svn commit: r188350 - in head/sys: amd64/amd64 arm/arm dev/usb2/core i386/i386 ia64/ia64 sys

[M. Warner Losh]

In message: <2EA5FEEB-E676-4D1B-9700-399C783F4590 at mac.com>
            Marcel Moolenaar <xcllnt at mac.com> writes:
: 
: On Feb 9, 2009, at 6:23 AM, John Baldwin wrote:
: 
: > On Monday 09 February 2009 12:37:53 am Marcel Moolenaar wrote:
: >>
: >> On Feb 8, 2009, at 5:00 PM, Scott Long wrote:
: >>
: >>> Busdma allows you to request bouncing for realignment.
: >>
: >> How exactly?
: >
: > The 'align' parameter to bus_dma_tag_create().  If your hardware  
: > needs buffers
: > to be aligned on a 4-byte boundary and you bus_dmamap_load() a buffer
: > where 'addr % 4 != 0', then the buffer is bounced.  Since by default  
: > the new
: > buffer starts on a page boundary, it satifies the 'addr % 4'.
: 
: But according to the man page, bounce buffering may not
: be implemented or not be applicable to a platform. It
: seems to me that you cannot depend on this side-effect
: in a generic driver. Are you guys talking only in terms
: of i386 or is this generally applicable?

There's no bounce buffering on MIPS right now, for example...  There
likely should be, but it isn't there now.

Warner