On 04/17/2015 02:11 AM, Alexandre Courbot wrote:
Tracking the PDE and PTE of each memory chunk can probably be avoided
if you change your unmapping strategy. Currently you are going through
the list of nvkm_vm_bp_list, but you know your PDE and PTE are always
going to be adjacent, since a nvkm_vma represents a contiguous block
in the GPU VA. So when unmapping, you can simply check for each PTE
entry whether the IOMMU bit is set, and unmap from the IOMMU space
after unmapping from the GPU VA space, in a loop similar to that of
nvkm_vm_unmap_at().
Then we only need priv. You are keeping the nvkm_mm_node of the IOMMU
space into it, and you need it to free the IOMMU VA space. If only we
could find another way to store it, we could get rid of the whole
structure and associated list_head in nvkm_vma...
I need to give it some more thoughts, and we will probably need to
change a few things in base.c to make the hooks more flexible, so
please give me some more time to think about it. :) I just wanted to
share my thoughts so far in case this puts you on track.
The way you described it would make GPU MMU and IOMMU mappings 1:1. So
when we map a buffer to GPU MMU, we always map page by page the buffer
also to IOMMU. There are disadvantages here.
IOMMU addresses are global, and uses in the GPU caches. When a buffer is
mapped multiple times to different graphics contexts, we want to avoid
cache aliasing by mapping the buffer only once to IOMMU. We also want to
unmap the buffer from IOMMU only once after all the instances of the
buffer have been unmapped, or only when the buffer is actually freed to
cache IOMMU mappings.
Doing IOMMU mapping for the whole buffer with dma_map_sg is also faster
than mapping page by page, because you can do only one TLB invalidate in
the end of the loop instead of after every page if you use dma_map_single.
All of these would talk for having IOMMU and GMMU mapping loops
separate. This patch set does not implement both the advantages above,
but your suggestion would take us further away from that than Vince's
version.
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