From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id A25D340B6D9; Mon, 15 Jun 2026 16:11:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781539875; cv=none; b=fr6zRFawMc6cosWiAGKkC4qM+y80oyJwXKPIjyAKfOsomy/bjgJ8BXjo7bIy6urGVvhSigwPNwzCJQssu4NTe0LXRmBmvHBKDShjVIyaIGnqvEM9wm1SetmJmAsuQGvgdZRIHoNapwDVLcGDfHY1WhujyOnqA2FG2Oh8nynqDPs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781539875; c=relaxed/simple; bh=IQn+EJfXSKnbywiAcRWsmiRkucdeNqtI6usBrzEOsOw=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=DuYmWKrnE17F0tH7atJwBrB86rlxxo4v22rnNQfQt7bphKJWfRPxWlyVjQpFdhbWEjngchoANuXx07pDQQGQ1AfkSVrnTBE8cKdmoburkb367mjgXTp9hD8b9Zzk6OXJ265wOGyR9izaRo9UkCF9Z8ofmfUFiK64uSu2vTNq0wE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SxSK0oMz; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="SxSK0oMz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8E6C91F00A3A; Mon, 15 Jun 2026 16:11:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1781539874; bh=47eLSZrGC8dbgWbXEjVWdFhdpbCm8wu5R48ilZUcy3g=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=SxSK0oMz+Inh9En/B/++WRvwwzyt0AQXoGQ2HB53cPc9/UR0pDwAhRJqg9jjVbLu8 JraPb5U0gDCRvdjBx7boKePeXuXbwRRsjupbxVNUACBueTKweQcBRdk4hmPwpQ/TOD rsLkGZwTe7JVw7LP+WoAqsaauzVHHEhtZidWFIR5eJsqv41ZdY59BAsOkEl0Z2MMUT SrxtWJtY9sQgVWppkYz6oT7RF/FVDyQ6IZn64ZJhzJxteoTNOiiYZbTpSUxx1bvtVh NnoHa4e32JB5pdYqMHDiz+hEW1Ws/PXs0nHkMhViborx99OL4E6423Cz9j0hUCBIE6 yN5S7S8MhSa0A== From: Vincent Mailhol Date: Mon, 15 Jun 2026 18:09:15 +0200 Subject: [PATCH 19/19] docs: document discoverable root partitions Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260615-discoverable-root_partitions-v1-19-39c78fac42e2@kernel.org> References: <20260615-discoverable-root_partitions-v1-0-39c78fac42e2@kernel.org> In-Reply-To: <20260615-discoverable-root_partitions-v1-0-39c78fac42e2@kernel.org> To: Jens Axboe , Davidlohr Bueso , Alexander Viro , Christian Brauner , Jan Kara Cc: linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, linux-efi@vger.kernel.org, linux-fsdevel@vger.kernel.org, Vincent Mailhol , Jonathan Corbet , Shuah Khan , linux-doc@vger.kernel.org X-Mailer: b4 0.14.3 X-Developer-Signature: v=1; a=openpgp-sha256; l=3354; i=mailhol@kernel.org; h=from:subject:message-id; bh=IQn+EJfXSKnbywiAcRWsmiRkucdeNqtI6usBrzEOsOw=; b=owGbwMvMwCV2McXO4Xp97WbG02pJDFkGytc0Z+74Kx67ebba8mDmqvsr1++Z+0t7qltorvp7/ TWL1EsrOyayMIhxMViKKbIsK+fkVugo9A479NcSZg4rE8gQaZEGBiBgYeDLTcwrNdIx0jPVNtQz BDJ0jBi4OAVgqufXMfyPldtTxCCrHnrj+sO1taszww99f1bc9DDgSZf7+W8Vhw6+ZGSYdEMvOmV xxYFPn4UlNmwI/xL2vT30yf7gxsV7P3vMzp/ICAA= X-Developer-Key: i=mailhol@kernel.org; a=openpgp; fpr=ED8F700574E67F20E574E8E2AB5FEB886DBB99C2 Document the automatic root block device discovery feature. Cc: Jonathan Corbet Cc: Shuah Khan Cc: linux-doc@vger.kernel.org Signed-off-by: Vincent Mailhol --- 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