On Mon, Dec 19, 2011 at 02:03:28PM +0530, Sumit Semwal wrote:
Hello Everyone,
This is RFC v3 for DMA buffer sharing mechanism - changes from v2 are in the changelog below.
Various subsystems - V4L2, GPU-accessors, DRI to name a few - have felt the need to have a common mechanism to share memory buffers across different devices - ARM, video hardware, GPU.
This need comes forth from a variety of use cases including cameras, image processing, video recorders, sound processing, DMA engines, GPU and display buffers, and others.
This RFC is an attempt to define such a buffer sharing mechanism- it is the result of discussions from a couple of memory-management mini-summits held by Linaro to understand and address common needs around memory management. [1]
A new dma_buf buffer object is added, with operations and API to allow easy sharing of this buffer object across devices.
The framework allows:
- a new buffer-object to be created with fixed size.
- different devices to 'attach' themselves to this buffer, to facilitate backing storage negotiation, using dma_buf_attach() API.
- association of a file pointer with each user-buffer and associated allocator-defined operations on that buffer. This operation is called the 'export' operation.
- this exported buffer-object to be shared with the other entity by asking for its 'file-descriptor (fd)', and sharing the fd across.
- a received fd to get the buffer object back, where it can be accessed using the associated exporter-defined operations.
- the exporter and user to share the scatterlist using map_dma_buf and unmap_dma_buf operations.
Documentation present in the patch-set gives more details.
This is based on design suggestions from many people at the mini-summits, most notably from Arnd Bergmann arnd@arndb.de, Rob Clark rob@ti.com and Daniel Vetter daniel@ffwll.ch.
The implementation is inspired from proof-of-concept patch-set from Tomasz Stanislawski t.stanislaws@samsung.com, who demonstrated buffer sharing between two v4l2 devices. [2]
References: [1]: https://wiki.linaro.org/OfficeofCTO/MemoryManagement [2]: http://lwn.net/Articles/454389
Patchset based on top of 3.2-rc3, the current version can be found at
http://git.linaro.org/gitweb?p=people/sumitsemwal/linux-3.x.git Branch: dma-buf-upstr-v2
Earlier versions: v2 at: https://lkml.org/lkml/2011/12/2/53 v1 at: https://lkml.org/lkml/2011/10/11/92
Best regards, ~Sumit Semwal
I think this is a really good v1 version of dma_buf. It contains all the required bits (with well-specified semantics in the doc patch) to implement some basic use-cases and start fleshing out the integration with various subsystem (like drm and v4l). All the things still under discussion like - userspace mmap support - more advanced (and more strictly specified) coherency models - and shared infrastructure for implementing exporters are imo much clearer once we have a few example drivers at hand and a better understanding of some of the insane corner cases we need to be able to handle.
And I think any risk that the resulting clarifications will break a basic use-case is really minimal, so I think it'd be great if this could go into 3.3 (maybe as some kind of staging/experimental infrastructure).
Hence for both patches: Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch