Re: [RFC] Helper to abstract vma handling in media layer

[Hans Verkuil <hverkuil@xs4all.nl>]

On 04/10/14 14:15, Jan Kara wrote:
> On Thu 10-04-14 13:07:42, Hans Verkuil wrote:
>> On 04/10/14 12:32, Jan Kara wrote:
>>>   Hello,
>>>
>>> On Thu 10-04-14 12:02:50, Marek Szyprowski wrote:
>>>> On 2014-03-17 20:49, Jan Kara wrote:
>>>>>   The following patch series is my first stab at abstracting vma handling
>>>> >from the various media drivers. After this patch set drivers have to know
>>>>> much less details about vmas, their types, and locking. My motivation for
>>>>> the series is that I want to change get_user_pages() locking and I want
>>>>> to handle subtle locking details in as few places as possible.
>>>>>
>>>>> The core of the series is the new helper get_vaddr_pfns() which is given a
>>>>> virtual address and it fills in PFNs into provided array. If PFNs correspond to
>>>>> normal pages it also grabs references to these pages. The difference from
>>>>> get_user_pages() is that this function can also deal with pfnmap, mixed, and io
>>>>> mappings which is what the media drivers need.
>>>>>
>>>>> The patches are just compile tested (since I don't have any of the hardware
>>>>> I'm afraid I won't be able to do any more testing anyway) so please handle
>>>>> with care. I'm grateful for any comments.
>>>>
>>>> Thanks for posting this series! I will check if it works with our
>>>> hardware soon.  This is something I wanted to introduce some time ago to
>>>> simplify buffer handling in dma-buf, but I had no time to start working.
>>>   Thanks for having a look in the series.
>>>
>>>> However I would like to go even further with integration of your pfn
>>>> vector idea.  This structure looks like a best solution for a compact
>>>> representation of the memory buffer, which should be considered by the
>>>> hardware as contiguous (either contiguous in physical memory or mapped
>>>> contiguously into dma address space by the respective iommu). As you
>>>> already noticed it is widely used by graphics and video drivers.
>>>>
>>>> I would also like to add support for pfn vector directly to the
>>>> dma-mapping subsystem. This can be done quite easily (even with a
>>>> fallback for architectures which don't provide method for it). I will try
>>>> to prepare rfc soon.  This will finally remove the need for hacks in
>>>> media/v4l2-core/videobuf2-dma-contig.c
>>>   That would be a worthwhile thing to do. When I was reading the code this
>>> seemed like something which could be done but I delibrately avoided doing
>>> more unification than necessary for my purposes as I don't have any
>>> hardware to test and don't know all the subtleties in the code... BTW, is
>>> there some way to test the drivers without the physical video HW?
>>
>> You can use the vivi driver (drivers/media/platform/vivi) for this.
>> However, while the vivi driver can import dma buffers it cannot export
>> them. If you want that, then you have to use this tree:
>>
>> http://git.linuxtv.org/cgit.cgi/hverkuil/media_tree.git/log/?h=vb2-part4
>   Thanks for the pointer that looks good. I've also found
> drivers/media/platform/mem2mem_testdev.c which seems to do even more
> testing of the area I made changes to. So now I have to find some userspace
> tool which can issue proper ioctls to setup and use the buffers and I can
> start testing what I wrote :)

Get the v4l-utils.git repository (http://git.linuxtv.org/cgit.cgi/v4l-utils.git/).
You want the v4l2-ctl tool. Don't use the version supplied by your distro,
that's often too old.

'v4l2-ctl --help-streaming' gives the available options for doing streaming.

So simple capturing from vivi is 'v4l2-ctl --stream-mmap' or '--stream-user'.
You can't test dmabuf unless you switch to the vb2-part4 branch of my tree.

If you need help with testing it's easiest to contact me on the #v4l irc
channel.

Regards,

	Hans

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