Date: Tue, 28 Sep 1999 11:45:44 -0600 From: Warner Losh <imp@village.org> To: Bruce Evans <bde@zeta.org.au> Cc: Takahashi Yoshihiro <nyan@FreeBSD.org>, winter@jurai.net, kato@FreeBSD.org, cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org, FreeBSD98-hackers@jp.freebsd.org Subject: Re: cvs commit: src/sys/conf files src/sys/i386/conf files.i386 Message-ID: <199909281745.LAA04687@harmony.village.org> In-Reply-To: Your message of "Wed, 29 Sep 1999 03:06:50 %2B1000." <Pine.BSF.4.10.9909290231150.7599-100000@alphplex.bde.org> References: <Pine.BSF.4.10.9909290231150.7599-100000@alphplex.bde.org>
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In message <Pine.BSF.4.10.9909290231150.7599-100000@alphplex.bde.org> Bruce Evans writes:
: 1) Only one bus type supported, and the tag for that is a compile-time
: constant. Otherwise, there will be an extra branch in the bus space
: routines for getting to the inirect case. This branch will cost
: more than indirection on most machines.
That's what the ifdefs do. They give you a zero branch function.
When you support more buses, then you have more things to support.
: 2) i386-style PIO access with the port address in a register, and offset
: 0. If the port address is in a softc, then there must be an indirection
: to load it, and the load should load the final address
: (sc->sc_bshtable[offset]). If the offset is nonzero, then on i386's
: there must be an instruction to add it, and this will take the same
: time as an indirection in many cases even if the base address is in
: a register.
I'm not sure I understand why this is bad. When a driver needs to use
indiction, it pays a small runtime price to do so (one extra add).
When a driver doesn't need to do so, ther eis no extra cost over the
current bus_space implementation.
: 3) MEMIO access with the port in a register, and the offset constant or in
: a register, and a machine that does suitable (base + index) addressing
: at no cost.
I'm not sure I understand this objection. In the current memio case
we have the same overhead we do for pio (namely the addition).
: > But, your code requires some ugly casts.
: > I suggest the following way to fix this problem.
: >
: > bus_space_handle_t is the union instead of u_int.
: >
: > union bus_space_handle {
: > bus_addr_t bsh_base;
: > bus_addr_t *bsh_iat;
: > };
: > typedef union bus_space_handle bus_space_handle_t;
:
: Doesn't this require changing all drivers that currently use the
: bus space functions to be changed to initialise their sc->sc_bsh.base
: instead of their sc->sc_bsh? This is uglier than casting.
No. There are remarkably few drivers that do math or other naughty
things with their bus handle. There are likely only 3-5 in the tree.
Doesn't sound like an undue burdon to me. I had an experimental bus
space implementation that had both bus_space_handle_t and
bus_space_tag_t as structs. The biggest breakage was with the tag_t
change since too many places still use the machine dependent
I386_BUS_SPACE_FOO tags and I was too lazy to fix it right (actually,
I didn't think of how to fix it right until just this instant).
bus_space_map/unmap would be easy to implement which would solve the
math problem, assuming that's all the do, and they don't mark the
region as in use/free. If they do, the implementations get uglier
much more quickly.
Warner
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