[Bug 102985] HDMI 2.0 4k EDIDs not available (Radeon RX Vega 56)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=102985

            Bug ID: 102985
           Summary: HDMI 2.0 4k EDIDs not available (Radeon RX Vega 56)
           Product: DRI
           Version: DRI git
          Hardware: x86-64 (AMD64)
                OS: Linux (All)
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/AMDgpu
          Assignee: dri-devel at lists.freedesktop.org
          Reporter: bicom at invergo.net

Created attachment 134476
  --> https://bugs.freedesktop.org/attachment.cgi?id=134476&action=edit
DRM/AMDGPU logs (journalctl)

I have a Radeon RX Vega 56 connected to a Sony 4K TV (kd-49x8305c) via a
"Premium" (certified) HDMI 2.0 cable. The TV's HDMI input is set to "Enhanced
Format". Thus, on the hardware side, everything's ready to go for 4k at 60Hz. On
the software side, I'm running Arch Linux with mesa-git (rev. 066d1dc951),
llvm-svn (rev. 313470), and Linux branch amd-staging-drm-next (4.13 rev.
dcae401d1113).  I'm running Gnome and I've tested both Wayland and Xorg.

While other functionality such as rendering and acceleration work with this
setup, I have been unable to access any 4k resolutions.

As you'll see in the attached logs, the maximum resolution detected from the
TV's EDID is 1920x1080. However, using the program edid-decode (output
attached), one can see that the EDID clearly advertises 4k modes, however,
they're in the Extension block at VIC indexes higher than 64.

Until recently, 64 was the highest supported by drivers/gpu/drm/drm_edid.c in
the kernel.  However, that has been fixed:

https://github.com/torvalds/linux/commit/8ec6e0755565192b328059b64f982adabbecda78#diff-f1b0089a97dd0195eb24305ab63a79e3

That commit has been merged into the amd-staging-drm-next kernel (but not
amd-staging), and the revision that I have built and installed includes that
code.  Nevertheless, I haven't had any luck in getting the computer to
recognize any of those extended modes.

I have tried adding the mode manually via xrandr, however when I try to switch
to that mode, I get the error "xrandr: Configure crtc 0 failed". Trying to set
it in a conf file in /etc/X11/xorg.conf.d also didn't work (I no longer have a
configuration file in there). I've also tried forcing it from the kernel
command line (video=3840x2160 at 60) to no avail.  Still, all of those seem like
workarounds when I think the problem is pretty clearly that the VIC 65+ modes
are not detected.

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