Olivier Langlois's blog

I have sorted out the IOMMU thing.

#if defined(CONFIG_AMD_IOMMU) || defined(CONFIG_DMAR)
#define FIREGL_DMA_REMAPPING
#endif

I have noticed CONFIG_DMAR does not exist anymore. It has been replaced by CONFIG_INTEL_IOMMU.

For people having a Intel chipset with VT-d. It might be worthwile to try. To enable FIREGL_DMA_REMAPPING,

1. replace CONFIG_DMAR by CONFIG_INTEL_IOMMU in firegl_public.c and rebuild the driver
2. Enable VT-d in BIOS
3. Edit kernel command line param in grub.cfg by adding 'intel_iommu=on'

The whole purpose FIREGL_DMA_REMAPPING is to avoid compatibility issues between different kernel version in regards to DMA API. On most distribution CONFIG_AMD_IOMMU and/or CONFIG_INTEL_IOMMU will be defined and when fglrx will be compiled, FIREGL_DMA_REMAPPING will be defined even if at runtime no HW IOMMU will be detected and it works fine anyway.

DMA API is fine. Interface is the same for HW and SW IOMMU.

DMA api ends up using functions in arch/x86/kernel/pci-nommu.c if no hardware IOMMU is on the system. So, unless proven wrong, I would say force FIREGL_DMA_REMAPPING for all setup.

That being said and after having done a fantastic journey in IOMMU and DMA kernel code, I have found out that my problem was much simpler than that:

In /etc/X11/xorg.conf, section "ServerLayout"

2 screens, each one associated to one of my 2 cards, were enumerated. I have removed the second screen of the section and by doing so, it has allowed me to activate Crossfire.

Now, X server starts but hard freeze as soon as I try to access opengl functionality. It can be something as simple as 'glxinfo'.