j713 (MacBook Air M4): appledrm idle cache freezes a console-only screen (fbcon never swaps) #409

On a J713 whose greeter is a text UI on the framebuffer console (tuigreet on tty1, through appledrm's fbdev emulation), the panel freezes about a second after KMS takes over. The rest of the system keeps running: keyboard input reaches the kernel and the greeter, and writes to /dev/tty1 complete, but nothing new is shown.

Cause

idle_cache_ms (default 1000) has the DCP cache the last frame once nothing has flipped for that long. It was added in aurora-silicon/linux a150a02 "drm/apple: m4: Cache a static frame while nothing is flipping", merged in aurora-silicon/linux#830.

fbcon draws straight into the scanout buffer and never submits a swap, so the DCP never leaves the cached frame. With appledrm.enable_verbose_logging=Y, a console write produces no appledrm or DCP activity at all.

Setting idle_cache_ms=0 at runtime doesn't help on its own, because it "applies from the next swap" and none comes. Forcing a swap restores a live screen straight away:

echo 0 > /sys/module/appledrm/parameters/idle_cache_ms
echo 1 > /sys/class/graphics/fb0/blank; echo 0 > /sys/class/graphics/fb0/blank

A compositor that flips regularly is presumably unaffected. This mainly hits console-only screens: text greeters, VTs, a graphical session that fails and falls back to fbcon, and early boot.

Workaround

appledrm.idle_cache_ms=0 on the kernel command line.

Possible fixes

• Don't engage the idle cache while the fbdev client (fbcon) owns the CRTC.
• Or have fbdev damage wake the DCP from the cached frame, for example a dirty or flush path that schedules a swap when the buffer changes.

Environment

• Mac: J713 (MacBook Air 13-inch, M4, 2025), Mac16,12.
• Firmware: iBoot1 mBoot-20457.41.3, iBoot2 (OS) mBoot-18000.161.10; macOS stub 26.6.2 (25G83).
• Loader: Aura 1.0.0b9 Stage 1 and Stage 2; U-Boot aurora-silicon/u-boot aurora-wip e1ef8b636f.
• Kernel: aurora-silicon/linux integration/aurora f5fa539 plus the J713 DCP physical-power series (drm/apple: m4: Physically power the J713 display domains down while blanked aurora-silicon/linux#1059), at EL2 in the CTXR window.
• appledrm.power_gate_ms=0, so the DCP power-down path is not involved.

Found while moving this Air to the monitorless b9 boot for suspend-to-RAM work.

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