The disk to search is identified by the Boot Loader Interface
+``LoaderDevicePartUUID`` EFI variable. If multiple partitions on that disk match
+the architecture root partition type UUID, the kernel selects the first match in
+block device enumeration order. Systems should not expose multiple eligible root
+partitions unless that ordering is intended.
+
+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.
+
+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 f68bf1cdb53b..c9bfa010883c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6696,6 +6696,11 @@ 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
 

-- 
2.53.0

Re: [PATCH 12/19] x86: define DPS root partition type UUIDs

[Dave Hansen]

On 6/15/26 09:09, Vincent Mailhol wrote:
> +#ifdef CONFIG_X86_64
> +#define DPS_ROOT_PARTITION_TYPE_UUID "4f68bce3-e8cd-4db1-96e7-fbcaf984b709"
> +#else
> +#define DPS_ROOT_PARTITION_TYPE_UUID "44479540-f297-41b2-9af7-d131d5f0458a"
> +#endif

This doesn't make a whole lot of sense to me. 64-bit kernels can run
32-bit userspace just fine.

But this #ifdef as proposed means that only a 32-bit *OR* 64-bit kernel
can auto-discover a given partition.

I kinda think you should just have an array of strings for these things,
maybe glued together with some preprocessor magic. Logically something
like this:

const char* const uuids[] = {
#ifdef CONFIG_ARM64
	"b921b045-1df0-41c3-af44-4c6f280d3fae"
#endif
#ifdef CONFIG_X86_64
	"4f68bce3-e8cd-4db1-96e7-fbcaf984b709",
#endif
#if defined(CONFIG_X86) && defined(CONFIG_COMPAT32)
	"44479540-f297-41b2-9af7-d131d5f0458a",
#endif
...
};

... and then search the array. I honestly don't think you need to
sprinkle UUIDs all over the architectures.

It could probably also be done almost entirely in Kconfig. This could be
in, say block/partitions/Kconfig, or arch/*/Kconfig:

config DPS_ROOT_PARTITION_TYPE_UUID_1
	string
        default "4f68bce3-e8cd-4db1-96e7-fbcaf984b709" if X86_64
	default "b921b045-1df0-41c3-af44-4c6f280d3fae" if ARM64
	...

config DPS_ROOT_PARTITION_TYPE_UUID_2
	string
        default "44479540-f297-41b2-9af7-..." if X86 && COMPAT_32

const char* const uuids[] = {
#ifdef CONFIG_DPS_ROOT_PARTITION_TYPE_UUID_1
	CONFIG_DPS_ROOT_PARTITION_TYPE_UUID_1
#endif
#ifdef CONFIG_DPS_ROOT_PARTITION_TYPE_UUID_2
	CONFIG_DPS_ROOT_PARTITION_TYPE_UUID_2
#endif
...
};

There are a lot of ways to do this. I'm just not a super big fan of the
current proposal.

So, boiling it down:

1. Should more than one UUID be supported per kernel build?
2. Should the UUIDs be defined in arch code or generic code?
3. Kconfig or #ifdefs?

Re: [PATCH 00/19] init: discoverable root partitions, a.k.a. an omittable "root=" cmdline option

[Al Viro]

On Mon, Jun 15, 2026 at 06:08:56PM +0200, Vincent Mailhol wrote:

> Tested with GRUB, which implements the LoaderDevicePartUUID EFI variable
> in its bli module [3]. With this, I was able to boot a kernel with a
> completely empty cmdline and no initrd.
> 
> [1] The Discoverable Partitions Specification (DPS)
> Link: https://uapi-group.org/specifications/specs/discoverable_partitions_specification/
> 
> [2] systemd-gpt-auto-generator
> Link: https://www.freedesktop.org/software/systemd/man/latest/systemd-gpt-auto-generator.html
> 
> [3] GRUB -- §16.2 bli
> Link: https://www.gnu.org/software/grub/manual/grub/html_node/bli_005fmodule.html

So what does that thing, tied to EFI as it is, have to do with architectures where
	* firmware is rather unlike EFI
	* firmware wouldn't know what to do with GPT
	* GRUB is *not* ported to, let alone used
such as, say it, the very first one mentioned at your [1]?

Or is that conditional upon "if anyone wants to design replacement firmware
for those, and if they agree to follow our wishlist"?

Re: [PATCH 12/19] x86: define DPS root partition type UUIDs

[Matthew Wilcox]

On Mon, Jun 15, 2026 at 09:46:41AM -0700, Dave Hansen wrote:
> There are a lot of ways to do this. I'm just not a super big fan of the
> current proposal.
> 
> So, boiling it down:
> 
> 1. Should more than one UUID be supported per kernel build?
> 2. Should the UUIDs be defined in arch code or generic code?
> 3. Kconfig or #ifdefs?

Further questions ... why do this in the kernel?  Seems perfectly
suited to be in initramfs where we can throw away the code after boot.