i915-related and general system freezes with specific kernel config// IOMMU

[Mihai Moldovan]

Hi Daniel, David and everyone else,

I'm experiencing system freezes on a box using the vanilla 3.7.2 (actually down
to 3.2 or something) kernel with a custom configuration.

There are two problems:

  [*] related to i915 with modeset enabled; upon loading the kernel module with
modeset=1, the box will instantly freeze.
  [*] seemingly unrelated to i915; the box will randomly freeze without any
clear indication of why and moreover no apparent trigger.

After months, nay, years of being "locked" into 3.0.2 for the random freezes and
i915 problems, I started playing with the kernel again and out of sheer
desperation installed the current debian testing kernel, based on 3.2.35.

From what I could see, it worked fine... no more crashes, neither when loading
i915, nor randomly after some time (well, at least not for a day.)

This time out of frustration, I ripped the config file used by debian to build
the kernel out of its deb package and rebuilt my (almost[1]) vanilla 3.7.2
kernel with this configuration exactly, updated via the oldconfig target and
changed to include AHCI, RAID and SCSI drivers statically, so that I wouldn't
need some initramfs to boot my system ... and ... with this config, I am not
experiencing any i915 problems nor system freezes?!

I then tried to spot any "obvious" differences between the two config files and
to "approximate" my config file to the debian config.

Comparing the dmesg output from 3.7.2 built with the slightly modified debian
config to my 3.7.2 built with my config, I came across IOMMU entries which
differed. My kernel config enables Intel IOMMU by default, while the debian
config doesn't.

Looking up IOMMU stuff in Documentation/, I found out that IOMMU *may* have bugs
with the internal graphics card and there is an option called
intel_iommu=igfx_off to disable IOMMU remapping for the integrated graphics card...

I tried booting "my" kernel with intel_iommu=igfx_off and lo and behold, no more
crashes when loading i915 with modeset enabled! Yay... but anyway, that's
definitely a kernel bug.

Next, regarding the random freezes... so did the kernel booted with
intel_iommu=igfx_off. It seems the random freeze issue is kind of decoupled of
the graphics issue.

Testing further, I rebooted using iommu=off and intel_iommu=off. So far, I had
no random crashes, but the system uptime of XXXXREPLACEMEXXXX minutes is too
small to draw conclusions yet.

Anyway, booting with both options made my USB ports unusable. Also, my PCIe and
PCI WiFi cards stopped working. Seems like the kernel can't enumerate those
devices due to... guess what, DMA remapping errors!

Note that the debian-config kernel with CONFIG_INTEL_IOMMU=y and
CONFIG_INTEL_IOMMU_DEFAULT_ON=n did not produce such errors. Both my USB and
WiFi cards have been working.

Any idea why is that?

As I'm not sure who to CC exactly, I'm adding both the i915 and Intel IOMMU
maintainers Daniel and David.

I have included several files:

  [*] the "debianish" config file
  [*] my current config file (IOMMU still on by default)
  [*] dmesg for the kernel built with the "debianish" config file
  [*] dmesg for the kernel built with "my" config file, no IOMMU options passed
  [*] dmesg for the kernel built with "my" config file, intel_iommu=igfx_off passed
  [*] dmesg for the kernel built with "my" config file, iommu=off and
intel_iommu=off passed

Hope we can squash those bugs!

Best regards,



Mihai




[1] only one "external" patch applied to ath9k, totally unrelated to the rest of
the system, just changing regulatory stuff.

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