2012/1/9 Daniel Vetter <daniel@xxxxxxxx>: > 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. 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. > 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 ;-) Ok, got it. thanks. :) > -- > Daniel Vetter > Mail: daniel@xxxxxxxx > Mobile: +41 (0)79 365 57 48 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>