Workaround to get HDMI working

Xen doesn't map any range of memory not explicitly specified in the device
tree. Unfortunately, the framebuffer memory region is not specified in the
device tree, so Xen doesn't map that region. Accesses to the framebuffer memory
were being trapped by Xen, leading to a Linux kernel panic.

Work around this by introducing a dummy device to get Xen to map the frame
buffer region.

Signed-off-by: Stewart Hildebrand <[email protected]>

README.md

@@ -18,12 +18,13 @@ When the script is finished, flash to SD card with (for example):
 Xen will print messages to the UART.
 Log in with username `dornerworks` password `dornerworks`.
 
+To install a graphical desktop, expand the rootfs partition, ensure that you have an internet connection, run the command `sudo apt install ubuntu-desktop`, and reboot.
+
 This script is a little bit like [https://github.com/mirage/xen-arm-builder](https://github.com/mirage/xen-arm-builder) and [https://github.com/RPi-Distro/pi-gen](https://github.com/RPi-Distro/pi-gen) but for Xen+Ubuntu instead of Raspbian.
 More info about Ubuntu Base is available here [https://wiki.ubuntu.com/Base](https://wiki.ubuntu.com/Base).
 
 ## Limitations
 
-* HDMI not working
 * Raspberry Pi kernel not suitable for domU
 * aux spi1 and aux spi2 are disabled
 

patches/linux/0001-Add-Xen-overlay-for-the-Pi-4.patch

@@ -4,10 +4,10 @@ Subject: [PATCH] Add Xen overlay for the Pi 4
 
 Signed-off-by: Corey Minyard <[email protected]>
 ---
- arch/arm/boot/dts/overlays/Makefile           |  4 +++-
- .../boot/dts/overlays/pi4-32-xen-overlay.dts  | 21 +++++++++++++++++++
- .../boot/dts/overlays/pi4-64-xen-overlay.dts  | 21 +++++++++++++++++++
- 3 files changed, 45 insertions(+), 1 deletion(-)
+ arch/arm/boot/dts/overlays/Makefile           |  4 +-
+ .../boot/dts/overlays/pi4-32-xen-overlay.dts  | 21 ++++++++++
+ .../boot/dts/overlays/pi4-64-xen-overlay.dts  | 39 +++++++++++++++++++
+ 3 files changed, 63 insertions(+), 1 deletion(-)
  create mode 100644 arch/arm/boot/dts/overlays/pi4-32-xen-overlay.dts
  create mode 100644 arch/arm/boot/dts/overlays/pi4-64-xen-overlay.dts
 
@@ -55,10 +55,10 @@ index 000000000000..47afa6ac24b7
 +};
 diff --git a/arch/arm/boot/dts/overlays/pi4-64-xen-overlay.dts b/arch/arm/boot/dts/overlays/pi4-64-xen-overlay.dts
 new file mode 100644
-index 000000000000..362e238576bf
+index 000000000000..6c499a831db6
 --- /dev/null
 +++ b/arch/arm/boot/dts/overlays/pi4-64-xen-overlay.dts
-@@ -0,0 +1,21 @@
+@@ -0,0 +1,39 @@
 +// Xen configuration for Pi 4
 +/dts-v1/;
 +/plugin/;
@@ -79,6 +79,24 @@ index 000000000000..362e238576bf
 +            };
 +        };
 +    };
++
++    /* Introduce a dummy device node to make Xen map the framebuffer */
++    fragment@1 {
++        target-path = "/";
++        __overlay__ {
++            fb_bus {
++                compatible = "simple-bus";
++                ranges;
++                #address-cells = <2>;
++                #size-cells = <1>;
++                fb_mem {
++                    compatible = "dummy";
++                    status = "okay";
++                    reg = <0x0 0x3b400000 0x04c00000>;
++                };
++            };
++        };
++    };
 +};
 -- 
 2.34.0