>>
>> 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 ?
>
> The actual allocation happens in videobuf2-vmalloc/dma-contig/dma-sg depending
> on the flavor of buffers you want (virtual memory, DMA into physically contiguous
> memory or DMA into scatter-gather memory). The alloc operation is the one that
> allocates the memory.
Thank you very much for the time.
Just to be sure I understand the general mechanism of DMA with regards
to the read() operation and in the case of using contiguous memory,
I try to draw the general sequence as I understand it from the code
and reading on this issue:
read() into user memory buffer ->
vb2_read() ->
__vb2_perform_fileio() ->
deaque buffer with: vb2_internal_dqbuf() into
contiguous DMA memory (kernel) ->
copy_to_user() will actually copy from
the contigious dma memory(kernel) into user buffer (userspace)
1. Is the above sequence correct ?
2. When talking about contiguous dma memory (or scatter-gatther) we
actually always refer to memory allocated in kernel, right ?
Best Regards,
Ran
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