On Thu, 2018-05-24 at 11:12 -0700, Steve Longerbeam wrote: [...] > > The following is required as well. Now the question is why we can't skip > > writing those odd UV rows. Anyway, with these 2 changes, I get a stable > > NTSC (and probably PAL) interlaced video stream. > > > > The manual says: Reduce Double Read or Writes (RDRW): > > This bit is relevant for YUV4:2:0 formats. For write channels: > > U and V components are not written to odd rows. > > > > How could it be so? With YUV420, are they normally written? > > Well, given that this bit exists, and assuming I understand it correctly > (1), > I guess the U and V components for odd rows normally are placed on the > AXI bus. Which is a total waste of bus bandwidth because in 4:2:0, > the U and V components are the same for odd and even rows. > > In other words for writing 4:2:0 to memory, this bit should _always_ be set. > > (1) OTOH I don't really understand what this bit is trying to say. > Whether this bit is set or not, the data in memory is correct > for planar 4:2:0: y plane buffer followed by U plane of 1/4 size > (decimated by 2 in width and height), followed by Y plane of 1/4 > size. > > So I assume it is saying that the IPU normally places U/V components > on the AXI bus for odd rows, that are identical to the even row values.
Whether they are identical depends on the input format. The IDMAC always gets fed AYUV32 from the CSI or IC. If the CSI captures YUV 4:2:x, odd and even lines will have the same chroma values. But if the CSI captures YUV 4:4:4 (or RGB, fed through the IC), we can have AYUV32 input with different chroma values on even and odd lines. In that case the IPU just writes the even chroma line and then overwrites it with the odd line, unless the double write reduction bit is set. > IOW somehow those identical odd rows are dropped before writing to > the U/V planes in memory. potentially identical. > Philipp please chime in if you have something to add here. I suppose the bit could be used to choose to write the chroma values of odd instead of even lines for 4:4:4 inputs, at the cost of increased memory bandwidth usage. > Steve > > > OTOH it seems that not only UV is broken with this bit set. > > Y is broken as well. Maybe scanline interlave and double write reduction can't be used at the same time? regards Philipp
