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diff --git a/Documentation/driver-api/media/drivers/pxa_camera.rst b/Documentation/driver-api/media/drivers/pxa_camera.rst new file mode 100644 index 000000000..46919919b --- /dev/null +++ b/Documentation/driver-api/media/drivers/pxa_camera.rst @@ -0,0 +1,194 @@ +.. SPDX-License-Identifier: GPL-2.0 + +PXA-Camera Host Driver +====================== + +Author: Robert Jarzmik <robert.jarzmik@free.fr> + +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 + Initially, the QCI interface is stopped. + When a buffer is queued, start_streaming is called and 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. + +c) Capture global finite state machine schema + +.. code-block:: none + + +----+ +---+ +----+ + | DQ | | Q | | DQ | + | v | v | v + +-----------+ +------------------------+ + | STOP | | Wait for capture start | + +-----------+ Q +------------------------+ + +-> | QCI: stop | ------------------> | QCI: run | <------------+ + | | DMA: stop | | DMA: stop | | + | +-----------+ +-----> +------------------------+ | + | / | | + | / +---+ +----+ | | + |capture list empty / | Q | | DQ | | QCI Irq EOF | + | / | v | v v | + | +--------------------+ +----------------------+ | + | | DMA hotlink missed | | Capture running | | + | +--------------------+ +----------------------+ | + | | QCI: run | +-----> | QCI: run | <-+ | + | | DMA: stop | / | DMA: run | | | + | +--------------------+ / +----------------------+ | Other | + | ^ /DMA still | | channels | + | | capture list / running | DMA Irq End | not | + | | not empty / | | finished | + | | / v | yet | + | +----------------------+ +----------------------+ | | + | | Videobuf released | | Channel completed | | | + | +----------------------+ +----------------------+ | | + +-- | QCI: run | | QCI: run | --+ | + | DMA: run | | DMA: run | | + +----------------------+ +----------------------+ | + ^ / | | + | no overrun / | overrun | + | / v | + +--------------------+ / +----------------------+ | + | Frame completed | / | Frame overran | | + +--------------------+ <-----+ +----------------------+ restart frame | + | QCI: run | | QCI: stop | --------------+ + | DMA: run | | DMA: stop | + +--------------------+ +----------------------+ + + Legend: - each box is a FSM state + - each arrow is the condition to transition to another state + - an arrow with a comment is a mandatory transition (no condition) + - arrow "Q" means : a buffer was enqueued + - arrow "DQ" means : a buffer was dequeued + - "QCI: stop" means the QCI interface is not enabled + - "DMA: stop" means all 3 DMA channels are stopped + - "DMA: run" means at least 1 DMA channel is still running + +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 transferring data into videobuffer RAM pages. + When all pages are transferred, 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 transferred 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 + +.. code-block:: none + + +------------+-----+---------------+-----------------+ + | 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, transferring 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 : + +.. code-block:: none + + Videobuffer 1 Videobuffer 2 + +---------+----+---+ +----+----+----+---+ + | d0 | .. | dN | l | | d0 | .. | dN | f | + +---------+----+-|-+ ^----+----+----+---+ + | | + +----+ + +After the chaining is finished, the chain looks like : + +.. code-block:: none + + 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 + +.. code-block:: none + + => + 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 !!!) + +.. code-block:: none + + 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. |