Date: Tue, 13 Jan 2015 09:29:51 -0700 From: Ian Lepore <ian@freebsd.org> To: Hans Petter Selasky <hps@selasky.org> Cc: kott <kmatpral@yahoo.com>, freebsd-usb@freebsd.org Subject: Re: usb_pc_cpu_flush Message-ID: <1421166591.14601.195.camel@freebsd.org> In-Reply-To: <54B54073.6000409@selasky.org> References: <1419359192795-5975583.post@n5.nabble.com> <5499E734.1070507@selasky.org> <1419392511197-5975691.post@n5.nabble.com> <549A811D.3060204@selasky.org> <1419416870924-5975752.post@n5.nabble.com> <1419423740820-5975763.post@n5.nabble.com> <549AB711.8070005@selasky.org> <1419431704871-5975773.post@n5.nabble.com> <549BF430.8000207@selasky.org> <1419877515606-5976832.post@n5.nabble.com> <1421133295061-5980199.post@n5.nabble.com> <1421160576.14601.175.camel@freebsd.org> <54B53956.4090708@selasky.org> <1421163656.14601.184.camel@freebsd.org> <54B54073.6000409@selasky.org>
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On Tue, 2015-01-13 at 16:57 +0100, Hans Petter Selasky wrote:
> On 01/13/15 16:40, Ian Lepore wrote:
> > On Tue, 2015-01-13 at 16:27 +0100, Hans Petter Selasky wrote:
> >> On 01/13/15 15:49, Ian Lepore wrote:
> >>> On Tue, 2015-01-13 at 00:14 -0700, kott wrote:
> >>>> Yes with cache disabled, this problem is not seen. Seems to be with a issue
> >>>> with l2 cache.
> >>>> Thanks kott
> >>>
> >>> Except that there are no known problems with l2 cache on armv7 right
> >>> now. There are known problems with the USB driver using the busdma
> >>> routines incorrectly, which accidentally works okay on x86 platforms but
> >>> likely not so well on others.
> >>>
> >>
> >> Hi,
> >>
> >> If there is a problem it is in "usb_pc_cpu_flush()" or
> >> "usb_pc_cpu_invalidate()":
> >>
> >> void
> >> usb_pc_cpu_flush(struct usb_page_cache *pc)
> >> {
> >> if (pc->page_offset_end == pc->page_offset_buf) {
> >> /* nothing has been loaded into this page cache! */
> >> return;
> >> }
> >> bus_dmamap_sync(pc->tag, pc->map, BUS_DMASYNC_PREWRITE);
> >> }
> >>
> >> USB has a very simple DMA sync language, either flush or invalidate.
> >> These are used correctly from what I can see with regard to the FreeBSD
> >> USB specification.
> >>
> >> If the "usb_pc_cpu_flush()" function does not cause the CPU cache to be
> >> written to RAM before the function returns, please let me know.
> >>
> >> --HPS
> >
> > You have an incomplete concept of how busdma sync operations work. It
> > isn't a simple "cpu cache written to ram" operation, there are bounce
> > buffers and other complexities involved that require that the sync
> > operations be done at the correct time in the correct order, and the
> > current usb driver doesn't do that. Instead it does things like
> >
> > bus_dmamap_sync(pc->tag, pc->map, BUS_DMASYNC_POSTREAD);
> > bus_dmamap_sync(pc->tag, pc->map, BUS_DMASYNC_PREREAD);
> >
> > And that's just nonsense that will lead to problems like delivering
> > random buffer garbage to/from a device.
> >
> > To the degree that USB works at all on non-x86 platforms it works by
> > accident. Usually. Except when it doesn't.
> >
> > -- Ian
> >
>
> Hi,
>
> Bounce buffers are perfectly fine with USB as long as the busdma does
> what it is told. If there is no easy way to do a simple "cache flush" or
> "cache invalide" or bounce buffer "flush" or bounce buffer "invalidate"
> multiple times in a row, then busdma cannot co-exist with USB. It is not
> because I'm stubborn, but because of the way USB DMA controllers are
> designed.
>
> With USB chipsets we sometimes need to read the RAM area for a single
> buffer multiple times to poll for updates. From what I've been told in
> the past BUSDMA does.
>
> --HPS
>
> --HPS
>
> --HPS
>
And so we reach the same old impasse, where you declare that USB is
special and doesn't have to behave like other drivers, even though it is
in no way unique in terms of having things like concurrent shared access
to descriptor memory, something that virtually every modern NIC has.
-- Ian
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