Question about iommu on x86_64 and radeon driver.

[Daniel Haid <d.haid@gogi.tv>]

 Hello,

 I have an x86_64 system with a VIA chipset and 4GB of RAM. The 
 mainboard
 is an ASUS M2V where the bios-setup has an option called "Map around
 memory hole" and I have an "ATI Technologies Inc RV710 [Radeon HD 
 4350]"
 graphics card according to lspci.

 Now one of the following things happen to my system:

 1) With the bios-option enabled and no kernel parameters I get the 
 following error:

 [drm:r600_ring_test] *ERROR* radeon: ring test failed 
 (scratch(0x8504)=0xCAFEDEAD)
 radeon 0000:02:00.0: disabling GPU acceleration

 and then I can not use any 3d-acceleration. I also get the message

 Looks like a VIA chipset. Disabling IOMMU. Override with iommu=allowed

 2) With the bios-option enabled and "mem=3072M" I can not use only 3GB 
 of RAM,
 but the radeon card works.

 3) With the bios-option enabled and "iommu=allowed" I get 4GB of RAM 
 and
 the radeon card works. But I wonder whether this can have any bad 
 effects?

 4) Without the bios-option (and without any kernel parameters) I do not
 get the "Looks like a VIA chipset. Disabling IOMMU. Override with 
 iommu=allowed"
 message, but strangely linux shows only about 3GB of RAM.

 I did not try other combinations. Now my questions are

 A) Is this a bug in the radeon driver? Or maybe not, since 
 Documentation/x86/x86_64/boot-options.txt
 seems to imply that for >3GB an iommu is required?

 B) Is it safe to use iommu=allowed in my case ? If not, what problems 
 will I encounter and what
 options should I use instead? Will I be stuck with 3GB of RAM?

 Please cc me if you answer, since I am not subscribed.

 Thank you.