Date: Sun, 29 Jun 2014 10:33:59 -0600 From: Warner Losh <imp@bsdimp.com> To: John-Mark Gurney <jmg@funkthat.com> Cc: "freebsd-arm@freebsd.org" <arm@freebsd.org> Subject: Re: arm alignment faults... Message-ID: <039F8A84-0193-424C-8093-E411957817E9@bsdimp.com> In-Reply-To: <20140629054039.GP1560@funkthat.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>
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[-- Attachment #1 --] 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… Warner > 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.. [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Comment: GPGTools - https://gpgtools.org iQIcBAEBCgAGBQJTsD/3AAoJEGwc0Sh9sBEAPEkP/2nWdgA6WJQzATfBDtpFYbBQ lvh6B4Updblfehjknav+Pt+kGp3LSR15Ev5BDHeffjMZt/HLJkG2yyaSiwS+QdSZ D9WMLPFeX+gJfNkul2/STV0XUwv/fcflRyYr2LaNnvccnPqvWRM8bZ++hLgssZmc CG9JTSq6mygTQwel/zISOiJRbAhEKa5iQ7UjvB1Ropli08QgMOJr/DkGQChe3fYU kg0KQXElRC5ser/W0vKTxM7pL4S5ITctXeKV+gMbg9CzxwgbM4Y+sDpqXh8e18kC oMx2B2DOQUgxycV5cO5p2srrz62VRklQmbTZhd2y6NfJjgjCPM1m6l/1Ek39lQ6b rW0Ct40i2R6dlMP/1UnwCV9piVA0Xy/eFr4kaQ04b4J/qadLo4rJlEFFBTD7cRu3 8knBYCuqggMlmOwXBrDV3w9YS845R+kSpHwMB2KvFShmD1AKioauGOmomo2oS1ec jwTAFohxRoF1rZi92I+6VzVN7RfdZI5Oq1LXuJHc5BbiKqre50GxYad/HVlIFji6 qPOrFSvZiAL0TJbR0KK9+heidF1Iq2neQl8MIsjZ0bWA9JbCsSHV/avIaH71UG9R qudby5xCMQpwaIc7EYed/UcV+18SBaQP1kyRcUdWJGacZG34WYWuH/emNxh2Ns2p dStTWQa0/mjvzC6drmU7 =Gi2m -----END PGP SIGNATURE-----help
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