Date: Tue, 18 Jun 2013 23:49:57 +0200 From: Marius Strobl <marius@alchemy.franken.de> To: Jung-uk Kim <jkim@niksun.com> Cc: "arch@freebsd.org" <arch@freebsd.org>, Robert Millan <rmh@freebsd.org>, Niclas Zeising <zeising@freebsd.org> Subject: Re: Bus space routines Message-ID: <20130618214957.GB53058@alchemy.franken.de> In-Reply-To: <51C0CFE7.6010906@niksun.com> References: <51C0345E.4000309@freebsd.org> <20130618111351.GA43938@alchemy.franken.de> <51C044DA.8030406@freebsd.org> <20130618124038.GV53058@alchemy.franken.de> <51C0A451.4010903@FreeBSD.org> <20130618205943.GA53058@alchemy.franken.de> <51C0CBE8.5030904@FreeBSD.org> <51C0CDBE.40501@FreeBSD.org> <51C0CFE7.6010906@niksun.com>
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On Tue, Jun 18, 2013 at 09:24:19PM +0000, Jung-uk Kim wrote: > On 2013-06-18 17:14:38 -0400, Jung-uk Kim wrote: > > On 2013-06-18 17:06:48 -0400, Jung-uk Kim wrote: > > 2013? 6? 18? 17:06, Jung-uk Kim ? ?:> On 2013-06-18 16:59:43 > > -0400, Marius Strobl wrote: > >>> On Tue, Jun 18, 2013 at 02:17:53PM -0400, Jung-uk Kim wrote: > >>>> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 > >>>> > >>>> On 2013-06-18 08:40:38 -0400, Marius Strobl wrote: > >>>>> On Tue, Jun 18, 2013 at 01:30:34PM +0200, Niclas Zeising > >>>>> wrote: > >>>>>> On 2013-06-18 13:13, Marius Strobl wrote: > >>>>>>> On Tue, Jun 18, 2013 at 12:20:14PM +0200, Niclas Zeising > >>>>>>> wrote: > >>>>>>>> This has been discussed before [1], but there seem to > >>>>>>>> still be a lack of consensus, so I'll ask again. > >>>>>>>> > >>>>>>>> Should in*/out* macros or bus_space* functions be used > >>>>>>>> in userland code? The background is that the port > >>>>>>>> devel/libpciaccess uses these routines on FreeBSD. In > >>>>>>>> a first incarnation it used the bus_space* routines, > >>>>>>>> see this patch: > >>>>>>>> > >>>>>>>> http://trillian.chruetertee.ch/ports/browser/trunk/devel/libpciaccess/files/patch-src-freebsd_pci.c?rev=591 > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>> > >>>>>>>> > > > >>>>>>>> > > This was later changed to use the in*/out* macros directly, with the > >>>>>>>> motivation that the bus_space* functions is a KPI that > >>>>>>>> shouldn't be used in userland. See following for an > >>>>>>>> updated patch: > >>>>>>>> > >>>>>>>> http://trillian.chruetertee.ch/ports/browser/trunk/devel/libpciaccess/files/patch-src-freebsd_pci.c?rev=815 > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>> > >>>>>>>> > > > >>>>>>>> > > The problem is that the in*/out* macros differ between FreeBSD and > >>>>>>>> Debian/kFreeBSD, and Debian/kFreeBSD want to switch > >>>>>>>> back to use bus_space* again. > >>>>>>>> > >>>>>>>> My question is simply, which one is correct, or should > >>>>>>>> libpciaccess be reworked in a completely different > >>>>>>>> way? > >>>>>>> > >>>>>>> The latter; in*/out*() are somewhat okay if you want to > >>>>>>> restrict this to work on x86 without PCI domains only. > >>>>>>> The above approach to using bus_space(9) is one big > >>>>>>> hack, though. The right way for employing that API is to > >>>>>>> allocate the corresponding bus resource of a particular > >>>>>>> device and then to obtain bus tag and handle via > >>>>>>> rman_get_bus{tag,handle}(9) - which won't work from > >>>>>>> userland, however. The shortcut to just stick in > >>>>>>> {AMD64,I386}_BUS_SPACE_IO instead is totally unportable > >>>>>>> as generally a bus tag isn't a mere constant and also > >>>>>>> may depend on which PCI bus and domain a particular > >>>>>>> device is located on/in. What we really need is a proper > >>>>>>> interface allowing userland to access PCI I/O and memory > >>>>>>> registers, f. e. via /dev/pci, and for libpciaccess to > >>>>>>> build upon that, i. e. essentially the same as things > >>>>>>> work on/with Linux and /sys/bus/pci/device. As a > >>>>>>> side-effect this then also permits to properly sanity > >>>>>>> check PCI accesses from userland within the kernel. > >>>>>>> > >>>>>> > >>>>>> That is true, however, it won't build itself today, and we > >>>>>> need to have this working in the meantime, so what do you > >>>>>> suggest we use for now? > >>>>> > >>>>> That's hard to say as architecturally neither in*/out*() nor > >>>>> bus_space(9) are the way to proceed. Tentatively, I'd go > >>>>> with abusing the latter as that approach _should_ allow to > >>>>> make things additionally work one another one or two > >>>>> architectures - in particular powerpc - without introducing > >>>>> an #ifdef hell. > >>>> > >>>> AFAIK, bus_space(9) can only work for amd64 and i386 in > >>>> userland by pure luck. It can never work for powerpc if I'm > >>>> reading the MD headers correctly. > > > >>> Actually, I think that by cloning bs_le_tag in userland as far as > >>> necessary, i. e. leaving out things like mapping/unmapping and > >>> allocation/deallocation etc., and using that as bus tag, > >>> bus_space(9) has a fairly good chance of working in userland for > >>> powerpc in this case. Obviously, that's harder to do than faking > >>> the bus tag for x86, though. > > > >> Please don't forget the point of this thread, i.e., finding simple > >> MI interface. ;-) > > > >>>> Also, I strongly disagree with abusing bus_space because it > >>>> gives a bad example. > > > >>> Well, I strongly believe that both in*/out*() and bus_space(9) > >>> give very bad examples for userland code :) > > > >> If you insist, we can simply use io(4). > > > > I went ahead and implemented it: > > > > http://people.freebsd.org/~jkim/libpciaccess.diff > > > > This should work with powerpc and other platforms with working io(4). > > I just realized powerpc does not have /dev/io, sorry. Anyway, my > point was there is userland API already and we don't have to reinvent > the wheel. > Essentially, the issue with io(4) is the same as with in*/out*(); you can't implement it in a sane way on architectures such as powerpc that have busses of different endianesses, apart from some other complications. IMO, we'll run in circles forever without a new userland interface or without extending an existing one to be able to deal with all these MD requirements (as such, thinking in the direction of the MI bus_space(9) at least in theory is the right thing but the problem is that it's an _K_PI in the first place). Marius
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