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

On Thursday 17 May 2007, David Brownell wrote:
> 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_*().

That's what I meant, yes. The idea is to vmalloc() a buffer instead of using 
kmalloc() or get_free_pages(). usb_sg_* creates a URB per sg entry. What I'd 
like to know is if this could be handled by the USB stack. Users would submit 
a single URB with a vmalloc buffer, and the buffer would be transparently 
split if required. Is that doable, too difficult or impossible (due to 
hardware constraints) ?

>  - 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.

And it wouldn't be that useful anyway.

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

Laurent Pinchart

-------------------------------------------------------------------------
This SF.net email is sponsored by DB2 Express
Download DB2 Express C - the FREE version of DB2 express and take
control of your XML. No limits. Just data. Click to get it now.
http://sourceforge.net/powerbar/db2/
_______________________________________________
[email protected]
To unsubscribe, use the last form field at:
https://lists.sourceforge.net/lists/listinfo/linux-usb-devel