Date: Sat, 8 Nov 2008 15:40:48 +0100 From: Hans Petter Selasky <hselasky@freebsd.org> To: Alexander Motin <mav@freebsd.org> Cc: Perforce Change Reviews <perforce@freebsd.org> Subject: Re: PERFORCE change 152649 for review Message-ID: <200811081540.50034.hselasky@freebsd.org> In-Reply-To: <4915A1F8.1010807@FreeBSD.org> References: <200811080910.mA89AgTZ048172@repoman.freebsd.org> <200811081259.08838.hselasky@freebsd.org> <4915A1F8.1010807@FreeBSD.org>
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On Saturday 08 November 2008, Alexander Motin wrote: > Hans Petter Selasky wrote: > > On Saturday 08 November 2008, Alexander Motin wrote: > >> Hans Petter Selasky wrote: > >>> http://perforce.freebsd.org/chv.cgi?CH=152649 > >>> > > > 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. > > > > Probably someone needs to sit down a think this through. Like it is now, > > it is very scary with regard to USB! > > This is a change to the one of basic kernel subsystems. Any change there > will affect ton's of drivers and may cause issues with too many of > broken/specific hardware. It should be done with maximum care without > quick fixes! > > > Maybe it is time to create a new busdma tag flag? BUS_DMA_NOREALIGN? > > > > If this flag is set, busdma should refuse to load the memory if the > > memory is not aligned already to the given aligment in the DMA tag ? > > What is the real reason to do that? If you are saying that you uses > 1-byte alignment then you should be able to accept _any_ address that > busdma will allocate to you! Why original data alignment should so > strictly affect hardware operation? Hi, The real reason is that the EHCI and OHCI only takes the full physical address of the _first_ page, and then for the subsequent pages only stores the upper bits of the physical address, leaving it impossible to program a per-page data start offset. The hardware assumes that the lower part of the physical address wraps modulo PAGE_SIZE, and that's it. Don't ask me why they designed it like that. Currently USB is broken on all architectures where bouncing is used. I see no other way around this than to change busdma. --HPS
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