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