CachyOS Kernel 7.0.12 Blank Screen — AMD Barcelo DMCUB Firmware Crash

After upgrading to linux-cachyos 7.0.12-1, my laptop boots to a blank screen. Not a kernel panic — the system starts, services load, but the display never initialises.

This is a known AMDGPU regression with the DMCUB (Display Micro Controller) firmware on certain APUs. Here’s the diagnosis and the practical question: is the mainline kernel worth the trouble?

The Failure

My GPU is an AMD Barcelo APU (1002:15e7, Ryzen 5000 series with Vega graphics). On kernel 7.0.12, the journal shows:

Jun 22 06:28:52 cachyos kernel: amdgpu 0000:03:00.0: [drm] Loading DMUB firmware via PSP: version=0x0101002B
Jun 22 06:29:00 cachyos kernel: amdgpu 0000:03:00.0: [drm] *ERROR* dc_dmub_srv_log_diagnostic_data: DMCUB error
Jun 22 06:29:01 cachyos kernel: amdgpu 0000:03:00.0: [drm] *ERROR* dpcd_set_link_settings: core_link_write_dpcd failed

The DMCUB firmware (version 0x0101002B, loaded via the Platform Security Processor) is too old for the kernel 7.x amdgpu driver. The display engine initialises, finds an incompatible firmware interface, and silently fails — blank screen.

The LTS kernel (linux-cachyos-lts 6.18.35-1) boots fine because its amdgpu driver expects the older firmware interface.

The Fix

At the Limine boot menu, highlight the latest kernel entry and press e to edit the command line. Add:

amdgpu.dc=0

This disables the new Display Core (DC) and falls back to the legacy display driver. Trade-offs:

✅ Boots correctly
❌ No DisplayPort Multi-Stream Transport (MST)
❌ No HDMI/DP audio via amdgpu
❌ Potentially lower power efficiency on displays

For a permanent fix, add it to the kernel cmdline permanently:

# Make amdgpu.dc=0 permanent
sudo sed -i 's/root=UUID=[^ ]*/& amdgpu.dc=0/' /boot/limine.conf

Or just boot the LTS kernel until the next kernel update which may carry a newer DMCUB firmware interface.

Can You Fix It Without Rebooting?

No. Kernel command-line parameters are parsed by the kernel at boot time, before userspace starts. You cannot set amdgpu.dc=0 dynamically — it’s a module parameter that controls early GPU initialisation.

You could try unloading and reloading the amdgpu module:

sudo modprobe -r amdgpu && sudo modprobe amdgpu dc=0

But this kills your display session. You’d need to do it from a TTY (Ctrl+Alt+F2), which defeats the purpose if you can’t get to a TTY because the display is blank.

The only practical approach is a one-time boot with amdgpu.dc=0 added at the Limine menu, then making it permanent if it works.

Is the Mainline Kernel Worth It?

Here’s what you get on linux-cachyos 7.0.12-1 vs linux-cachyos-lts 6.18.35-1:

Feature	Mainline (7.0.12)	LTS (6.18.35)
Scheduler	EEVDF + Cachy Sauce (BORE)	EEVDF + Cachy Sauce
LTO	✅ Full LTO	❌
AutoFDO	✅ Profile-guided	❌
Propeller	✅ Propeller PGO	❌
Sched-ext	✅	✅
Newer drivers	✅ Latest amdgpu, etc.	❌ Backported fixes
Stability	⚠️ May have regressions	✅ Battle-tested

The main kernel has LTO (Link Time Optimization), AutoFDO, and Propeller — all profile-guided optimizations that the compiler uses to reorder hot code paths for better cache locality and branch prediction. In practice:

CPU-bound workloads: ~5-15% faster (compilation, encoding, scientific computing)
Desktop responsiveness: BORE scheduler gives snappier interactive feel
Gaming: Better frametimes, lower 1% lows
Daily browsing/dev: Imperceptible difference

For a developer who spends most of their time in a browser, terminal, and editor, the LTS kernel is perfectly adequate. The main kernel matters if you:

Compile code frequently
Run CPU-bound batch jobs
Game on the machine
Need the very latest hardware support (newer WiFi, GPU, NVMe)

Bottom Line

If the blank screen is annoying and you don’t need the main kernel’s optimisations, run the LTS kernel:

# In /boot/limine.conf
default_entry: 2        # Points to linux-cachyos-lts
remember_last_entry: no  # Don't change after manual selection

The LTS kernel is the upstream kernel.org LTS branch with CachyOS’s sauce on top. It’s stable, well-tested, and your GPU works. The main kernel’s LTO/AutoFDO/Propeller gains are real but marginal for everyday desktop use.

If you want the main kernel, add amdgpu.dc=0 and verify it works before making it permanent. Report the DMCUB firmware regression to the CachyOS issue tracker — a future kernel update may ship with a compatible firmware interface.

Hardware: AMD Barcelo APU (1002:15e7). CachyOS April 2026 release. linux-cachyos 7.0.12-1 / linux-cachyos-lts 6.18.35-1. Firmware: linux-firmware 20260519-1.