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

[Dave Airlie <airlied@gmail.com>]

On Mon, Jan 9, 2012 at 8:10 AM, Daniel Vetter <daniel@ffwll.ch> wrote:
> On Mon, Jan 09, 2012 at 03:20:48PM +0900, InKi Dae wrote:
>> I has test dmabuf based drm gem module for exynos and I found one problem.
>> you can refer to this test repository:
>> http://git.infradead.org/users/kmpark/linux-samsung/shortlog/refs/heads/exynos-drm-dmabuf
>>
>> at this repository, I added some exception codes for resource release
>> in addition to Dave's patch sets.
>>
>> let's suppose we use dmabuf based vb2 and drm gem with physically
>> continuous memory(no IOMMU) and we try to share allocated buffer
>> between them(v4l2 and drm driver).
>>
>> 1. request memory allocation through drm gem interface.
>> 2. request DRM_SET_PRIME ioctl with the gem handle to get a fd to the
>> gem object.
>> - internally, private gem based dmabuf moudle calls drm_buf_export()
>> to register allocated gem object to fd.
>> 3. request qbuf with the fd(got from 2) and DMABUF type to set the
>> buffer to v4l2 based device.
>> - internally, vb2 plug in module gets a buffer to the fd and then
>> calls dmabuf->ops->map_dmabuf() callback to get the sg table
>> containing physical memory info to the gem object. and then the
>> physical memory info would be copied to vb2_xx_buf object.
>> for DMABUF feature for v4l2 and videobuf2 framework, you can refer to
>> this repository:
>> git://github.com/robclark/kernel-omap4.git drmplane-dmabuf
>>
>> after that, if v4l2 driver want to release vb2_xx_buf object with
>> allocated memory region by user request, how should we do?. refcount
>> to vb2_xx_buf is dependent on videobuf2 framework. so when vb2_xx_buf
>> object is released videobuf2 framework don't know who is using the
>> physical memory region. so this physical memory region is released and
>> when drm driver tries to access the region or to release it also, a
>> problem would be induced.
>>
>> for this problem, I added get_shared_cnt() callback to dma-buf.h but
>> I'm not sure that this is good way. maybe there may be better way.
>> if there is any missing point, please let me know.
>
> The dma_buf object needs to hold a reference on the underlying
> (necessarily reference-counted) buffer object when the exporter creates
> the dma_buf handle. This reference should then get dropped in the
> exporters dma_buf->ops->release() function, which is only getting called
> when the last reference to the dma_buf disappears.
>
> If this doesn't work like that currently, we have a bug, and exporting the
> reference count or something similar can't fix that.
>
> Yours, Daniel
>
> PS: Please cut down the original mail when replying, otherwise it's pretty
> hard to find your response ;-)

And also the importer needs to realise it doesn't own the pages in the
sg_table and when its freeing its backing memory it shouldn't free
those pages. So for GEM objects we have to keep track if we allocated
the pages or we got them from an dma buf.

Dave.

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