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v2/content/posts/2020/linux-amdgpu-pixel-format.md
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title = "Setting the amdgpu HDMI Pixel Format on Linux"
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date = 2020-05-30T08:48:30+10:00
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#[extra]
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#updated = 2020-03-27T21:53:53+11:00
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This week I discovered some details of digital display technology that I was
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previously unaware of: pixel formats. I have two [Dell P2415Q displays][P2415Q]
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connected to [my computer][ryzen9-pc]. One via DisplayPort, the other via HDMI.
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The HDMI connected one was misbehaving and showing a dull picture. It turned
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out I needed to force the HDMI port of my RX560 graphics card to use RGB output
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instead of YcBCr. However, the `amdgpu` driver does not expose a means to do
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this. So, I used an EDID hack to make it look like the display only supported
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RGB.
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**tl;dr** You can't easily configure the pixel format of the Linux `amdgpu`
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driver but you can hack the EDID of your display so the driver chooses RGB.
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[Jump to the instructions](#the-fix).
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Previously I had one display at work and one at home, both using DisplayPort
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and all was well. However, when I started working from home at the start of
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2020 (pre-pandemic) the HDMI connected one has always been a bit flakey. The
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screen would go blank for second, then come back on. I tried 3 different HDMI
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cables each more premium (and hopefully shielded than the last) without
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success.
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This week the frustration boiled over and I vented to some friends. I was on
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the brink of just rage buying a new graphics card with multiple DisplayPorts,
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since I'd never had any trouble with that connection. I received one suggestion
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to swap the cables between the two, to rule out a fault with the HDMI connected
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display. I was quite confident the display was ok but it was a sensible thing
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to try before dropping cash on a new graphics card. So I swapped the cables
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over.
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After performing [the magical incantation to enable HDMI 2.0][incantation] and
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get 4K 60Hz on the newly HDMI connected display I immediately noticed lag. I
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even captured it in a slow motion video on my phone to prove I wasn't going
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crazy. Despite `xrandr` reporting a 60Hz connection it seemed as though it was
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updating at less than that. This led me to compare the menus of the two
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displays. It was here I noticed that good one reported an input colour format
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of RGB, than the other [YPbPr].
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This led to more reading about pixel formats in digital displays — a thing I
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was not previously aware of. Turns out that ports like HDMI support multiple
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ways of encoding the pixel data, some sacrificing dynamic range for lower
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bandwidth. I found this article particularly helpful,
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[DisplayPort vs. HDMI: Which Is Better For Gaming?](https://www.tomshardware.com/features/displayport-vs-hdmi-better-for-gaming).
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My hypothesis at this point was that the lag was being introduced by my display
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converting the YPbPr input to its native RGB. So, I looked for a way to change
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the pixel format output from the HDMI port of my RX560 graphics card. Turns out
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this is super easy on Windows, but [the `amdgpu` driver on Linux does not
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support changing it][amdgpu-bug].
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In trying various suggestions in that bug report I rebooted a few times and the
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lag mysteriously went away but the pixel format remained the same. At this
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point I noticed the display had a grey cast to it especially on areas of white.
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This had been present on the other display when it was connected via HDMI too
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but I just put it down to being a couple of years older. With my new pixel
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format knowledge in hand I knew this was was the source of lack of brightness.
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So, I was still determined to find a way to force the HDMI output to RGB.
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### The Fix
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It was at this point I found [this Reddit post][reddit-pixel-format] with a
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fairly terrible hack: Copy the [EDID] of the display and modify it to make it
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seem like the display only supports RGB. The `amdgpu` driver then chooses that
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format instead. Amazingly enough it worked! I also haven't experienced the
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screen blanking issue since swapping cables. I can't say for sure if that is
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fixed but the HDMI cable is now further away from interference from my Wi-Fi
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router, so perhaps that helped.
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The following are the steps I took on Arch Linux to use a modified EDID:
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1. Install [wxEDID from the AUR][wxEDID].
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1. Make a copy of the EDID data: `cp /sys/devices/pci0000:00/0000:00:03.1/0000:09:00.0/drm/card0/card0-HDMI-A-1/edid Documents/edid.bin`
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1. Edit `edid.bin` with wxEDID and change these values:
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1. Find SPF: Supported features -> vsig_format -> replace 0b01 wih 0b00
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1. Find CHD: CEA-861 header -> change the value of YCbCr420 and YCbCr444 to 0
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1. Recalculate the checksum: Options > Recalc Checksum.
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1. Save the file.
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**Note:** I had to attempt editing the file a few times as wxEDID kept
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segfaulting. Eventually it saved without crashing though.
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Now we need to get the kernel to use the modified file:
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1. `sudo mkdir /lib/firmware/edid`
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1. `sudo mv edid.bin /lib/firmware/edid/edid.bin`
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1. Edit the kernel command line. I use [systemd-boot], so I edited `/boot/loader/entries/arch.conf` and added `drm_kms_helper.edid_firmware=edid/edid.bin` to the command line, making the full file look like this:
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title Arch Linux
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linux /vmlinuz-linux
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initrd /amd-ucode.img
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initrd /initramfs-linux.img
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options root=PARTUUID=2f693946-c278-ed44-8ba2-67b07c3b6074 resume=UUID=524c0604-c307-4106-97e4-1b9799baa7d5 resume_offset=4564992 drm_kms_helper.edid_firmware=edid/edid.bin rw
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1. Regenerate the initial RAM disk: `sudo mkinitcpio -p linux`
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1. Reboot
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After rebooting the display confirmed it was now using RGB and visually it was
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looking much brighter! 🤞 the display blanking issue remains fixed as well.
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[P2415Q]: https://www.dell.com/en-au/shop/dell-24-ultra-hd-4k-monitor-p2415q/apd/210-anfp/monitors-monitor-accessories
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[ryzen9-pc]: https://bitcannon.net/page/ryzen9-pc/
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[incantation]: https://www.dell.com/support/article/en-au/sln306595/setting-up-the-p2415q-p2715q-monitors-with-hdmi-2-0-that-support-4k-x-2k-60hz?lang=en
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[amdgpu-bug]: https://gitlab.freedesktop.org/drm/amd/-/issues/476
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[YPbPr]: https://en.wikipedia.org/wiki/YPbPr
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[reddit-pixel-format]: https://www.reddit.com/r/Amd/comments/8bwul6/how_to_switch_the_pixel_format_for_amdgpu_on_linux/dxaef7a/
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[EDID]: https://en.wikipedia.org/wiki/Extended_Display_Identification_Data
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[wxEDID]: https://aur.archlinux.org/packages/wxedid
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[systemd-boot]: https://wiki.archlinux.org/index.php/Systemd-boot
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