Date: Mon, 30 Jun 2014 15:11:26 +0200 From: Bernd Walter <ticso@cicely7.cicely.de> To: Warner Losh <imp@bsdimp.com> Cc: "freebsd-arm@freebsd.org" <arm@freebsd.org> Subject: Re: arm alignment faults... Message-ID: <20140630131126.GB55631@cicely7.cicely.de> In-Reply-To: <039F8A84-0193-424C-8093-E411957817E9@bsdimp.com> References: <20140629033823.GN1560@funkthat.com> <CAJ-VmokxO5vfOOSvPrTfnda6gSKOPpJQF3kto3AdgUhvbFgNYg@mail.gmail.com> <20140629040150.GO1560@funkthat.com> <CAJ-Vmon1iFeJdJ-nBnz%2B4bqannshT4nct2dFKep1EZzCEM62Jw@mail.gmail.com> <86E20F1C-C161-45B2-AC82-11738596E004@bsdimp.com> <20140629054039.GP1560@funkthat.com> <039F8A84-0193-424C-8093-E411957817E9@bsdimp.com>
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On Sun, Jun 29, 2014 at 10:33:59AM -0600, Warner Losh wrote: > > On Jun 28, 2014, at 11:40 PM, John-Mark Gurney <jmg@funkthat.com> wrote: > > > Warner Losh wrote this message on Sat, Jun 28, 2014 at 22:53 -0600: > >> > >> On Jun 28, 2014, at 10:52 PM, Adrian Chadd <adrian@freebsd.org> wrote: > >> > >>> On 28 June 2014 21:01, John-Mark Gurney <jmg@funkthat.com> wrote: > >>>> Adrian Chadd wrote this message on Sat, Jun 28, 2014 at 20:44 -0700: > >>>>> On 28 June 2014 20:38, John-Mark Gurney <jmg@funkthat.com> wrote: > >>>>>> So, one of the little projects I'd like to see is the removal of > >>>>>> ETHER_ALIGN from the tree.. This bogosity can (and does) cause the use > >>>>>> of bouncing durning DMA ops on all ethernet frames... > >>>> > >>>> Now that I think about it, total removal may not be necessary, just > >>>> the requirement to use it... If the ethernet dma engine can do half > >>>> word aligned dma, then there would be benifit on those to keep > >>>> ETHER_ALIGN... > >>>> > >>>>> Well, as long as you're not doing it by forcing the various CPUs to > >>>>> handle unaligned accesses. > >>>> > >>>> Hard to do on armv4 which I don't believe supports unaligned access... > >>>> > >>>>> The cost of those unaligned accesses on some CPUs that support them is > >>>>> not trivial. We benchmarked some of the ARM cores at Qualcomm back > >>>>> when looking to migrate stuff to ARM and it wasn't very quick. > >>>> > >>>> I plan on fixing the TCP/IP stack to copy data to an aligned buffer > >>>> (maybe only if the original buffer isn't aligned) on the stack when > >>>> __NO_STRICT_ALIGNMENT is not defined... I can't see how copying the > >>>> entire packet is cheaper than copying 20 bytes or so... > >>> > >>> There's lots of other stupid corner cases that screw you. > >>> > >>> VLAN headers add extra bytes. > >>> > >>> 802.11 headers can offset things depending upon the 802.11 frame type > >>> (3-addr, 4-addr, vlan, no vlan, etc.) > >>> > >>> There's no guarantee all ethernet DMA engines can do the alignment as > >>> required. :( > >> > >> The ate driver for Atmel?s AT91RM9200 is one such beast. > > > > Are you sure? The tag for data says alignment 1: > > if (bus_dma_tag_create(bus_get_dma_tag(dev), 1, 0, > > BUS_SPACE_MAXADDR_32BIT, BUS_SPACE_MAXADDR, NULL, NULL, MCLBYTES, > > 1, MCLBYTES, 0, busdma_lock_mutex, &sc->sc_mtx, &sc->mtag)) > > > > Or is that a limitation on the parent? > > It is a limitation in hardware. You have to DMA to a 4 byte boundary. That might be a bug in the above line though? It is a limitation of the AT91RM9200. More recent ate hardware (e.g. AT91SAM9260) don't have this limitation. > > I did an audit of the arm drivers (not mips), but I didn't see any that > > have the restriction... The one that I'm thinking of was the Cirrus > > EP9302 in the TS-7200 port that I did years ago.. > -- B.Walter <bernd@bwct.de> http://www.bwct.de Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm.help
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