Sorry for the late answer to this. > > This allocator is needed because some device (like STA2X11 VIP) cannot > > work > > with DMA sg or DMA coherent. Some other device (like the one used by > > Jonathan when he proposes vb2-dma-nc allocator) can obtain much better > > performance with DMA streaming than coherent. > > Ok, please add such explanations at the patch's descriptions, as it is > important not only for me, but to others that may need to use it.. OK > >> 2) why vb2-dma-config can't be patched to use dma_map_single > >> > >> (eventually using a different vb2_io_modes bit?); > > > > I did not modify vb2-dma-contig because I was thinking that each DMA > > memory > > allocator should reflect a DMA API. > > The basic reason for having more than one VB low-level handling (vb2 was > inspired on this concept) is that some DMA APIs are very different than > the other ones (see vmalloc x DMA S/G for example). > > I didn't make a diff between videobuf2-dma-streaming and > videobuf2-dma-contig, so I can't tell if it makes sense to merge them or > not, but the above argument seems too weak. I was expecting for a technical > reason why it wouldn't make sense for merging them. I cannot work on this now. But I think that I can do an integration like the one that I pushed some month ago (a8f3c203e19b702fa5e8e83a9b6fb3c5a6d1cce4). Wind River made that changes to videobuf-contig and I tested, fixed and pushed. > >> 3) what are the usecases for it. > >> > >> Could you please detail it? Without that, one that would be needing to > >> write a driver will have serious doubts about what would be the right > >> driver for its usage. Also, please document it at the driver itself. I don't have a full understand of the board so I don't know exactly why dma_alloc_coherent does not work. I focused my development on previous work by Wind River. I asked to Wind River (which did all the work on this board) for the technical explanation about why coherent doesn't work, but they do not know. That's why I made the new allocator: coherent doesn't work and HW doesn't support SG. > I'm not a DMA performance expert. As such, from that comment, it sounded to > me that replacing dma-config/dma-sg by dma streaming will always give > "performance optimizations the hardware allow". me too, I'm not a DMA performance expert. I'm just an user of the DMA API. On my hardware simply it works only with that interface, it is not a performance problem. > On a separate but related issue, while doing DMABUF tests with an Exynos4 > hardware, using a s5p sensor, sending data to s5p-tv, I noticed a CPU > consumption of about 42%, which seems too high. Could it be related to > not using the DMA streaming API? As I wrote above, I'm not a DMA performance expert. I skip this -- Federico Vaga -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
