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. 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
