[Bug 251018] sdhc errors on growfs with RPI3

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=251018

Mark Johnston <markj at FreeBSD.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |Andrew at FreeBSD.org,
                   |                            |markj at FreeBSD.org
             Status|New                         |Open

--- Comment #2 from Mark Johnston <markj at FreeBSD.org> ---
I looked at this a bit.  For whatever reason, growfs is triggering a DMA
transfer of length 512 on a buffer that is not cacheline size-aligned.  Because
both the beginning and end of the buffer address range are not cacheline
size-aligned, the loop in _bus_dmamap_count_pages() concludes that it needs 2
pages in order to bounce the request, but clearly that's not true since the
entire request is only 512 bytes.

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