9020sff/mt: say how to get graphics cards working

Signed-off-by: Leah Rowe <info@minifree.org>

site/docs/hardware/dell9020.md

@@ -91,6 +91,24 @@ Kukri's patch is here:
 This patch, at this revision (patchset 31), is what Libreboot uses for this
 port.
 
+Graphics cards
+--------------
+
+On current lbmk master, graphics cards *do* work. The option to hide PEG
+devices from MRC was disabled. Now when you insert a graphics card, the
+onboard Intel GPU is disabled and the graphics card is used instead.
+
+NOTE: You must set `iommu=off` in your linux cmdline. For instance, set this
+in `/etc/default/grub` and regenerate your GRUB config. With the IOMMU turned
+off, graphics cards work fine. Otherwise, with IOMMU turned on, you might get
+this error:
+
+<https://av.vimuser.org/error.jpg>
+
+NOTE2: You *can* use the onboard Intel GPU (without a graphics card inserted),
+and leave IOMMU turned on. You only need to disable the IOMMU when a graphics
+card is inserted. This bug will be investigated and fixed at a later date.
+
 7020 compatibility
 ------------------