linux-kernel - [PATCH] um: Document dtb command line option

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Message-Id: <20220104104457.282828-1-anton.ivanov@cambridgegreys.com>
Date:   Tue,  4 Jan 2022 10:44:57 +0000
From:   anton.ivanov@...bridgegreys.com
To:     linux-um@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
        kernel@...s.com, robh+dt@...nel.org, jdike@...toit.com,
        johannes@...solutions.net, vincent.whitchurch@...s.com,
        richard@....at, linux-doc@...r.kernel.org,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>
Subject: [PATCH] um: Document dtb command line option

From: Anton Ivanov <anton.ivanov@...bridgegreys.com>

Add documentation for the dtb command line option and the
ability to load/parse device trees.

Signed-off-by: Anton Ivanov <anton.ivanov@...bridgegreys.com>
---
 .../virt/uml/user_mode_linux_howto_v2.rst     | 20 +++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/Documentation/virt/uml/user_mode_linux_howto_v2.rst b/Documentation/virt/uml/user_mode_linux_howto_v2.rst
index 2cafd3c3c6cb..81986fe014d4 100644
--- a/Documentation/virt/uml/user_mode_linux_howto_v2.rst
+++ b/Documentation/virt/uml/user_mode_linux_howto_v2.rst
@@ -1189,6 +1189,26 @@ E.g. ``os_close_file()`` is just a wrapper around ``close()``
 which ensures that the userspace function close does not clash
 with similarly named function(s) in the kernel part.
 
+Using UML as a Test Platform
+============================
+
+UML is an excellent test platform for device driver development. As
+with most things UML, "some user assembly may be required". It is
+up to the user to build their emulation environment. UML at present
+provides only the kernel infrastructure.
+
+Part of this infrastructure is the ability to load and parse fdt
+device tree blobs as used in Arm or Open Firmware platforms. These
+are supplied as an optional extra argument to the kernel command
+line::
+
+    dtb=filename
+
+The device tree is loaded and parsed at boottime and is accessible by
+drivers which query it. At this moment in time this facility is
+intended solely for development purposes. UML's own devices do not
+query the device tree.
+
 Security Considerations
 -----------------------
 
-- 
2.30.2