Date: Sun, 7 Dec 2008 10:25:58 +0100 From: Hans Petter Selasky <hselasky@c2i.net> To: freebsd-current@freebsd.org Cc: Marcel Moolenaar <xcllnt@mac.com>, freebsd-usb@freebsd.org, Alfred Perlstein <alfred@freebsd.org> Subject: Re: [Serious] busdma bug in -current in relation to USB hardware - review wanted Message-ID: <200812071026.00497.hselasky@c2i.net> In-Reply-To: <031DE609-3E5B-4508-BAB0-95800B7F02F4@mac.com> References: <20081107082740.GA1334@icarus.home.lan> <200812061334.55365.hselasky@c2i.net> <031DE609-3E5B-4508-BAB0-95800B7F02F4@mac.com>
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On Saturday 06 December 2008, Marcel Moolenaar wrote: > 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? Hi Marcel, I think you might be right there. There is one case in which I don't understand what is the correct busdma behaviour. If the DMA tag has an alignment of 4 bytes, and the memory loaded is not aligned to four bytes, then should a bounce page be used? If yes, then you will need to clear the page offset. Else not. NOTE: We are not talking about allocating DMA memory, only loading it. --HPS
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