On Nov 8, 2008, at 4:53 AM, Hans Petter Selasky wrote:

> On Saturday 08 November 2008, Alexander Motin wrote:
>> Hans Petter Selasky wrote:
>>> http://perforce.freebsd.org/chv.cgi?CH=152649
>>> Change 152649 by hselasky@hselasky_laptop001 on 2008/11/08 09:09:50
>>> Fix a serious BUSDMA bug: The initial virtual page offset is
>>> not retained on the bounce pages, which is what the EHCI and
>>> the OHCI expects.

>> I'm sorry, but are you sure with this? If bounce page will have the
>> same
>> page offset as original data, then how bus_dma_tag_create()'s
>> alignment
>> parameter expected to work? What if data are misaligned, but hardware
>> require alignment?

> Hi,
> I'm not saying that my patch is fully correct for all cases, but at
> least it
> solves the USB case. USB uses an alignment of 1-byte for data.

Another way to look at it is that USB (at least EHCI) uses page-aligned
data and that for the first descriptor you can optionally add an offset
within the page. Effectively this gives you 1-byte alignment, but it
comes with the restriction that on crossing the page boundary, all data
comes from offset 0 on subsequent pages.

Marcel Moolenaar

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