Re: [linux-usb-devel] Re: [Bug 1412] Copy from USB1 CF/SM readerstalls, no actual content is read (only directory structure)
From: David Brownell
Date: Fri Nov 07 2003 - 18:58:00 EST
Jens Axboe wrote:
No that looks alright, given you are allocating low memory pages. The
devices can probably do full 32-bit dma I bet, though.
Typically ... most usb host controllers you'll see are on
PCI (OHCI, UHCI, EHCI) with no restrictions, and only some
EHCI controllers can do 64-bit DMA. That's all visible in
the the dma_mask for each interface in the device with the
mass storage support, usually still at its "32-bit dma is ok"
pci controller default.
But it seems that most current 2.6 DMA API implementations
have some problems in those areas. See for example:
That second patch is a partial workaround for the first patch
presumably not getting applied before 2.6.0-final. Net result,
some systems with gobs of memory and no IOMMU may do needless
buffer copies during USB I/O.
Though a quick glance suggested to me that SCSI infrastructure
is consulting dma_mask directly, instead of using the DMA API
calls which do that. I'm not sure I'd trust it to be any
more correct, given GIGO ...
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