Kernel Panic with external LG 27gm950b Monitor

[Zahni]

Hi,

I'm using a MacBook Pro M5 (with M5 Pro) and a new LG 5K monitor (model 27gm950b). I tested the monitor (partly to be able to use DSC 2.1 and 165 Hz) with a USB-to-DP adapter (which supports this speed) and also with the included high-speed HDMI cable. Unfortunately, the device crashes at least once a day with a kernel panic. I’ve now switched to the “standard” USB-C connection, which, however, only supports DSC 1.4 and a maximum of 144 Hz. I can’t say yet whether the crash will occur again.

Unfortunately, it also happened today even with the latest macOS update.

Apple’s diagnostics (via CTRL-D) don’t find any errors.

Any ideas?

{"bug_type":"210","timestamp":"2026-08-18 11:05:14.00 +0200","os_version":"macOS 26.6.2 (25G83)","roots_installed":0,"incident_id":"F266D688-E248-4422-B22C-EB058BB9B5AE"}
{
  "crashReporterKey" : "04C69EBC-3A66-7221-07A5-A303F1AE65ED",
  "panicProcessingFlags" : "0x0",
  "product" : "Mac17,9",
  "kernel" : "Darwin Kernel Version 25.6.0: Fri Jul 31 19:19:08 PDT 2026; root:xnu-12377.161.14~5\/RELEASE_ARM64_T6050",
  "socRevision" : "11",
  "panicString" : "panic(cpu 1 caller 0xfffffe00486b885c): DCPEXT1--  - iomfb_mailbox(53)\napt firmware: dual_pipe.c:317 sync_pipe_end_of_config() -- \nRTKit: RTKit-3255.160.4.release - Client: AppleDCP-1041.120.7~2715-t605xdcp.RELEASE\n!UUID: 949b8260-458e-3c0a-b28c-f12b225454f8\nASLR slide: 0x0000000000285000\nTime: 0x00000018183ebf07\n\nFaulting task  53 Call Stack: 0x00000000042aec18 0x00000000042ae988 0x00000000042ae7f4 0x000000000429e9ec 0x000000000441b488 0x000000000445b450 0x000000000445b1d8 0x0000000004457bf4 0x00000000042af12c 0x00000000042a1ec0 000000000000000000\nRTKit Task List:\n   name                    | pri     | stack use | status     | resource |

 

[Bigwaff]

Launch the Console app. Are there any Crash Reports related to this panic? Possible to post contents of the report instead of the limited JSON you posted earlier?

 

[benholt]

DCPEXT1 iomfb_mailbox is almost always a display coprocessor choking on the DSC handshake — the DCP renegotiates the link on every wake and if the pref refresh rate needs DSC but the recovered link doesn't come back the same way, you get that panic class. Couple of things I'd try:

Force the monitor OSD to a fixed refresh below the DSC threshold (120Hz instead of 144/165). If the LG lets you pick "DSC off" separately, do that and see if a day passes without a crash. That isolates whether it's the DSC negotiation vs the higher pixel clock.

Second, if you're on a passive USB-C to DP adapter, that's worth swapping. On my 16" M3 Max I had DCPEXT panics on wake with a cheap passive USB-C-to-DP going to an LG ultrawide until I switched to an active StarTech DP 2.1 adapter — panics stopped that week. Cable ordering matters too, some monitors negotiate cleaner with a certified DP 2.1 cable straight into a TB dock than through USB-C.