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

[Daniel Vetter]

On Mon, Jan 09, 2012 at 09:06:56PM +0900, InKi Dae wrote:
> 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:
> http://cgit.freedesktop.org/~airlied/linux/log/?h=drm-prime-dmabuf

I haven't really looked at Dave's latest prime patches, but he reported
some reference counting issues last time around we chatted about it on
irc. So maybe you're just right and the dma_buf_put is indeed missing from
drm_prime_gem_destroy ;-) But as I've said, haven't really reviewed the
code, so I'm likely completely wrong.

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