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Date: Tue, 18 Jun 2013 15:36:07 -0400
From: Jung-uk Kim <jkim@FreeBSD.org>
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To: Warner Losh <imp@bsdimp.com>
Subject: Re: Bus space routines
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Cc: arch@freebsd.org, Niclas Zeising <zeising@freebsd.org>,
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On 2013-06-18 15:17:13 -0400, Warner Losh wrote:
> On Jun 18, 2013, at 12:50 PM, Jung-uk Kim wrote:
>> On 2013-06-18 06:56:15 -0400, Robert Millan wrote:
>>> I think the BSD world did the right thing by introducing new 
>>> semantics. Plus they're also more portable (on the hardware 
>>> sense), have a look, e.g.:
>> ...
>>> So why not just use those? It seems very natural to me that if
>>> you have something which is unambigous and reliable, you use
>>> this instead of something else which is prone to nasty errors.
>> 
>> bus_space(9!) is KPI and it must not be used on userland.
>> Actually, it only works on X86 by pure luck, e.g.,
>> bus_space_tag_t is an integral type, it has very simple
>> instructions to directly access I/O space, etc.
> 
> There's nothing preventing a bus_space implementation in user
> space. It's just that we don't have one yet, except on x86 where it
> works by luck.

Yeah, that's actually what I meant to say.

> On most architectures other than x86, however, it would likely be
> tricky to implement.

Linux implementation just uses native instructions for x86 and arm via
sys/io.h.  Other architectures (ab)use sysfs to expose PCI BARs to
userland.

http://cgit.freedesktop.org/xorg/lib/libpciaccess/tree/src/linux_sysfs.c

We can simply use io(4) for non-X86 platforms.

Jung-uk Kim
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