Re: [RFC] remove bus_memio.h and bus_pio.h

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Date: Sun, 29 May 2005 11:36:07 -0600
To: "M. Warner Losh" <imp@bsdimp.com>

M. Warner Losh wrote:

> In message: <20050525.212009.71136852.nyan@jp.FreeBSD.org>
> Takahashi Yoshihiro <nyan@jp.FreeBSD.org> writes:
> : The bus_memio.h and bus_pio.h for a micro-optimization depend on the
> : implementation of the bus_space on i386 and amd64, so they are
> : meaningless files on the other archs. I'd like to remove a MD part
> : like this from MI drivers at least.
> :
> : I think that a increasing performance by using this method is very
> : trivial on recent machines. If there is not strong objection, I'll
> : remove bus_{mem,p}io.h and related code from all archs.
> :
> : Comments?
>
> Short answer:
>
> Great idea. aac and bfe should be tested after the change to
> see if there is any benefit for them. Other drivers almost
> certainly will see no benefit from this.
>
> Longer, more detailed answer.
>
> The original idea was to provide a hint to busspace that this driver
> only ever used a certain subset of the available mappings so it should
> assume that subset and agressively optimize the code. The assumption
> was that one could know at compile time that one would never use
> certain features. In an i386 centric world, this made good sense,
> especially since the bus_space_* macros expanded to inb or whatever
> and nothing else (compiler technology innovations may have changed
> this over time).
>
> You are correct in that other architectures might have more than two
> kinds of address space, might have other complicating factors. pc98
> has, as you know, an indirect vector because devices on the
> motherboard and cbus are rarely mapped at contiguous locations due to
> the dual 8-bit bus nature of the internal buses. In that case it
> makes no sense to do any optimization at all, and these files should
> be empty for such an implementation.
>
> Alpha, sparc64, powerpc and arm all have much more complex bus space
> implementations due to their greater intra-architectural differences,
> as well as their large difference with i386. To similarly optimize
> these architectures, one would need additional MD info to know how to
> inline things. None of them have chosen to support this level of
> optimization. It is unclear to me how big a win such optimiztion
> would be, even on the slower CPUs some of these platforms support.
>
> The lowest end of FreeBSD/i386 these days[*] is likely a Pentium II
> running at 300MHz or a soekris box. The 4510 box is still only
> 166MHz. However, the only device that it has that are likely to
> benefit from this is sio. Well, in extreme cases, one could make the
> case for any pci card or pccard, but I think that's too extreme to
> consider. Since the soekris box has only one free serial port, we
> need only keep up with a ppp connection on that serial port, so I'm
> pretty sure we're OK.
>
> A number of drivers include only one of these two include files:
> ti, bfe, trm, stg, scd, aac, kbd, ie, idt, hfa, gfb, fb, dpt,
> cnw, aic, aha, ahb, adv
> and some of the mii phy drivers, plus some other trivial uses.
>
> The only ones on the list that stand out are bfe and aac (the dpt
> optimization is only for EISA cards, and only for the EISA specific
> portions of the driver). I do not know how much this optimization
> helps these devices, but they are the only ones that I see might be
> affected. Simple benchmarks should be easy enough to do on aac and
> bfe.
>
> Warner
>
> [*] Yes, I know that slower CPUs are supported, and do still perform
> decently if you have enough memory. This is an arbitrary cutoff for
> the cost-benefit analysis.

This kind of makes me sad. I don't see how this was harming anything,
it just wasn't documented so people didn't know how to use it. If it
didn't apply to non-i386 and amd64, fine, just don't implement it for
those platform. This optimization might have seemed trivial, but it's
all of the little trivial optimizations that add up to make a nice
system. I'm guessing that Justin only put effort into this originally
because he did see a benefit; discounting it without doing any testing
of your own is a bit disingenuous.

Scott
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