Date: Mon, 22 Feb 2010 17:03:45 +0000 (UTC) From: "Justin T. Gibbs" <gibbs@FreeBSD.org> To: cvs-src-old@freebsd.org Subject: cvs commit: src/sys/amd64/amd64 busdma_machdep.c Message-ID: <201002221704.o1MH468Z017599@repoman.freebsd.org>
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gibbs 2010-02-22 17:03:45 UTC
FreeBSD src repository
Modified files:
sys/amd64/amd64 busdma_machdep.c
Log:
SVN rev 204214 on 2010-02-22 17:03:45Z by gibbs
Enforce stronger semantics for bus-dma alignment (currently only on amd64).
Now all contiguous regions returned from bus-dma will be aligned to the
alignment constraint and all but the last region are guaranteed to be
a multiple of the alignment in length. This also means that the relative
alignment of two adjacent bytes in the I/O stream have a difference of 1
even if they are not physically contiguous.
The old code, when needing to perform a copy in order to align data, only
copied the amount of data needed to reach the next page boundary. This
often left an unaligned end to the segment. Drivers such as Xen's blkfront
can't deal with such segments.
The downside to this approach is that, once an unaligned region is encountered,
the remainder of the I/O will be bounced. However, bouncing should be rare.
It is typically caused by non-performance critical userland programs that
don't bother to align their I/O buffers (e.g. bsdlabel). In-kernel I/O
buffers are always aligned to at least a page boundary.
Reviewed by: scottl
MFC after: 2 weeks
Revision Changes Path
1.92 +19 -12 src/sys/amd64/amd64/busdma_machdep.c
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