Date: Sat, 25 Apr 2015 08:04:27 +0200 From: Hans Petter Selasky <hps@selasky.org> To: Ian Lepore <ian@freebsd.org>, Svatopluk Kraus <onwahe@gmail.com> Cc: freebsd-current@freebsd.org Subject: Re: [patch] fix add_bounce_page() for BUS_DMA_KEEP_PG_OFFSET Message-ID: <553B2E6B.20406@selasky.org> In-Reply-To: <553B2D56.1000405@selasky.org> References: <CAFHCsPVgYHL0OyMnn1b2DcNiQO-20k1aDvJNvFjwU21YxB29Cw@mail.gmail.com> <1429882188.1157.2.camel@freebsd.org> <553B2D56.1000405@selasky.org>
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On 04/25/15 07:59, Hans Petter Selasky wrote: > On 04/24/15 15:29, Ian Lepore wrote: >> On Fri, 2015-04-24 at 13:34 +0200, Svatopluk Kraus wrote: >>> The add_bounce_page() can be called from _bus_dmamap_load_phys(). >>> Client virtual address is zero (not valid) in that case. So, client >>> physical address must be used to get an offset when >>> BUS_DMA_KEEP_PG_OFFSET flag is set. Note that client physical address >>> is always valid. >>> >>> Svatopluk Kraus >> >> The only user of BUS_DMA_KEEP_PG_OFFSET in the system is USB. If the >> flag didn't exist, we could do bouncing with a whole lot more >> efficiency, at least in the ARM world. I wonder if it would be possible >> to just eliminate the flag (and of course, USB's need for it, which I >> fully admit I don't understand even a little bit). >> > > Hi, > > USB (EHCI/UHCI/OHCI) needs this flag when computing scatter/gather > lists. Please ensure it works like expected. The good news is XHCI needs > it less than the EHCI. > > Ian: The patch looks good - will you commit it? > The reason is that the EHCI/UHCI/OHCI doesn't have separate length fields for each data chunk and somtimes use the "0x1000 - (addr & 0xFFF)" formula to compute how many bytes are left of a transfer when a transfer wraps around 0x1000. Thank you for you understanding this is not a USB problem in general. --HPS
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