Re: What is scatter/ gather. I am making a frame grabber with DMA .

["Vashek Weis" <vashek@ast.co.il> (by way of Daniel Weaver <dan.weaver@znyx.com>)]

Irv, Daniel,

Everything in the explanation below is correct except the fact that the 
blocks may be bigger than 4K. For example see PLX bridges, each block size 
can be up to 8 MBytes.

Vashek
========

At 16:53 20-05-2002, you wrote:
>Daniel,
>
>To continue on Drews explanation, most PCI bridges with DMA controllers 
>support scatter/gather transfers.  Typically they use a linked link of 
>discriptors in which the DMA controller will traverse.  Each descriptor 
>will tell the controller the location of each 4K buffer to transfer 
>to/from on both sides of the bridge.  When completed, the descriptor will 
>contain a link to the next descriptor.  This processes continues until the 
>DMA controller reaches an "end of transfer" descriptor. This mechanism 
>allows the DMA controller to access these discontiguous memory allocations 
>as if it were contiguous to the local bus.
>
>Irv
>
>-----Original Message-----
>From: Drew Eckhardt [mailto:drew@PoohSticks.ORG]
>Sent: Saturday, May 18, 2002 6:26 PM
>To: Support@PixelSmart.com
>Cc: pci-sig@znyx.com
>Subject: Re: What is scatter/ gather. I am making a frame grabber with DMA.
>
>
>In message <008a01c1fde8$0c00bee0$4c65fea9@dan>, support@pixelsmart.com
>writes:
> >I am making a frame grabber with DMA. I assumed I could write 256K to =
> >one contiguous address. Then I leared a little bit about scatter gather. =
> >What is it and how does it limit my 256K image capture ?
>
>During normal operation, modern operating systems allocate memory on page
>(almost always 4K) boundaries.  If you want to DMA directly into this memory
>(which may be mapped contiguously within a processes' address space) rather
>than into a contiguous buffer allocated on startup which then gets copied
>as necessary you need to be able to accomodate DMAs into discontigous
>memory locations.

========
Vashek Weis

Advanced Semiconductor Technology Ltd.
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