[Serious] busdma bug in -current in relation to USB hardware - review wanted

Marcel Moolenaar xcllnt at mac.com
Sat Dec 6 17:45:06 UTC 2008

On Dec 6, 2008, at 4:34 AM, Hans Petter Selasky wrote:

> Hi,
> After various feedback from several people I have made a new patch  
> proposal
> that will fix the busdma problem.
> See:
> http://perforce.freebsd.org/chv.cgi?CH=154181
> Review wanted!

The USB stack has a fixed page size of 4K. On our 64-bit platforms
PAGE_SIZE is at least 8K. Your change is sloppy in that respect
and doesn't make the distinction. That makes the patch a kluge.
The definition of BUS_DMA_NO_REALIGN is based on circumstantial
evidence only and as such, works as a side-effect. I don't think
that's a good design.

I don't think there's any reason not to preserve the page offset
in all cases. So far all hardware worked whether or not their
DMA pages were bounced and the non-bounced pages would have a
possible non-zero page offset, whereas the bounced pages would
always have a zero page offset. In short: it works either way.
In particular, it works with the page offset preserved. Why not
preserve it always? What's the downside?

Marcel Moolenaar
xcllnt at mac.com

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