Ubuntu 20.04使用5.15.0-74以上内核时开机黑屏求助（iMac 2011中配+Mobility Radeon HD 4670显卡）

各位大佬好，我遇到了一个棘手的问题，想请大家帮忙排查下：

我用的是21英寸mid 2011款iMac，配置为Core i3 540处理器、16GB内存，显卡是Mobility Radeon HD 4670，安装的是Ubuntu 20.04系统。现在只要使用5.15.0-74及以上版本的内核，开机就会出现黑屏情况，目前只能把内核锁定在5.15.0-73-generic才能正常使用。

具体症状是：开机完成磁盘检查后，屏幕直接陷入黑屏状态，但系统其实是正常启动的——我可以从其他机器通过SSH连接到这台iMac。

我分别提取了最新的5.15.0-84-generic内核和能正常使用的5.15.0-73-generic内核下，执行journalctl -b --no-pager | grep radeon的输出，发现新版本内核里systemd会停止屏幕背光服务，这应该是问题的关键所在？

5.15.0-84-generic内核日志输出 #

Sep 24 21:33:13 ubuntu2004-imac kernel: [drm] radeon kernel modesetting enabled.
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: vgaarb: deactivate vga console
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: No more image in the PCI ROM
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: VRAM: 256M 0x0000000000000000 - 0x000000000FFFFFFF (256M used)
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: GTT: 1024M 0x0000000010000000 - 0x000000004FFFFFFF
Sep 24 21:33:13 ubuntu2004-imac kernel: [drm] radeon: 256M of VRAM memory ready
Sep 24 21:33:13 ubuntu2004-imac kernel: [drm] radeon: 1024M of GTT memory ready.
Sep 24 21:33:13 ubuntu2004-imac kernel: [drm] radeon: dpm initialized
Sep 24 21:33:13 ubuntu2004-imac kernel: [drm] enabling PCIE gen 2 link speeds, disable with radeon.pcie_gen2=0
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: WB enabled
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: fence driver on ring 0 use gpu addr 0x0000000010000c00
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: fence driver on ring 3 use gpu addr 0x0000000010000c0c
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: fence driver on ring 5 use gpu addr 0x000000000005c598
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: radeon: MSI limited to 32-bit
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: radeon: using MSI.
Sep 24 21:33:13 ubuntu2004-imac kernel: [drm] radeon: irq initialized.
Sep 24 21:33:13 ubuntu2004-imac kernel: [drm] radeon atom DIG backlight initialized
Sep 24 21:33:13 ubuntu2004-imac kernel: fbcon: radeondrmfb (fb0) is primary device
Sep 24 21:33:13 ubuntu2004-imac kernel: radeon 0000:01:00.0: [drm] fb0: radeondrmfb frame buffer device
Sep 24 21:33:13 ubuntu2004-imac kernel: [drm] Initialized radeon 2.50.0 20080528 for 0000:01:00.0 on minor 0
Sep 24 21:33:06 ubuntu2004-imac systemd[1]: Starting Load/Save Screen Backlight Brightness of backlight:radeon_bl0...
Sep 24 21:33:06 ubuntu2004-imac systemd[1]: Finished Load/Save Screen Backlight Brightness of backlight:radeon_bl0.
Sep 24 21:34:05 ubuntu2004-imac systemd[1]: Stopping Load/Save Screen Backlight Brightness of backlight:radeon_bl0...
Sep 24 21:34:05 ubuntu2004-imac systemd[1]: systemd-backlight@backlight:radeon_bl0.service: Succeeded.
Sep 24 21:34:05 ubuntu2004-imac systemd[1]: Stopped Load/Save Screen Backlight Brightness of backlight:radeon_bl0.

5.15.0-73-generic内核日志输出 #

Sep 24 21:35:27 ubuntu2004-imac kernel: [drm] radeon kernel modesetting enabled.
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: vgaarb: deactivate vga console
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: No more image in the PCI ROM
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: VRAM: 256M 0x0000000000000000 - 0x000000000FFFFFFF (256M used)
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: GTT: 1024M 0x0000000010000000 - 0x000000004FFFFFFF
Sep 24 21:35:27 ubuntu2004-imac kernel: [drm] radeon: 256M of VRAM memory ready
Sep 24 21:35:27 ubuntu2004-imac kernel: [drm] radeon: 1024M of GTT memory ready.
Sep 24 21:35:27 ubuntu2004-imac kernel: [drm] radeon: dpm initialized
Sep 24 21:35:27 ubuntu2004-imac kernel: [drm] enabling PCIE gen 2 link speeds, disable with radeon.pcie_gen2=0
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: WB enabled
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: fence driver on ring 0 use gpu addr 0x0000000010000c00
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: fence driver on ring 3 use gpu addr 0x0000000010000c0c
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: fence driver on ring 5 use gpu addr 0x000000000005c598
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: radeon: MSI limited to 32-bit
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: radeon: using MSI.
Sep 24 21:35:27 ubuntu2004-imac kernel: [drm] radeon: irq initialized.
Sep 24 21:35:27 ubuntu2004-imac kernel: [drm] radeon atom DIG backlight initialized
Sep 24 21:35:27 ubuntu2004-imac kernel: fbcon: radeondrmfb (fb0) is primary device
Sep 24 21:35:27 ubuntu2004-imac kernel: radeon 0000:01:00.0: [drm] fb0: radeondrmfb frame buffer device
Sep 24 21:35:27 ubuntu2004-imac kernel: [drm] Initialized radeon 2.50.0 20080528 for 0000:01:00.0 on minor 0
Sep 24 21:35:17 ubuntu2004-imac systemd[1]: Starting Load/Save Screen Backlight Brightness of backlight:radeon_bl0...
Sep 24 21:35:17 ubuntu2004-imac systemd[1]: Finished Load/Save Screen Backlight Brightness of backlight:radeon_bl0.
Sep 24 21:36:11 ubuntu2004-imac /usr/lib/gdm3/gdm-x-session[2595]:         loading driver: radeon
Sep 24 21:36:11 ubuntu2004-imac /usr/lib/gdm3/gdm-x-session[2595]: (==) Matched radeon as autoconfigured driver 0
Sep 24 21:36:11 ubuntu2004-imac /usr/lib/gdm3/gdm-x-session[2595]: (II) LoadModule: "radeon"
Sep 24 21:36:11 ubuntu2004-imac /usr/lib/gdm3/gdm-x-session[2595]: (II) Loading /usr/lib/xorg/modules/drivers/radeon_drv.so
Sep 24 21:36:11 ubuntu2004-imac /usr/lib/gdm3/gdm-x-session[2595]: (II) Module radeon: vendor="X.Org Foundation"

备注：内容来源于stack exchange，提问作者hakanekat