[PATCH v2 6/6] docs/system/arm/aspeed: Document the VGA display controller

[Jamin Lin <jamin_lin@aspeedtech.com>]

The display controller was listed as missing, which it no longer is. It
does not belong with the rest of the machines' devices though: the host
reaches it as a PCI VGA endpoint, so it is plugged into whichever machine
stands in for the host rather than being part of the BMC machine.

Describe it that way, with the two flavours, the property that sizes the
framebuffer aperture, the option ROM, and how to reach the screen.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
---
 docs/system/arm/aspeed.rst | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/docs/system/arm/aspeed.rst b/docs/system/arm/aspeed.rst
index 2d51ceeb84..0900b4202d 100644
--- a/docs/system/arm/aspeed.rst
+++ b/docs/system/arm/aspeed.rst
@@ -75,7 +75,6 @@ Missing devices
  * Slave GPIO Controller
  * Super I/O Controller
  * PCI-Express 1 Controller
- * Graphic Display Controller
  * MCTP Controller
  * Mailbox Controller
  * Virtual UART
@@ -319,7 +318,6 @@ Missing devices
  * Slave GPIO Controller
  * Super I/O Controller
  * PCI-Express 1 Controller
- * Graphic Display Controller
  * MCTP Controller
  * Mailbox Controller
  * Virtual UART
@@ -512,3 +510,38 @@ To boot a kernel directly from a Zephyr build tree:
 
   $ qemu-system-arm -M ast1030-evb -nographic \
         -kernel zephyr.bin
+
+VGA display controller
+======================
+
+The display controller of the Aspeed SoCs is reached by the host as a PCI
+VGA endpoint, so it is modelled as a device to be plugged into whichever
+machine plays that host rather than as part of the BMC machines above:
+
+- ``ast2600-vga``          Aspeed AST2600 VGA
+- ``ast2700-vga``          Aspeed AST2700 VGA
+
+The size of the framebuffer aperture is set with ``vgamem_mb``, 32 MiB by
+default. The device comes up with the standard VGA BIOS; pass ``romfile``
+to use Aspeed's own option ROM instead, which can be downloaded from
+https://www.aspeedtech.com/support_driver/
+
+To plug one into an ``aarch64`` guest standing in for the host:
+
+.. code-block:: bash
+
+  $ qemu-system-aarch64 \
+        -M virt \
+        -cpu neoverse-n1 -smp 8 -m 4G \
+        -bios edk2-aarch64-code.fd \
+        -device ast2700-vga,vgamem_mb=32,romfile=uefi_arm_2700_vga.rom \
+        -drive file=noble-server-cloudimg-arm64.img,format=qcow2 \
+        -device qemu-xhci,id=xhci \
+        -device usb-kbd,bus=xhci.0 \
+        -device usb-tablet,bus=xhci.0 \
+        -display vnc=0.0.0.0:0
+
+The disk image is an Ubuntu cloud image from
+https://cloud-images.ubuntu.com/. The output goes to the VNC backend on
+display 0, which listens on TCP port 5900, so point a VNC viewer at that
+port to see the screen.
-- 
2.53.0