97 lines
3.6 KiB
Plaintext
97 lines
3.6 KiB
Plaintext
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This page describes the structures and procedures used by the cx2341x DMA
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engine.
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Introduction
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============
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The cx2341x PCI interface is busmaster capable. This means it has a DMA
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engine to efficiently transfer large volumes of data between the card and main
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memory without requiring help from a CPU. Like most hardware, it must operate
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on contiguous physical memory. This is difficult to come by in large quantities
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on virtual memory machines.
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Therefore, it also supports a technique called "scatter-gather". The card can
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transfer multiple buffers in one operation. Instead of allocating one large
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contiguous buffer, the driver can allocate several smaller buffers.
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In practice, I've seen the average transfer to be roughly 80K, but transfers
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above 128K were not uncommon, particularly at startup. The 128K figure is
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important, because that is the largest block that the kernel can normally
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allocate. Even still, 128K blocks are hard to come by, so the driver writer is
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urged to choose a smaller block size and learn the scatter-gather technique.
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Mailbox #10 is reserved for DMA transfer information.
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Note: the hardware expects little-endian data ('intel format').
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Flow
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====
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This section describes, in general, the order of events when handling DMA
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transfers. Detailed information follows this section.
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- The card raises the Encoder interrupt.
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- The driver reads the transfer type, offset and size from Mailbox #10.
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- The driver constructs the scatter-gather array from enough free dma buffers
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to cover the size.
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- The driver schedules the DMA transfer via the ScheduleDMAtoHost API call.
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- The card raises the DMA Complete interrupt.
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- The driver checks the DMA status register for any errors.
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- The driver post-processes the newly transferred buffers.
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NOTE! It is possible that the Encoder and DMA Complete interrupts get raised
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simultaneously. (End of the last, start of the next, etc.)
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Mailbox #10
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===========
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The Flags, Command, Return Value and Timeout fields are ignored.
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Name: Mailbox #10
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Results[0]: Type: 0: MPEG.
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Results[1]: Offset: The position relative to the card's memory space.
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Results[2]: Size: The exact number of bytes to transfer.
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My speculation is that since the StartCapture API has a capture type of "RAW"
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available, that the type field will have other values that correspond to YUV
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and PCM data.
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Scatter-Gather Array
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====================
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The scatter-gather array is a contiguously allocated block of memory that
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tells the card the source and destination of each data-block to transfer.
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Card "addresses" are derived from the offset supplied by Mailbox #10. Host
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addresses are the physical memory location of the target DMA buffer.
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Each S-G array element is a struct of three 32-bit words. The first word is
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the source address, the second is the destination address. Both take up the
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entire 32 bits. The lowest 18 bits of the third word is the transfer byte
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count. The high-bit of the third word is the "last" flag. The last-flag tells
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the card to raise the DMA_DONE interrupt. From hard personal experience, if
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you forget to set this bit, the card will still "work" but the stream will
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most likely get corrupted.
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The transfer count must be a multiple of 256. Therefore, the driver will need
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to track how much data in the target buffer is valid and deal with it
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accordingly.
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Array Element:
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- 32-bit Source Address
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- 32-bit Destination Address
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- 14-bit reserved (high bit is the last flag)
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- 18-bit byte count
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DMA Transfer Status
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===================
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Register 0x0004 holds the DMA Transfer Status:
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Bit
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0 read completed
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1 write completed
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2 DMA read error
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3 DMA write error
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4 Scatter-Gather array error
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