From 0aa0a46bb28d5a36601bfabb90a17c392950386d Mon Sep 17 00:00:00 2001
From: Christoph Haag <christoph.haag@collabora.com>
Date: Wed, 3 Feb 2021 22:12:17 +0100
Subject: [PATCH] direct-mode: Add VkDisplayKHR backend documentation
---
direct-mode.md | 14 +++++++-
tegra.md | 86 ++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 99 insertions(+), 1 deletion(-)
create mode 100644 tegra.md
diff --git a/direct-mode.md b/direct-mode.md
index c06abe7..27ebaed 100644
--- a/direct-mode.md
+++ b/direct-mode.md
@@ -91,7 +91,19 @@ The `VK_EXT_acquire_xlib_display`/`VK_EXT_direct_mode_display` Vulkan extensions
For standalone XR systems or systems that don't support these extensions, it is possible to use `VK_KHR_display` and `VK_KHR_display_swapchain` directly on KMS/DRM without running X.Org altogether.
-This is currently not implemented in Monado, but will be supported eventually.
+Monado v21.0.0 introduces such a VkDisplayKHR based backend. Its internal structure is very similar to the nvidia backend.
+
+This backend requires the Vulkan driver to gain exclusive ("drm master") control over the display hardware. This means that an X server or wayland compositor must not be control the "active" vt, it must be a text vt (an X server running "in the background" can still be running in the background). The active vt can be switched with Ctrl+Alt+F1 to F12 or `sudo chvt 1` to 12.
+
+This backend is only enabled when the environment variable `XRT_COMPOSITOR_FORCE_VK_DISPLAY=0` is used. Its value is the nth display connector. Using a too big number like `XRT_COMPOSITOR_FORCE_VK_DISPLAY=100` will print the available display connectors and the names of the connected displays.
+
+Note that X servers may not handle well switching to them while the monado compositor is running. Switching to another text vt should work fine.
+
+Many OpenXR applications will require to be started with an X server - however X servers that are "running in the background" may not accept connections for applications that use the GPU. See [Tegra]({% link tegra.md %}) for an example of how to get it working on the proprietary nvidia driver.
+
+An OpenXR application that does not require an X server is hello_xr with the Vulkan backend: `hello_xr -G Vulkan`.
+
+This implementation has been tested on amd (radv) and the proprietary nvidia driver.
# Extended Mode in Monado (Linux)
diff --git a/tegra.md b/tegra.md
new file mode 100644
index 0000000..343a621
--- /dev/null
+++ b/tegra.md
@@ -0,0 +1,86 @@
+---
+title: "Monado - Running on Tegra"
+layout: main
+---
+
+glslang is not available on Ubuntu 18.04. It can be compiled and installed with the usual procedure:
+
+```
+https://github.com/KhronosGroup/glslang.git
+cd glslang
+cmake . -G Ninja -DCMAKE_INSTALL_PREFIX=/usr -Bbuild
+ninja -C build install
+```
+
+Otherwise the instructions from [Getting Started]({% link getting-started.md %}) work on Tegra for extended mode on X11.
+
+# Direct Mode
+
+The Vulkan driver for nvidia Tegra does not support vulkan based direct mode (VK_EXT_acquire_xlib_display, VK_EXT_direct_mode_display).
+
+[Response from nvidia](https://forums.developer.nvidia.com/t/vk-ext-direct-mode-display-not-present-in-vulkan-1-2-on-jetpack-4-4/161593/2):
+
+> VK_EXT_direct_mode_display is not required for doing direct mode through Vulkan on Jetson. VK_KHR_display and VK_KHR_display_swapchain are supported and those can be used for direct display mode.
+
+See the `Vulkan on KMS/DRM` section in [Direct Mode]({% link direct-mode.md %}) for details of the implementation in Monado.
+
+## Applications that require X11
+
+Graphical applications do not properly connect to a normally running X server.
+
+It is possible to run an offscreen X server that runs graphical applications.
+
+Save a modified copy of the xorg.conf file as
+
+/etc/X11/xorg.conf.offscreen
+```
+# Copyright (c) 2011-2013 NVIDIA CORPORATION. All Rights Reserved.
+
+#
+# This is the minimal configuration necessary to use the Tegra driver.
+# Please refer to the xorg.conf man page for more configuration
+# options provided by the X server, including display-related options
+# provided by RandR 1.2 and higher.
+
+Section "ServerLayout"
+ Identifier "Layout0"
+ Screen 0 "Screen0"
+EndSection
+
+Section "Monitor"
+ Identifier "Monitor0"
+ VendorName "Unknown"
+ ModelName "Unknown"
+ HorizSync 28.0 - 33.0
+ VertRefresh 43.0 - 72.0
+ Option "DPMS"
+EndSection
+
+Section "Screen"
+ Identifier "Screen0"
+ Device "Device0"
+ Monitor "Monitor0"
+ SubSection "Display"
+ Virtual 1920 1080
+ EndSubSection
+ Option "UseDisplayDevice" "none"
+EndSection
+
+Section "Module"
+ Disable "dri"
+ SubSection "extmod"
+ Option "omit xfree86-dga"
+ EndSubSection
+EndSection
+
+Section "Device"
+ Identifier "Tegra0"
+ Driver "nvidia"
+# Allow X server to be started even if no display devices are connected.
+ Option "AllowEmptyInitialConfiguration" "true"
+EndSection
+```
+
+Then run an X server with the command `Xorg -config ./xorg.conf.offscreen :10`. It should drop to a text vt console.
+
+Now it is possible to use both `XRT_COMPOSITOR_FORCE_VK_DISPLAY=0` to let the Monado compositor use the display, and to start an OpenGL application that requires an X server with `DISPLAY=:10`, for example `DISPLAY=:10 hello_xr -G OpenGL`.
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