In Documentation/DMA-mapping.txt:
If you acquired your memory via the page allocator
(i.e. __get_free_page*()) or the generic memory allocators
(i.e. kmalloc() or kmem_cache_alloc()) then you may DMA to/from
that memory using the addresses returned from those routines.
This means specifically that you may _not_ use the memory/addresses
returned from vmalloc() for DMA. It is possible to DMA to the
_underlying_ memory mapped into a vmalloc() area, but this requires
walking page tables to get the physical addresses, and then
translating each of those pages back to a kernel address using
something like __va().
My Question:
1. when any generic hardware (non-CPU) attempts to write to the memory, they will access it via the physical address - right?
2. when the CPU attempt to access the memory, they can access it via the virtual address + MMU + pagetable mechanism to retrieve the data - right?
3.. I don't understand this part - "this requires walking page tables to get the physical addresses, and then translating each of those pages back to a kernel address using something like __va(). ".
what is the difference between get_free_page()/kmem_cache_alloc() and vmalloc() that makes the former dma-able, and latter not?
