On 01/30/2016 12:55 PM, Ran Shalit wrote: > 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 ) ? Yes. 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
