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

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hello,

On Friday, December 23, 2011 10:22 AM Ming Lei wrote:

> On Thu, Dec 22, 2011 at 5:28 PM, Marek Szyprowski
> <m.szyprowski@samsung.com> 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@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.

It can be easily increased for particular boards, there is no problem with this.

> > 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]

Your driver doesn't access video data inside kernel space, so this is also not an issue.
 
> > 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 *'.

Then you MUST use cookie for it. vaddr method should return kernel virtual address 
to the buffer video data. Some parts of videobuf2 relies on this - it is used by file
io emulator (read(), write() calls) and mmap equivalent for non-mmu systems.

Manual casting in the driver is also a bad idea, that's why there are helper functions
defined for both dma_contig and dma_sg allocators: vb2_dma_contig_plane_dma_addr() and
vb2_dma_sg_plane_desc().

Best regards
-- 
Marek Szyprowski
Samsung Poland R&D Center