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

On Fri, 18 May 2007, David Brownell wrote:

> On Friday 18 May 2007, jidong xiao wrote:
> > Regarding this, I have one question:
> > "except at the beginning and the end, scatter-gather buffers follow page
> > boundaries", why? i.e. Is there any specific reason that sg buffers should
> > follow page boundaries? Then why the beinging and the end are exceptions?
> 
> You should ask the person who wrote that comment; I can't
> make sense of it myself.  What's "follow" etc.

I wrote it.  Stating it more verbosely:

	Each entry in a scatter-gather list maps a group of physically
	contiguous pages.  The buffer itself occupies only a portion of 
	those pages, as given by the .offset and .length fields in the 
	entry.

	For each entry in the list (except possibly the first),
	.offset is 0 and hence the buffer starts at a page boundary.	

	For each entry in the list (except possibly the last),
	.offset + .length is equal to the total size of the pages
	mapped by the entry, and hence the buffer ends at a page 
	boundary.

Alan Stern


-------------------------------------------------------------------------
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