1. 24 Aug, 2015 1 commit
    • Robert Jarzmik's avatar
      lib: scatterlist: add sg splitting function · f8bcbe62
      Robert Jarzmik authored
      Sometimes a scatter-gather has to be split into several chunks, or sub
      scatter lists. This happens for example if a scatter list will be
      handled by multiple DMA channels, each one filling a part of it.
      
      A concrete example comes with the media V4L2 API, where the scatter list
      is allocated from userspace to hold an image, regardless of the
      knowledge of how many DMAs will fill it :
       - in a simple RGB565 case, one DMA will pump data from the camera ISP
         to memory
       - in the trickier YUV422 case, 3 DMAs will pump data from the camera
         ISP pipes, one for pipe Y, one for pipe U and one for pipe V
      
      For these cases, it is necessary to split the original scatter list into
      multiple scatter lists, which is the purpose of this patch.
      
      The guarantees that are required for this patch are :
       - the intersection of spans of any couple of resulting scatter lists is
         empty.
       - the union of spans of all resulting scatter lists is a subrange of
         the span of the original scatter list.
       - streaming DMA API operations (mapping, unmapping) should not happen
         both on both the resulting and the original scatter list. It's either
         the first or the later ones.
       - the caller is reponsible to call kfree() on the resulting
         scatterlists.
      Signed-off-by: 's avatarRobert Jarzmik <robert.jarzmik@free.fr>
      Signed-off-by: 's avatarJens Axboe <axboe@fb.com>
      f8bcbe62