[PATCH v2 8/8] docs: Add device tree overlay documentation

[Henry Wang <xin.wang2@xxxxxxx>]

From: Vikram Garhwal <fnu.vikram@xxxxxxxxxx>

Signed-off-by: Vikram Garhwal <fnu.vikram@xxxxxxxxxx>
Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
Signed-off-by: Henry Wang <xin.wang2@xxxxxxx>
---
v2:
- Update the content based on the changes in this version.
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 docs/misc/arm/overlay.txt | 99 +++++++++++++++++++++++++++++++++++++++
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 create mode 100644 docs/misc/arm/overlay.txt

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+# Device Tree Overlays support in Xen
+
+Xen now supports dynamic device assignment to running domains,
+i.e. adding/removing nodes (using .dtbo) to/from Xen device tree, and
+attaching/detaching them to/from a running domain with given $domid.
+
+Dynamic node assignment works in two steps:
+
+## Add/Remove device tree overlay to/from Xen device tree
+
+1. Xen tools check the dtbo given and parse all other user provided arguments
+2. Xen tools pass the dtbo to Xen hypervisor via hypercall.
+3. Xen hypervisor applies/removes the dtbo to/from Xen device tree.
+
+## Attach/Detach device from the DT overlay to/from domain
+
+1. Xen tools check the dtbo given and parse all other user provided arguments
+2. Xen tools pass the dtbo to Xen hypervisor via hypercall.
+3. Xen hypervisor attach/detach the device to/from the user-provided $domid by
+   mapping/unmapping node resources in the DT overlay.
+
+# Examples
+
+Here are a few examples on how to use it.
+
+## Dom0 device add
+
+For assigning a device tree overlay to Dom0, user should firstly properly
+prepare the DT overlay. More information about device tree overlays can be
+found in [1]. Then, in Dom0, enter the following:
+
+    (dom0) xl dt-overlay add overlay.dtbo
+
+This will allocate the devices mentioned in overlay.dtbo to Xen device tree.
+
+To assign the newly added device from the dtbo to Dom0:
+
+    (dom0) xl dt-overlay attach overlay.dtbo 0
+
+Next, if the user wants to add the same device tree overlay to dom0
+Linux, execute the following:
+
+    (dom0) mkdir -p /sys/kernel/config/device-tree/overlays/new_overlay
+    (dom0) cat overlay.dtbo > 
/sys/kernel/config/device-tree/overlays/new_overlay/dtbo
+
+Finally if needed, the relevant Linux kernel drive can be loaded using:
+
+    (dom0) modprobe module_name.ko
+
+## Dom0 device remove
+
+For removing the device from Dom0, first detach the device from Dom0:
+
+    (dom0) xl dt-overlay detach overlay.dtbo 0
+
+NOTE: The user is expected to unload any Linux kernel modules which
+might be accessing the devices in overlay.dtbo before detach the device.
+Detaching devices without unloading the modules might result in a crash.
+
+Then remove the overlay from Xen device tree:
+
+    (dom0) xl dt-overlay remove overlay.dtbo
+
+## DomU device add/remove
+
+All the nodes in dtbo will be assigned to a domain; the user will need
+to prepare the dtb for the domU. For example, the `interrupt-parent` property
+of the DomU overlay should be changed to the Xen hardcoded value `0xfde8`.
+Below assumes the properly written DomU dtbo is `overlay_domu.dtbo`.
+
+User will need to create the DomU with below properties properly configured
+in the xl config file:
+- `iomem`
+- `passthrough` (if IOMMU is needed)
+
+User will also need to modprobe the relevant drivers.
+
+Example for domU device add:
+
+    (dom0) xl dt-overlay add overlay.dtbo            # If not executed before
+    (dom0) xl dt-overlay attach overlay.dtbo $domid
+    (dom0) xl console $domid                         # To access $domid console
+
+Next, if the user needs to modify/prepare the overlay.dtbo suitable for
+the domU:
+
+    (domU) mkdir -p /sys/kernel/config/device-tree/overlays/new_overlay
+    (domU) cat overlay_domu.dtbo > 
/sys/kernel/config/device-tree/overlays/new_overlay/dtbo
+
+Finally, if needed, the relevant Linux kernel drive can be probed:
+
+    (domU) modprobe module_name.ko
+
+Example for domU overlay remove:
+
+    (dom0) xl dt-overlay detach overlay.dtbo $domid
+    (dom0) xl dt-overlay remove overlay.dtbo
+
+[1] https://www.kernel.org/doc/Documentation/devicetree/overlay-notes.txt
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