Re: [PATCH] videobuf-dma-contig: zero copy USERPTR support V3

[Magnus Damm <magnus.damm@gmail.com>]

On Mon, May 11, 2009 at 10:36 PM, Mauro Carvalho Chehab
<mchehab@infradead.org> wrote:
> Em Fri, 8 May 2009 13:06:58 -0700
> Andrew Morton <akpm@linux-foundation.org> escreveu:
>
>> On Fri, 08 May 2009 17:53:10 +0900
>> Magnus Damm <magnus.damm@gmail.com> wrote:
>>
>> > From: Magnus Damm <damm@igel.co.jp>
>> >
>> > This is V3 of the V4L2 videobuf-dma-contig USERPTR zero copy patch.
>> >
>> > Since videobuf-dma-contig is designed to handle physically contiguous
>> > memory, this patch modifies the videobuf-dma-contig code to only accept
>> > a user space pointer to physically contiguous memory. For now only
>> > VM_PFNMAP vmas are supported, so forget hotplug.
>> >
>> > On SuperH Mobile we use this with our sh_mobile_ceu_camera driver
>> > together with various multimedia accelerator blocks that are exported to
>> > user space using UIO. The UIO kernel code exports physically contiguous
>> > memory to user space and lets the user space application mmap() this memory
>> > and pass a pointer using the USERPTR interface for V4L2 zero copy operation.
>> >
>> > With this approach we support zero copy capture, hardware scaling and
>> > various forms of hardware encoding and decoding.
>> >
>> > Signed-off-by: Magnus Damm <damm@igel.co.jp>
>
> Acked-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Thank you!

>> What does it do, how does it do it and why does it do it?
>
> A good documentation is a really good idea here. There videobuf internals are
> very complex. A good documentation for it is very important to keep it updated.

I've just posted a little patch that adds function descriptions,
hopefully that is one step in the right direction.

> I would also suggest if you could also take a look at videobuf-vmalloc and implement a
> similar method to provide USERPTR. The vmalloc flavor can easily be tested with
> the virtual (vivi) video driver, so it helps people to better understand how
> videobuf works. It will also help the USB drivers that use videobuf to use USERPTR.

Yeah, supporting USERPTR with vivi sounds like a good plan. I'm not
sure how much work it involves though. The comment in the
videobuf-vmalloc header says that the buffer code assumes that the
driver does not touch the data, but I think that's exactly how vivi
generates the frame data for us. =)

I need to figure out the best way to grab references to user space
pages and map them virtually contiguous like vmalloc does. This will
take a bit of time, so don't expect anything submitted in time for
v2.6.31. I've put it fairly high on my TODO list.

Thanks for your help!

/ magnus

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