Re: Is usb_buffer_alloc a generic URB buffer allocator ? (was Split Bulk Transfers)

On Thursday 17 May 2007, Laurent Pinchart wrote:

> Shouldn't we implement scatter-gather for EHCI (and possibly other) 
> controllers ?

If by "scatter-gather" you mean "struct scatterlist", we've
had that support for some time now ... in usb_sg_*() routines.
That's the usual definition inside Linux.

But I suspect you mean something else, not well defined.  A
few things I've heard called "scatter gather" in the past:

 - Single copy I/O to buffers specified by kernel (or user)
   virtual addresses.  If the page boundaries are transfer
   buffer boundaries, that's handled already ... given a bit
   of glue code to pin the pages and dma-map them, and the
   same I/O queue mechanisms used by usb_sg_*().

 - Combining N buffers, which aren't aligned on wMaxPacketSize
   boundaries ... e.g. buffers of 10, 500, and 2 bytes combined
   into one 512 byte transfer.  This violates the fundamental
   model of what an URB signifies, so it wouldn't be at easy.

So for most definitions we *do* already handle scatter/gather.

- Dave

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