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

[InKi Dae]

2012/1/10 Rob Clark <rob@xxxxxx>:
> On Mon, Jan 9, 2012 at 7:34 PM, InKi Dae <daeinki@xxxxxxxxx> wrote:
>> 2012/1/10 Rob Clark <rob@xxxxxx>:
>>> On Mon, Jan 9, 2012 at 4:10 AM, InKi Dae <daeinki@xxxxxxxxx> 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
>

I think that we also used same concept as your. for this, you can
refer to Dave's repository below and see the drm_prime_gem_destroy
function.
http://cgit.freedesktop.org/~airlied/linux/commit/?h=drm-prime-dmabuf&id=7cb374d6642e838e0e4836042e057e6d9139dcad

but when it comes to releasing resources, I mistakely understood some
parts of dmabuf concept so thank you for Rob and Sumit. that is very
useful.

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