contiguous memory allocation problem
Hans Petter Selasky
hselasky at c2i.net
Sun Jul 2 09:38:08 UTC 2006
On Saturday 01 July 2006 21:44, David Malone wrote:
> On Sat, Jul 01, 2006 at 10:44:54AM +0200, Hans Petter Selasky wrote:
> > > The latest concensus seems to be that the USB system should make use of
> > > the scatter-gather facilities in the hardware to avoid the need to
> > > allocate large contiguous memory chunks. iedowse@ had mostly finished
> > > implementing this in mid May.
> > Yes, but scatter and gather will add extra complexity to the driver, and
> > maybe an extra memory copy in most cases. The idea is to allocate less
> > than or equal to a page of memory, and then avoid the problem?
> I believe the USB drivers in -current grew scatter/gather support
> about a month ago. See the commit message below. Is this likely to
But there is one problem, that has been overlooked, and that is High speed
isochronous transfers, which are not supported by the existing USB system. I
don't think that the EHCI specification was designed for scatter and gather,
when you consider this:
8 transfers of 0xC00 bytes has to fit on 7 pages. If this is going to work,
and I am right, one page has to contain two transfers. (see page 43 of
Currently the PAGE_SIZE is 0x1000, right? As you can see "(0xC00 * 2) >
So is it possible to change the PAGE_SIZE ?
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