On Tue, Jan 19, 2016 at 9:33 AM, Hans Verkuil <hverkuil@xxxxxxxxx> wrote: > On 01/19/2016 05:45 AM, Ran Shalit wrote: >> On Mon, Jan 18, 2016 at 7:45 PM, Hans Verkuil <hverkuil@xxxxxxxxx> wrote: >>> On 01/18/2016 05:26 PM, Ran Shalit wrote: >>>> Hello, >>>> >>>> I am trying to understand how to implement dma transfer correctly >>>> using videobuf2 APIs. >>>> >>>> Normally, application will do semthing like this (video test API): >>>> >>>> xioctl(fd, VIDIOC_DQBUF, &buf) >>>> process_image(buffers[buf.index].start, buf.bytesused); >>>> xioctl(fd, VIDIOC_QBUF, &buf) >>>> >>>> Therefore, in the driver below I will assume that: >>>> 1. VIDIOC_DQBUF - trigger dma to start >>> >>> No. DMA typically starts when VIDIOC_STREAMON is called, although depending >>> on the hardware the start of the DMA may be delayed if insufficient buffers >>> have been queued. When the start_streaming op is called you know that STREAMON >>> has been called and that at least min_buffers_needed buffers have been queued. >>> >>>> 2. interrupt handler in driver - stop dma >>> >>> ??? No, this just passes the buffer that has been filled by the DMA engine >>> to vb2 via vb2_buffer_done. The DMA will continue filling the next queued buffer. >>> >> >> Hi, >> Just to be sure I got it all right: >> "The DMA will continue filling the next queued buffer" is usually the >> responsibility of the interrupt handler . >> Interrrupt will get the new buffer from list and trigger next dma >> transaction with that buffer. >> Is that Right ? > > Usually that's true. Again, I can't give absolutes because it depends on > the DMA hardware details. There tend to be two main types of DMA: > > 1) the DMA engine has pointers to the current and next frame: in that case > the irq handler has to update the pointers once the current frame finishes > the DMA. > > 2) the DMA engine has a list of descriptors describing each frame and that > links each frame to the next one. In that case the list of descriptors is > updated whenever a new buffer is queued. For engines like this the interrupt > function just returns the DMAed buffer and doesn't have to do anything else. > Hi, Is the first dma engine type above is usually used with contigous dma (videobuf2-dma-contig.h), while the second type referese to sg (scatter-gather) mode (videobuf2-dma-sg.h ) ? Thank you very much, Ran -- 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
