Need Information on setting up Hardware Memory Buffers

[Carlos Rafael Giani]

On 2013-08-12 19:55, Stirling Westrup wrote:
> I've never before had to delve into the depths on how gstreamer 
> allocates and passes video buffers along, but its looking like I will 
> soon need to know this.
>
> I am assisting in porting OpenCL to gstreamer 1.0, and we'll need some 
> method of allocating and passing around OpenCL buffers. Ideally, for 
> my purposes, we'll want to build a pipeline that passes a single 
> unchanging buffer pointer, but attaches different metadata to describe 
> transformations. When someone needs an actual frame of data, we'll 
> combine the transformations and  perform them with an OpenCL kernel 
> that writes to a standard memory buffer.
>
> The documentation on how memory allocation and passing works in 
> gstreamer is kinda scant though, and I was hoping folks could point me 
> to useful tutorials and/or example code that I can learn from.
>
> -- 
> Stirling Westrup
> Programmer, Entrepreneur.
> https://www.linkedin.com/e/fpf/77228
> http://www.linkedin.com/in/swestrup
> http://technaut.livejournal.com
> http://sourceforge.net/users/stirlingwestrup
>
>
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Hello, I have recently written a plugin which also passes around 
hardware buffers, so here are my comments.

You need these components: a buffer pool, an allocator, a metadata type, 
and a memory type.

The memory type is derived from GstMemory, and extends it by whatever 
you need for the OpenCL buffers (id's, addresses etc.)
The allocator is used to allocate your custom memory blocks.
The metadata is necessary to let downstream elements know that the 
buffers you are sending them contain OpenCL specific stuff.
The buffer pool uses the allocator to allocate memory with your 
allocator. In GStreamer 1.0 , a GstBuffer (which is what a buffer pool 
produces) is a container for metadata and memory blocks. You allocate 
one memory block with your allocator, and append it to the GstBuffer. 
You also create the OpenCL metadata, and add it to the buffer.

Now you can use the buffer pool to create GstBuffers. The buffer pool 
logic takes care of reclaiming buffers that are no longer used etc. One 
important limitation of buffer pools is that they can only be used for 
buffers with the same total memory size (= the sum of all of the buffer 
memory block sizes) and the same caps. If this limitation is 
unacceptable, omit the buffer pool, and manually create buffers + append 
memory blocks to them.

Keep in mind that memory blocks may be shared. This typically happens 
when subbuffers are created. Decide whether or not this is something you 
want to allow.

cheers
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