DMA, CMA, coherence and performance

[Peter Teoh]

I think this discussion should help you:

http://e2e.ti.com/support/embedded/linux/f/354/t/89419.aspx

other failures:

http://stackoverflow.com/questions/14625919/allocating-a-large-dma-buffer

and some guideline here:

https://www.kernel.org/doc/Documentation/DMA-API.txt

https://lkml.org/lkml/2011/3/25/19

As I don't have any specific crashdump or error information, nothing I can
comment further about your problem.   It is quite difficult to make general
comment.



On Fri, Jan 3, 2014 at 7:20 AM, Steven Bell <botsnlinux at gmail.com> wrote:

> Hi,
>
> I'm working on a device driver for a video device which continuously reads
> and writes image frames using DMA. The frames are fairly large, in the
> range of 2-8MB, and I would like the buffers for them to be contiguous
> because of my hardware. My understanding is that using the contiguous
> memory allocator is the current "right way" to get the buffers, and that
> CMA operates entirely behind the scenes when calls are made to
> dma_alloc_coherent().
>
> However, it seems that for this system, a streaming DMA setup would be
> more appropriate.  The buffer gets filled with data once, handed to the
> device, and then isn't touched again until it gets reused with new data.
> The resources I've read have hinted that streaming DMA has some performance
> benefits over coherent DMA, so this seems like the way to go.  But I
> haven't seen any discussion of how to use CMA with streaming DMA (or
> whether such a thing is even necessary).
>
> Does the CMA also work behind get_free_pages, or other kernel memory
> allocation methods?  Does it matter?  The kernel newbies page on memory
> allocation (http://kernelnewbies.org/KernelMemoryAllocation) says that
> get_free_pages up to about 8MB are ok.  Is that a generalization based on
> typical memory fragmentation, or a guarantee?
>
> Thanks,
> Steven
>
> _______________________________________________
> Kernelnewbies mailing list
> Kernelnewbies at kernelnewbies.org
> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
>
>


-- 
Regards,
Peter Teoh
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.kernelnewbies.org/pipermail/kernelnewbies/attachments/20140106/c4a3da2f/attachment.html