android_kernel_xiaomi_sm8350/Documentation/video4linux/pxa_camera.txt

                              PXA-Camera Host Driver
                              ======================

Constraints
-----------
  a) Image size for YUV422P format
     All YUV422P images are enforced to have width x height % 16 = 0.
     This is due to DMA constraints, which transfers only planes of 8 byte
     multiples.


Global video workflow
---------------------
  a) QCI stopped
     Initialy, the QCI interface is stopped.
     When a buffer is queued (pxa_videobuf_ops->buf_queue), the QCI starts.

  b) QCI started
     More buffers can be queued while the QCI is started without halting the
     capture.  The new buffers are "appended" at the tail of the DMA chain, and
     smoothly captured one frame after the other.

     Once a buffer is filled in the QCI interface, it is marked as "DONE" and
     removed from the active buffers list. It can be then requeud or dequeued by
     userland application.

     Once the last buffer is filled in, the QCI interface stops.


DMA usage
---------
  a) DMA flow
     - first buffer queued for capture
       Once a first buffer is queued for capture, the QCI is started, but data
       transfer is not started. On "End Of Frame" interrupt, the irq handler
       starts the DMA chain.
     - capture of one videobuffer
       The DMA chain starts transfering data into videobuffer RAM pages.
       When all pages are transfered, the DMA irq is raised on "ENDINTR" status
     - finishing one videobuffer
       The DMA irq handler marks the videobuffer as "done", and removes it from
       the active running queue
       Meanwhile, the next videobuffer (if there is one), is transfered by DMA
     - finishing the last videobuffer
       On the DMA irq of the last videobuffer, the QCI is stopped.

  b) DMA prepared buffer will have this structure

     +------------+-----+---------------+-----------------+
     | desc-sg[0] | ... | desc-sg[last] | finisher/linker |
     +------------+-----+---------------+-----------------+

     This structure is pointed by dma->sg_cpu.
     The descriptors are used as follows :
      - desc-sg[i]: i-th descriptor, transfering the i-th sg
        element to the video buffer scatter gather
      - finisher: has ddadr=DADDR_STOP, dcmd=ENDIRQEN
      - linker: has ddadr= desc-sg[0] of next video buffer, dcmd=0

     For the next schema, let's assume d0=desc-sg[0] .. dN=desc-sg[N],
     "f" stands for finisher and "l" for linker.
     A typical running chain is :

         Videobuffer 1         Videobuffer 2
     +---------+----+---+  +----+----+----+---+
     | d0 | .. | dN | l |  | d0 | .. | dN | f |
     +---------+----+-|-+  ^----+----+----+---+
                      |    |
                      +----+

     After the chaining is finished, the chain looks like :

         Videobuffer 1         Videobuffer 2         Videobuffer 3
     +---------+----+---+  +----+----+----+---+  +----+----+----+---+
     | d0 | .. | dN | l |  | d0 | .. | dN | l |  | d0 | .. | dN | f |
     +---------+----+-|-+  ^----+----+----+-|-+  ^----+----+----+---+
                      |    |                |    |
                      +----+                +----+
                                           new_link

  c) DMA hot chaining timeslice issue

     As DMA chaining is done while DMA _is_ running, the linking may be done
     while the DMA jumps from one Videobuffer to another. On the schema, that
     would be a problem if the following sequence is encountered :

      - DMA chain is Videobuffer1 + Videobuffer2
      - pxa_videobuf_queue() is called to queue Videobuffer3
      - DMA controller finishes Videobuffer2, and DMA stops
      =>
         Videobuffer 1         Videobuffer 2
     +---------+----+---+  +----+----+----+---+
     | d0 | .. | dN | l |  | d0 | .. | dN | f |
     +---------+----+-|-+  ^----+----+----+-^-+
                      |    |                |
                      +----+                +-- DMA DDADR loads DDADR_STOP

      - pxa_dma_add_tail_buf() is called, the Videobuffer2 "finisher" is
        replaced by a "linker" to Videobuffer3 (creation of new_link)
      - pxa_videobuf_queue() finishes
      - the DMA irq handler is called, which terminates Videobuffer2
      - Videobuffer3 capture is not scheduled on DMA chain (as it stopped !!!)

         Videobuffer 1         Videobuffer 2         Videobuffer 3
     +---------+----+---+  +----+----+----+---+  +----+----+----+---+
     | d0 | .. | dN | l |  | d0 | .. | dN | l |  | d0 | .. | dN | f |
     +---------+----+-|-+  ^----+----+----+-|-+  ^----+----+----+---+
                      |    |                |    |
                      +----+                +----+
                                           new_link
                                          DMA DDADR still is DDADR_STOP

      - pxa_camera_check_link_miss() is called
        This checks if the DMA is finished and a buffer is still on the
        pcdev->capture list. If that's the case, the capture will be restarted,
        and Videobuffer3 is scheduled on DMA chain.
      - the DMA irq handler finishes

     Note: if DMA stops just after pxa_camera_check_link_miss() reads DDADR()
     value, we have the guarantee that the DMA irq handler will be called back
     when the DMA will finish the buffer, and pxa_camera_check_link_miss() will
     be called again, to reschedule Videobuffer3.

--
Author: Robert Jarzmik <robert.jarzmik@free.fr>