[RFC PATCH v2 4/8] media: videobuf2: introduce VIDEOBUF2_PAGE memops

[Ming Lei]

On Thu, Dec 22, 2011 at 5:28 PM, Marek Szyprowski
<m.szyprowski at samsung.com> wrote:
> Hello,
>
> On Wednesday, December 14, 2011 3:00 PM Ming Lei wrote:
>
>> DMA contig memory resource is very limited and precious, also
>> accessing to it from CPU is very slow on some platform.
>>
>> For some cases(such as the comming face detection driver), DMA Streaming
>> buffer is enough, so introduce VIDEOBUF2_PAGE to allocate continuous
>> physical memory but letting video device driver to handle DMA buffer mapping
>> and unmapping things.
>>
>> Signed-off-by: Ming Lei <ming.lei at canonical.com>
>
> Could you elaborate a bit why do you think that DMA contig memory resource
> is so limited? If dma_alloc_coherent fails because of the memory fragmentation,
> the alloc_pages() call with order > 0 will also fail.

For example, on ARM, there is very limited kernel virtual address space reserved
for DMA coherent buffer mapping, the default size is about 2M if I
don't remember
mistakenly.

>
> I understand that there might be some speed issues with coherent (uncached)
> userspace mappings, but I would solve it in completely different way. The interface

Also there is poor performance inside kernel space, see [1]

> for both coherent/uncached and non-coherent/cached contig allocator should be the
> same, so exchanging them is easy and will not require changes in the driver.
> I'm planning to introduce some design changes in memory allocator api and introduce
> prepare and finish callbacks in allocator ops. I hope to post the rfc after
> Christmas. For your face detection driver using standard dma-contig allocator
> shouldn't be a big issue.
>
> Your current implementation also abuses the design and api of videobuf2 memory
> allocators. If the allocator needs to return a custom structure to the driver

I think returning vaddr is enough.

> you should use cookie method. vaddr is intended to provide only a pointer to
> kernel virtual mapping, but you pass a struct page * there.

No, __get_free_pages returns virtual address instead of 'struct page *'.


thanks,
--
Ming Lei

[1], http://marc.info/?t=131198148500001&r=1&w=2