[RFC v2 1/2] dma-buf: Introduce dma buffer sharing mechanism

[InKi Dae]

2012/1/10 Semwal, Sumit <sumit.semwal at ti.com>:
> On Tue, Jan 10, 2012 at 7:44 AM, Rob Clark <rob at ti.com> wrote:
>> On Mon, Jan 9, 2012 at 7:34 PM, InKi Dae <daeinki at gmail.com> wrote:
>>> 2012/1/10 Rob Clark <rob at ti.com>:
>>> at least with no IOMMU, the memory information(containing physical
>>> memory address) would be copied to vb2_xx_buf object if drm gem
>>> exported its own buffer and vb2 wants to use that buffer at this time,
>>> sg table is used to share that buffer. and the problem I pointed out
>>> is that this buffer(also physical memory region) could be released by
>>> vb2 framework(as you know, vb2_xx_buf object and the memory region for
>>> buf->dma_addr pointing) but the Exporter(drm gem) couldn't know that
>>> so some problems would be induced once drm gem tries to release or
>>> access that buffer. and I have tried to resolve this issue adding
>>> get_shared_cnt() callback to dma-buf.h but I'm not sure that this is
>>> good way. maybe there would be better way.
> Hi Inki,
> As also mentioned in the documentation patch, importer (the user of
> the buffer) - in this case for current RFC patches on
> v4l2-as-a-user[1] vb2 framework - shouldn't release the backing memory
> of the buffer directly - it should only use the dma-buf callbacks in
> the right sequence to let the exporter know that it is done using this
> buffer, so the exporter can release it if allowed and needed.

thank you for your comments.:) and below are some tables about dmabuf
operations with ideal use and these tables indicate reference count of
when buffer is created, shared and released. so if there are any
problems, please let me know. P.S. these are just simple cases so
there would be others.


in case of using only drm gem and dmabuf,

operations                       gem refcount    file f_count   buf refcount
------------------------------------------------------------------------------------------------
1. gem create                   A:1                                   A:0
2. export(handle A -> fd)    A:2                A:1              A:0
3. import(fd -> handle B)    A:2, B:1         A:2              A:1
4. file close(A)                  A:2, B:1         A:1              A:1
5. gem close(A)                A:1, B:1         A:1              A:1
6. gem close(B)                A:1, B:0         A:1              A:0
7. file close(A)                  A:0                A:0
-----------------------------------------------------------------------------------------------
3. handle B shares the buf of handle A.
6. release handle B but its buf.
7. release gem handle A and dmabuf of file A and also physical memory region.


and in case of using drm gem, vb2 and dmabuf,

operations                  gem, vb2 refcount    file f_count   buf refcount
------------------------------------------------------------------------------------------------
1. gem create                   A:1                 A:0
   (GEM side)
2. export(handle A -> fd)    A:2                 A:1              A:0
   (GEM side)
3. import(fd -> handle B)    A:2, B:1          A:2              A:1
   (VB2 side)
4. file close(A)                  A:2, B:1          A:1              A:1
   (VB2 side)
5. vb2 close(B)                 A:2, B:0          A:1              A:0
   (VB2 side)
6. gem close(A)                A:1                A:1              A:0
   (GEM side)
7. file close(A)                  A:0                A:0
   (GEM side)
------------------------------------------------------------------------------------------------
3. vb2 handle B is shared with the buf of gem handle A.
5. release vb2 handle B and decrease refcount of the buf pointed by it.
7. release gem handle A and dmabuf of file A and also physical memory region.

>>
>> the exporter (in this case your driver's drm/gem bits) shouldn't
>> release that mapping / sgtable until the importer (in this case v4l2)
>> calls dma_buf_unmap fxn..
>>
>> It would be an error if the importer did a dma_buf_put() without first
>> calling dma_buf_unmap_attachment() (if currently mapped) and then
>> dma_buf_detach() (if currently attached).  Perhaps somewhere there
>> should be some sanity checking debug code which could be enabled to do
>> a WARN_ON() if the importer does the wrong thing.  It shouldn't really
>> be part of the API, I don't think, but it actually does seem like a
>> good thing, esp. as new drivers start trying to use dmabuf, to have
>> some debug options which could be enabled.
>>
>> It is entirely possible that something was missed on the vb2 patches,
>> but the way it is intended to work is like this:
>> https://github.com/robclark/kernel-omap4/blob/0961428143cd10269223e3d0f24bc3a66a96185f/drivers/media/video/videobuf2-core.c#L92
>>
>> where it does a detach() before the dma_buf_put(), and the vb2-contig
>> backend checks here that it is also unmapped():
>> https://github.com/robclark/kernel-omap4/blob/0961428143cd10269223e3d0f24bc3a66a96185f/drivers/media/video/videobuf2-dma-contig.c#L251
>
> The proposed RFC for V4L2 adaptation at [1] does exactly the same
> thing; detach() before dma_buf_put(), and check in vb2-contig backend
> for unmapped() as mentioned above.
>
>>
>> BR,
>> -R
>>
> BR,
> Sumit.
>
> [1]: V4l2 as a dma-buf user RFC:
> http://comments.gmane.org/gmane.linux.drivers.video-input-infrastructure/42966