Working Around The Intel Core i7 5775C Broadwell Stability Issue On Linux

On Friday I mentioned I was running into stability issues on Linux with the Core i7 5775C, Intel's new socketed, high-end Broadwell processor with Iris 6200 graphics. The issue was quite perplexing, but I seem to have finally figured out a workaround for this problem that seems to be plaguing other early i7-5775C Linux users too.

The aforementioned article covered the kernel panics and issues encountered in trying to test this long-awaited Broadwell desktop CPU under Linux. Back then I wasn't able to succeed in running Linux performance tests as the i7-5775C was too unstable on various versions of the Linux kernel on Ubuntu. I hadn't heard anything at that point from the Intel Linux developers about this issue given the major US holiday weekend. The comments on that earlier article also confirmed at least another Linux user managing to having his hands on this still hard-to-find retail CPU also is experiencing similar issues.

For the past few days I've been trying many combinations to figure out the problem... Disabling the intel_idle driver, disabling Intel P-State, swapping RAM, changing RAM XMP profiles, disabling Enhanced Intel SpeedStep Technology, disabling various ACPI / power management settings, booting the kernel without APIC support, booting different kernel versions, trying a discrete graphics card, booting without kernel mode-setting, and plenty of other combinations in trying to figure out the problem of the Core i7 5775C always running into a kernel panic shortly after boot-up. However, nothing seemed to work and almost seemed like a hardware issue with the processor, as mentioned before, the latest motherboard BIOS was in use and a Core i7 4790K played great on the same setup.

However, today, I at least managed to find a workaround. As shared on Twitter, I got this Core i7 5775C system finally running without any problems!!! After spending almost the entire holiday weekend trying to debug this issue and going through all of the other settings (and several alcoholic drinks in the process to make it bearable), I tried the "CPU OC Fixed Mode" of the ASRock UEFI setup. The CPU OC Fixed mode is designed for running the system at its specified multiplier without ever down-clocking. This setting also disables Turbo Boost and EIST. While I was not overclocking this system. and had already tried disabling EIST and/or Turbo Boost together, this option did the trick!

Since enabling the CPU OC Fixed Mode from the ASRock Z97 motherboard, the Core i7 Broadwell system is now happily running Ubuntu without any kernel panics to speak of. So while I tried disabling Intel P-State and Intel Idle and other CPU clocking related settings for the kernel and BIOS/UEFI, this CPU OC Fixed Mode managed to workaround the issue.

So far so good and benchmarks have been stressing the system for a while. The only downside to enabling this CPU Fixed OC Mode is that the processor will always be forced to run at its highest frequency / state, which leads to greater power consumption while idling along with greater heat being generated... But at least now will be able to run benchmarks under whatever i7-5775C issue around its re-clocking with modern versions of the Linux kernel can be addressed.

I hope to hear more from Intel soon as the official business week begins. Assuming this "workaround" continues working, I'll also start pushing out some preliminary i7-5775C performance figures in the coming hours for those -- like I -- who have been incredibly curious how this Broadwell i7 LGA-1150 CPU with Iris 6200 graphics can perform under Linux.