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    SubjectRe: USB transfer_buffer allocations on 64bit systems
    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...

    thanks,

    greg k-h


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