There are some monitors, in my case Dell U2413, that report having YCbCr support when plugged in over HDMI. My AMD Radeon RX 570 Series video card sees this YCbCr pixel format and then prefers that over the RGB pixel format. The result is that fonts, graphics and other visuals are pixelated and not smooth in Ubuntu.

This actually is not just a Linux problem. With the same monitor hooked up over HDMI in macOS you get the same behavior. In fact an article by John Ruble on the Atomic Object blog called Fixing the External Monitor Color Problem with My 2018 MacBook Pro discusses and attempts to explain how to force RGB using EDID patching.

What follows here is essentially EDID patching on Linux. I’m not going to explain EDID, John did a better job, so I’m just going to cut to the chase and explain what I did to fix it on Ubuntu (19.10 and 20.04).

Solution

All of the articles I could find exploring this topic advocate patching the EDID for the monitor. Unfortunately the macOS solution would not work here. Luckily I found a Reddit post that covered how to get it working.

Install Patched EDID

Install the patched EDID (this example uses the pre-patched EDID attached here) and modify GRUB to use the new EDID.

$ sudo mkdir -p /lib/firmware/edid
$ cd /lib/firmware/edid
$ wget https://gist.github.com/RLovelett/171c374be1ad4f14eb22fe4e271b7eeb/raw/edid.bin

Create initramfs hook to copy the new EDID

$ sudo tee "/etc/initramfs-tools/hooks/edid" > /dev/null <<'EOF'
#!/bin/sh
PREREQ=""
prereqs()
{
    echo "$PREREQ"
}

case $1 in
prereqs)
    prereqs
    exit 0
    ;;
esac

. /usr/share/initramfs-tools/hook-functions
# Begin real processing below this line
mkdir -p "${DESTDIR}/lib/firmware/edid"
cp -a /lib/firmware/edid/edid.bin "${DESTDIR}/lib/firmware/edid/edid.bin"
exit 0
EOF
$ chmod +x /etc/initramfs-tools/hooks/edid
$ sudo update-initramfs -u

Modify the GRUB configuration to use the new EDID

Edit /etc/default/grub and add drm_kms_helper.edid_firmware=edid/edid.bin to the end of GRUB_CMDLINE_LINUX_DEFAULT.

For example:

--- /etc/default/grub	2020-03-19 15:27:24.350222700 -0400
+++ /etc/default/grub	2020-03-19 14:22:58.052179120 -0400
@@ -7,7 +7,7 @@
 GRUB_TIMEOUT_STYLE=hidden
 GRUB_TIMEOUT=0
 GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
-GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
+GRUB_CMDLINE_LINUX_DEFAULT="quiet splash drm_kms_helper.edid_firmware=edid/edid.bin"
 GRUB_CMDLINE_LINUX=""
 
 # Uncomment to enable BadRAM filtering, modify to suit your needs

After saving the changes to /etc/default/grub run sudo grub-mkconfig -o /boot/grub/grub.cfg and reboot.

Creating a Patched EDID

Get Unpatched EDID

Find your current EDID and copy it to the current working directory.

$ sudo find /sys/devices/pci*/*/*/*/*/*HDMI* -name "*edid*" | head -1 | xargs -I{} cp {} edid.bin

Compile wxEDID Utility

$ sudo apt install -y libwxgtk3.0-dev
$ wget https://sourceforge.net/projects/wxedid/files/wxedid-0.0.19.tar.gz/download
$ tar xvf wxedid-0.0.19.tar.gz
$ cd wxedid-0.0.19
$ ./configure --prefix=$HOME
$ make
$ make install

Run the wxEDID Utility

$HOME/bin/wxEDID

Edit/Patch EDID with wxEDID Utility

  1. Open the edid.bin file with wxEDID
  2. Find SPF: Supported features -> vsig_format -> replace 0b01 wih 0b00
  3. Find CHD: CEA-861 header -> change the value of YCbCr420 and YCbCr444 to 0
  4. Find VSD: Vendor Specific Data Block -> Change the value of DC_Y444 to 0
  5. Click Option on the panel-> Recalc Checksum
  6. Save patched EDID and exit

Backup

EDID edit