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

[Rob Clark]

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>:
>> On Mon, Jan 9, 2012 at 4:10 AM, InKi Dae <daeinki at gmail.com> wrote:
>>> note : in case of sharing a buffer between v4l2 and drm driver, the
>>> memory info would be copied vb2_xx_buf to xx_gem or xx_gem to
>>> vb2_xx_buf through sg table. in this case, only memory info is used to
>>> share, not some objects.
>>
>> which v4l2/vb2 patches are you looking at?  The patches I was using,
>> vb2 holds a reference to the 'struct dma_buf *' internally, not just
>> keeping the sg_table
>>
>
> yes, not keeping the sg_table. I mean... see a example below please.
>
> static void vb2_dma_contig_map_dmabuf(void *mem_priv)
> {
>    struct sg_table *sg;
>     ...
>     sg = dma_buf_map_attachment(buf->db_attach, dir);
>     ...
>     buf->dma_addr = sg_dma_address(sg->sgl);
>     ...
> }
>
> 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.

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

BR,
-R

> Thanks.
>
>> BR,
>> -R
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