Re: videobuf-dma-contig - buffer allocation at init time ?

["Magnus Damm" <magnus.damm@xxxxxxxxx>]

Hi there,

On Sat, Oct 4, 2008 at 2:41 AM, Jadav, Brijesh R <brijesh.j@xxxxxx> wrote:
> Hi,
>
> Even dma_alloc_coherent fails after some time because of the memory fragmentation for the large buffer, which is required for the HD modes. This is typically in the Embedded devices because they have less memory. Is it possible to have a hook from the video-buf layer to the driver for allocating contiguous buffer so that allocation and de-allocation is managed in the driver completely?

Yes, you are correct that fragmentation will be an issue. To avoid
that you could allocate a static buffer at boot up and use that buffer
with dma_declare_coherent_memory().

Look at how platform_resource_setup_memory() is being used in the
board specific code here:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=arch/sh/boards/mach-migor/setup.c;h=714dce91cc9bbb30684764c33eef963c09844866;hb=HEAD

This SuperH specific function is used at boot up time to allocate a
chunk of physically contiguous memory for on-chip IP blocks:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=arch/sh/mm/consistent.c;h=64b8f7f96f9aed038e87be11ee374d5b0797f239;hb=HEAD

This memory chunk is passed to the V4L2 driver as a platform resource.
The V4L2 driver performs a dma_declare_coherent_memory() call to make
sure all future dma_alloc_coherent() calls will be allocated from this
memory.

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=blob;f=drivers/media/video/sh_mobile_ceu_camera.c;h=76838091dc66ce05140b63912ec106422e2bc542;hb=HEAD

Look at Documentation/DMA-API.txt for more information. Not sure if
your architecture support this mechanism though.

/ magnus

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