[PATCH v4 05/12] memory: Add NVIDIA Tegra memory controller support

[Alexandre Courbot]

On Thu, Oct 30, 2014 at 7:18 PM, Terje Bergström <tbergstrom at nvidia.com> 
wrote:
 > On 30.10.2014 12:03, Alexandre Courbot wrote:
 >> I had to change the .reg of TEGRA_SWGROUP_GPU to 0xaac to get the IOMMU
 >> to work with GK20A. The reason is still not completely clear to me, but
 >> if you look at the TRM you see that 0xaa8 is basically constant, with
 >> the SMMU translation bit hardcoded to DISABLE (and the ASID field being
 >> meaningless in that case). However right after that register you have a
 >> functional one named GPUB instead of GPU, and this one is fully
 >> writeable (and has the expected effect).
 >
 > GPU has two SW group IDs, because it accesses memory both with and
 > without translation. The bit 34 in addresses in f.ex. PTE chooses
 > between the two.

Indeed, to enable SMMU translation I have to program 0xaac correctly 
*and* set the bit 34 of every address that needs to go through the SMMU.

 > GPU is hard-wired to disable translation. For GPUB translation can be
 > enabled.

So should I understand that the GPU group is for addresses without bit 
34 set (hence forcibly disabled) while GPUB is used when that bit is 
set? Or is it something else?