Semaphores Continue Biting Intel Linux Users

It turns out the semaphore issues for Intel Sandy Bridge Linux users continues to be present and it's resulting in the patch from the recent Intel merge having to be changed at the last minute within the Linux 3.2 kernel.

With the Linux 3.2-rc6 kernel, the use of semaphores were flipped on when the Intel I/O re-mapping is turned off. But it turns out that semaphores on Sandy Bridge hardware is still problematic.

For those that don't recall, semaphore issues borked the Linux 2.6.39 kernel at the last minute for Sandy Bridge owners too. Semaphores also caused problems during the Linux 2.6.38 kernel. With the Linux 3.2 kernel, it was attempted to flip it back on again for Sandy Bridge and Ivy Bridge hardware, but there's at least one report of hardware hanging when this feature is enabled.

This option though can at least be easily toggled using the i915.semaphores=1 kernel module parameter. (The 2.6.39 Phoronix article explains the issue in greater detail.)

Due to this problem, Keith Packard just sent in a Friday afternoon patch to turn off semaphores by default for Sandy. Semaphores for Ivy Bridge is still left on, which hopefully means the support is more reliable there or at least with their current pre-production development systems as the IVB hardware is still a few months from shipping to the public.

The patch changing the default can be found on the Linux kernel mailing list.