Dirty page-tacking with TTM

[Thomas Hellstrom]

Hi, Christian,

I'm looking at adding a utility for tracking dirty pages for TTM. It's 
usable for non-coherent drivers to emulate coherent memory, for example 
for fbdev or if vmwgfx were to emulate coherent mappings, like required 
by Vulkan or by user-space dma-buf mappings.

Basically one would need to hook up the vm_ops page_mkwrite() and 
pfn_mkwrite() into ttm_bo_vm.c. That can be done with small stubs in 
ttm_bo_vm.c which call new functions in the ttm_bo driver. Alternatively 
we could have the driver provide the TTM vm ops, with the current as the 
default. I figure the rest can be done with helpers that initially can 
be part of vmwgfx and moved to TTM should other drivers be interested.

Does this sound OK with you?

/Thomas