[Linaro-mm-sig] [RFCv1 2/4] v4l:vb2: add support for shared buffer (dma_buf)
m.szyprowski at samsung.com
Mon Jan 23 09:06:57 UTC 2012
On Friday, January 20, 2012 5:29 PM Laurent Pinchart wrote:
> On Friday 20 January 2012 17:20:22 Tomasz Stanislawski wrote:
> > >> IMO, One way to do this is adding field 'struct device *dev' to struct
> > >> vb2_queue. This field should be filled by a driver prior to calling
> > >> vb2_queue_init.
> > >
> > > I haven't looked into the details, but that sounds good to me. Do we have
> > > use cases where a queue is allocated before knowing which physical
> > > device it will be used for ?
> > I don't think so. In case of S5P drivers, vb2_queue_init is called while
> > opening /dev/videoX.
> > BTW. This struct device may help vb2 to produce logs with more
> > descriptive client annotation.
> > What happens if such a device is NULL. It would happen for vmalloc
> > allocator used by VIVI?
> Good question. Should dma-buf accept NULL devices ? Or should vivi pass its
> V4L2 device to vb2 ?
I assume you suggested using struct video_device->dev entry in such case.
It will not work. DMA-mapping API requires some parameters to be set for the
client device, like for example dma mask. struct video_device contains only an
artificial struct device entry, which has no relation to any physical device
and cannot be used for calling DMA-mapping functions.
Performing dma_map_* operations with such artificial struct device doesn't make
any sense. It also slows down things significantly due to cache flushing
(forced by dma-mapping) which should be avoided if the buffer is accessed only
with CPU (like it is done by vb2-vmalloc style drivers).
IMHO this case perfectly shows the design mistake that have been made. The
current version simply tries to do too much.
Each client of dma_buf should 'map' the provided sgtable/scatterlist on its own.
Only the client device driver has all knowledge to make a proper 'mapping'.
Real physical devices usually will use dma_map_sg() for such operation, while
some virtual ones will only create a kernel mapping for the provided scatterlist
(like vivi with vmalloc memory module).
Samsung Poland R&D Center
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