|This page describes the structures and procedures used by the cx2341x DMA
|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').
|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.)
|The Flags, Command, Return Value and Timeout fields are ignored.
|Name: Mailbox #10
|Results: Type: 0: MPEG.
|Results: Offset: The position relative to the card's memory space.
|Results: 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.
|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
|- 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:
|0 read completed
|1 write completed
|2 DMA read error
|3 DMA write error
|4 Scatter-Gather array error