On Wed, 2 Jan 2002, David Brownell wrote:
> > > > I presume there is some overhead in bouncing to lowmem? I imagine that
> > > > highmem support for the HCDs wouldn't be that difficult -- they are just
> > > > PCI devices, after all.
> > >
> > > I'm unclear on what "bouncing to lowmem" involves, but I'd rather avoid
> > > teaching all three HCDs a second model for addressing transfer buffers.
> >
> > AFAIK bouncing means a plain, physical copy.
>
> Probably not going to impact the USB 1.1 bandwidth utilization that
> Matt was concerned with ... :)
It might hurt with swapping.
> > Either the HCDs can do 64bit DMA or they can't.
> > Do you really expect there to be a significant number of 32bit machines
> > whose HCD can do 64bit DMA ?
>
> Highmem doesn't only have to do with 64bit DMA. At least in the 2.4
> series there's a model that addresses over about 880MBytes are also
> classed as "highmem".
Is there an impact of the full 32bit DMA patches Jens has done for 2.4
which might concivably be included into 2.5 ?
> > If not, it's IMHO not worth doing it as you'd have either two kinds of
> > urbs or overhead in the common case.
>
> I think some special accomodation for the block I/O layer is going to
> have to happen.
The block io layer should have adaptions to provide for the capability
devices provide. We should not have to deal with copying >32bit pages
ourselves.
> > On 64Bit machines we might have to deal with HCDs who can do 32Bit DMA
> > only. Perhaps there should be a gfp field in the usb_device struct
> > to export knowledge about the memory the HCD can cope with.
>
> Shouldn't be needed.
How do we deal with the combination of 32bit OHCI and 64bit EHCI ?
Regards
Oliver
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