[Linaro-mm-sig] [RFC v3 0/2] Introduce DMA buffer sharing mechanism

[Dave Airlie]

>>
>> 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 at arndb.de>, Rob Clark <rob at ti.com> and
>> Daniel Vetter <daniel at ffwll.ch>.
>>
>> The implementation is inspired from proof-of-concept patch-set from
>> Tomasz Stanislawski <t.stanislaws at 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 at ffwll.ch>

Yeah I'm with Daniel, I like this one, I can definitely build the drm
buffer sharing layer on top of this.

How do we see this getting merged? I'm quite happy to push it to Linus
if we don't have an identified path, though it could go via a Linaro
tree as well.

so feel free to add:
Reviewed-by: Dave Airlie <airlied at redhat.com>

Dave.