On 11/20/2015 05:14 PM, Ran Shalit wrote: >>> >>> 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 ? Yes. > 2. When talking about contiguous dma memory (or scatter-gatther) we > actually always refer to memory allocated in kernel, right ? Usually. With the V4L2_MEMORY_USERPTR stream I/O mode it is userspace that allocates the memory, but when using physically contiguous DMA this particular streaming mode is normally not supported. With V4L2_MEMORY_MMAP it is always the kernel that allocates the memory. Regards, Hans -- 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
