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Diffstat (limited to 'Documentation/video4linux/pxa_camera.txt')
| -rw-r--r-- | Documentation/video4linux/pxa_camera.txt | 125 |
1 files changed, 125 insertions, 0 deletions
diff --git a/Documentation/video4linux/pxa_camera.txt b/Documentation/video4linux/pxa_camera.txt new file mode 100644 index 00000000000..b1137f9a53e --- /dev/null +++ b/Documentation/video4linux/pxa_camera.txt @@ -0,0 +1,125 @@ + 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> |