Re: USB transfer_buffer allocations on 64bit systems

[Robert Hancock]

On 04/07/2010 06:33 PM, Greg KH wrote:
> On Wed, Apr 07, 2010 at 03:13:11PM -0400, Alan Stern wrote:
> > On Wed, 7 Apr 2010, Takashi Iwai wrote:
> >
> > > > Ok, I'll write some dummies for usb_malloc() and usb_zalloc() which
> > > > will just call kmalloc() with GFP_DMA32 for now.
> > >
> > > Can't we provide only zalloc() variant?  Zero'ing doesn't cost much,
> > > and the buffer allocation shouldn't be called too often.
> >
> > Linus specifically requested us to avoid using kzalloc in usbfs.  I
> > can't find the message in the email archives, but Greg KH should be
> > able to confirm it.
> >
> > As long as we're imitating kmalloc for one use, we might as well make
> > it available to all.
> >
> > > > And while at it,
> > > > usb_alloc_buffer() will be renamed to usb_alloc_consistent().
> > >
> > > Most of recent functions are named with "coherent".
> >
> > Yes, the terminology got a little confused between the PCI and DMA
> > realms.  I agree, "coherent" is better.
> >
> > BTW, although some EHCI controllers may support 64-bit DMA, the driver
> > contains this:
> >
> > 	if (HCC_64BIT_ADDR(hcc_params)) {
> > 		ehci_writel(ehci, 0,&ehci->regs->segment);
> > #if 0
> > // this is deeply broken on almost all architectures
> > 		if (!dma_set_mask(hcd->self.controller, DMA_BIT_MASK(64)))
> > 			ehci_info(ehci, "enabled 64bit DMA\n");
> > #endif
> > 	}
> >
> > I don't know if the comment is still true, but until the "#if 0" is
> > removed, ehci-hcd won't make use of 64-bit DMA.
>
> I think someone tried to remove it recently, but I wouldn't let them :)
>
> What a mess, hopefully xhci will just take over and save the world from
> this whole thing...

True.. except for the fact that the xhci driver currently doesn't do 
64-bit DMA either, nor does it support MSI even though the HW supports 
it (surprisingly enough the NEC Windows driver does, MSI-X even). At 
this point only Intel likely knows how to do this properly, though, 
since AFAICS the spec isn't publicly available yet.
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