[PATCH 2.6.34] ehci-hcd: add option to enable 64-bit DMA support

[Robert Hancock <hancockrwd@gmail.com>]

Add a module parameter to allow the user to enable 64-bit DMA support in EHCI,
which has been forcibly disabled since 2003 - see:

http://www.mail-archive.com/linux-usb-devel@lists.sourceforge.net/msg17230.html

At that time the comment was "it'd only matter on a few big Intel boxes anyway",
however the situation is much different today when many new machines have 4GB
or more of RAM and IOMMU/SWIOTLB are thus needlessly required for USB transfers.
For now, the support remains disabled by default and is controlled by an
allow_64bit module parameter.

Note that some USB device drivers may require updates to pass the DMA
capabilities up to their higher layers to avoid unnecessary IOMMU or bounce-
buffer use (i.e. networking layer NETIF_F_HIGHDMA). Some of these checks were
disabled by the patch listed above, and more may be required again today.
However, those previous checks were done incorrectly using dma_supported,
which checks to see whether a device's DMA mask can be validly set to a given
mask, not whether its previously set mask will accomodate the mask passed in.

Signed-off-by: Robert Hancock <hancockrwd@gmail.com>

diff --git a/drivers/usb/host/ehci-hcd.c b/drivers/usb/host/ehci-hcd.c
index 1ec3857..f527e15 100644
--- a/drivers/usb/host/ehci-hcd.c
+++ b/drivers/usb/host/ehci-hcd.c
@@ -101,6 +101,10 @@ static int ignore_oc = 0;
 module_param (ignore_oc, bool, S_IRUGO);
 MODULE_PARM_DESC (ignore_oc, "ignore bogus hardware overcurrent indications");
 
+static int allow_64bit;
+module_param(allow_64bit, bool, S_IRUGO);
+MODULE_PARM_DESC(allow_64bit, "allow 64-bit DMA");
+
 #define	INTR_MASK (STS_IAA | STS_FATAL | STS_PCD | STS_ERR | STS_INT)
 
 /*-------------------------------------------------------------------------*/
@@ -644,11 +648,9 @@ static int ehci_run (struct usb_hcd *hcd)
 	hcc_params = ehci_readl(ehci, &ehci->caps->hcc_params);
 	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)))
+		if (allow_64bit &&
+		    !dma_set_mask(hcd->self.controller, DMA_BIT_MASK(64)))
 			ehci_info(ehci, "enabled 64bit DMA\n");
-#endif
 	}