Hello,
On Tuesday, March 20, 2012 8:25 AM Marek Szyprowski wrote:
Hi Linus,
Please pull the dma-mapping framework updates for v3.4 since commit c16fa4f2ad19908a47c63d8fa436a1178438c7e7:
Linux 3.3
with the top-most commit e749a9f707f1102735e02338fa564be86be3bb69
common: DMA-mapping: add NON-CONSISTENT attribute
from the git repository at:
git://git.infradead.org/users/kmpark/linux-samsung dma-mapping-next
Those patches introduce a new alloc method (with support for memory attributes) in dma_map_ops structure, which will later replace dma_alloc_coherent and dma_alloc_writecombine functions.
I've been pointed out that this summary is quite short and misses the main rationale for the proposed changes.
A few limitations have been identified in the current dma-mapping design and its implementations for various architectures. There exist more than one function for allocating and freeing the buffers: currently these 3 are used dma_{alloc, free}_coherent, dma_{alloc,free}_writecombine, dma_{alloc,free}_noncoherent.
For most of the systems these calls are almost equivalent and can be interchanged. For others, especially the truly non-coherent ones (like ARM), the difference can be easily noticed in overall driver performance. Sadly not all architectures provide implementations for all of them, so the drivers might need to be adapted and cannot be easily shared between different architectures. The provided patches unify all these functions and hide the differences under the already existing dma attributes concept. The thread with more references is available here: http://www.spinics.net/lists/linux-sh/msg09777.html
These patches are also a prerequisite for unifying DMA-mapping implementation on ARM architecture with the common one provided by dma_map_ops structure and extending it with IOMMU support. More information is available in the following thread: http://thread.gmane.org/gmane.linux.kernel.cross-arch/12819
More works on dma-mapping framework are planned, especially in the area of buffer sharing and managing the shared mappings (together with the recently introduced dma_buf interface: commit d15bd7ee445d0702ad801fdaece348fdb79e6581 "dma-buf: Introduce dma buffer sharing mechanism" ).
Best regards