Hello,
>
> No. All video capture/output devices all use DMA since it would be
> prohibitively
> expensive for the CPU to do otherwise. So just dig in and implement it.
I am trying to better understand how read() operation actually use the
dma, but I can't yet understand it from code.
>
> No. The vmalloc variant is typically used for USB devices. For PCI(e) you'll
> use videobuf2-dma-contig if the DMA engine requires physically contiguous DMA,
> or videobuf2-dma-sg if the DMA engine supports scatter-gather DMA. You can
> start with dma-contig since the DMA code tends to be simpler, but it is
> harder to get the required physically contiguous memory if memory
> fragmentation
> takes place. So you may not be able to allocate the buffers. dma-sg works much
> better with virtual memory.
>
>
1. I tried to understand the code implementation of videobuf2 with
regards to read():
read() ->
vb2_read() ->
__vb2_perform_fileio()->
vb2_internal_dqbuf() & copy_to_user()
Where is the actual allocation of dma contiguous memory ? Is done with
the userspace calloc() call in userspace (as shown in the v4l2 API
example) ? As I understand the calloc/malloc are not guaranteed to be
contiguous.
How do I know if the try to allocate contigious memory has failed or not ?
2. Is the call to copy_to_user results is performance degredation of
read() in compare to mmap() method ?
Best Regards,
Ran
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