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

InKi Dae daeinki at gmail.com
Mon Jan 9 07:06:56 EST 2012

2012/1/9 Daniel Vetter <daniel at ffwll.ch>:
> On Mon, Jan 09, 2012 at 07:10:25PM +0900, InKi Dae wrote:
>> 2012/1/9 Daniel Vetter <daniel at ffwll.ch>:
>> > 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.
>> >
>> when the exporter creates the dma_buf handle(for example, gem -> fd),
>> I think the refcount of gem object should be increased at this point,
>> and decreased by dma_buf->ops->release() again because when the
>> dma_buf is created and dma_buf_export() is called, this dma_buf refers
>> to the gem object one time. and in case of inporter(fd -> gem),
>> file->f_count of the dma_buf is increased and then when this gem
>> object is released by user request such as drm close or
>> drn_gem_close_ioctl, dma_buf_put() should be called by
>> dma_buf->ops->detach() to decrease file->f_count again because the gem
>> object refers to the dma_buf. for this, you can refer to my test
>> repository I mentioned above. but the problem is that when a buffer is
>> released by one side, another can't know whether the buffer already
>> was released or not.
> Nope, dma_buf_put should not be called by ->detach. The importer gets his
> reference when importing the dma_buf and needs to drop that reference
> himself when it's done using the buffer by calling dma_buf_put (i.e. after
> the last ->detach call).

I'm afraid that there may be my missing points. I'm confusing. who is
Importer and who is Exporter you think? Importer is fd goes to private
buffer and Exporter is private buffer goes to fd? if so, yes, when the
importer needs to drop that reference(the importer want to release
that buffer), dma_buf_put() should be called somewhere and in my case,
that function is called by drm_prime_gem_destory(). this function is
included at Dave's patch sets and also dma_buf_detatch() is called
there. and I just thought that here is right place. I didn't find the
place dma_buf_put() is called anywhere. could you please tell me where
dma_buf_put() should be called at you think?.

for this, you can refer to Dave's repository:

> I think adding separate reference counting to
> ->attach and ->detach is a waste of time and only papers over buggy
> importers.

 I mean  when fd goes to private buffer, that reference(file->f_count)
would be increased by dma_buf_get() and  only ->detach is used to drop
that reference.

> Additionally the importer does _not_ control the lifetime of an dma_buf
> object and it's underlying backing storage. It hence may _never_ free the
> backing storage itself, that's the job of the exporter.

yes, my understanding is that if user app requested close(fd) call to
the Importer, after close(fd), file->f_count would be decreased but
would still has 1. because file->f_count already was 1 by the

> With that cleared up, referencing the exporters underlying buffer object
> from the dma_buf will just do the right thing.
>> 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.
> Hm, maybe I need to take a look at the currently proposed v4l dma_buf
> patches ;-) atm I don't have an idea what exactly you're talking about.

I need to stop the confusing. thank you for your answer. :)

> Yours, Daniel
> --
> Daniel Vetter
> Mail: daniel at ffwll.ch
> Mobile: +41 (0)79 365 57 48

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