Re: [PATCH v3 2/4] videobuf2-dma-streaming: new videobuf2 memory allocator

[Federico Vaga <federico.vaga@gmail.com>]

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