Usage of dma-buf sg-tables

[Thomas Hellstrom <thellstrom@vmware.com>]

Hi!

Considering that the linux DMA-API states that information in an sg-list 
may be destroyed when it's mapped,
it seems to me that at least  one of the drm prime functions use invalid 
assumptions.

In particular, I don't think it's safe to assume that pages in a single 
sg-list segment are contigous after mapping, so
if we want struct page pointers we should use

pfn = dma_to_phys((sg_dma_address(sg) + p_offset*PAGE_SIZE)) >> PAGE_SHIFT

and if the pfn is valid, convert it to a struct page.

(Incorrect code is, for example, in drm_prime_sg_to_page_addr_arrays)

Or does dma-buf require that page info in sg-lists need to be kept 
across the map operation?

BTW this brings up another question: It's stated that the above function 
is needed by the TTM driver in order to do
correct fault handling. This seems odd, TTM shouldn't be able to mmap() 
or fault an imported dma-buf, right?

Thanks,
Thomas