Mpp buffer design (2016.10.12)
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================================================================================
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Mpp buffer is the warpper of the buffer used by hardware. Hardware usually can
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not use the buffer malloc by cpu. Then we design MppBuffer for different memory
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allocator on different platform. Currently it is designed for ion buffer on
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Android and drm buffer on Linux. Later may support vb2_buffer in v4l2 devices.
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In order to manage buffer usage in different user mpp buffer module introduces
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MppBufferGroup as bufffer manager. All MppBuffer will connect to its manager.
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MppBufferGroup provides allocator service and buffer reuse ability.
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Different MppBufferGroup has different allocator. And besides normal malloc/free
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function the allocator can also accept buffer from external file descriptor.
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MppBufferGroup has two lists, unused buffer list and used buffer list. When
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buffer is free buffer will not be released immediately. Buffer will be moved to
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unused list for later reuse. There is a good reason for doing so. When video
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resolution comes to 4K the buffer size will be above 12M. It will take a long
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time to allocate buffer and generate map table for it. So reusing the buffer
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will save the allocate/free time.
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Here is the diagram of Mpp buffer status transaction.
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+----------+ +---------+
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| create | | commit |
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+-----+----+ +----+----+
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| +----v----+
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+----------+ unused <-----------+
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| +---------+ |
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| | | |
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+-----v----+ | | +-----+----+
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| malloc | | | | free |
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+----------+ | | +----------+
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| inc_ref | +---------+ | dec_ref |
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+-----+----+ +-----^----+
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| +---------+ |
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+----------> used +-----------+
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+---------+
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+----^----+
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+----+----+
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| import |
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+---------+
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