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Date: Tue, 21 Dec 1999 18:48:18 -0500
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From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Linus Torvalds <torvalds@transmeta.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, Jes Sorensen <Jes.Sorensen@cern.ch>, wendling@ganymede.isdn.uiuc.edu, linux-kernel@vger.rutgers.edu
In-reply-to: Linus Torvalds's message of Sat, 18 Dec 1999 15:50:19 -0800 (PST), <Pine.LNX.4.10.9912181546480.1964-100000@penguin.transmeta.com>
Subject: Re: [patch] read[bwl] and ioremap problem
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   Date:   Sat, 18 Dec 1999 15:50:19 -0800 (PST)
   From: Linus Torvalds <torvalds@transmeta.com>

   THAT case is certainly a rather strong argument for using something
   like "gsc_read[bwl]()" on HP-PA.

The other strong reason for doing gsc_read[bwl] on HP-PA is that some
PA-RISC boxes have both a GSC bus *and* a PCI bus, and you need to
access devices on both buses....

Something to consider is that for certain drivers, such as the serial
driver, I'm already having to do a serial_inp() which dispatches to the
proper {inb,readb,gsc_readb} already.  Yes I take a overhead/performance
hit for doing this, but it's the only clean way to support both ISA and
PCI serial boards in a single i386 box, or to support multiple buses in
the HP-PA scenario.

We've historically said that this kind of thing is horrible for
performance reasons, and the SCO and NetBSD approaches of doing
parameterized I/O has been derided for that reason.  However, it's
something that perhaps we should rethink; on modern CPU's, the extra
procedure activation/deactivation isn't *that* expensive, and it ends up
making the drivers much more portable and easier to support multiple
architectures.  The alternative is that each driver author ends up
writing their own I/O dispatch routines, such as what's currently in the
serial driver.  While this approach does have some advantages, in that
each driver author can decide whether or not he/she wishes to pay the
indirection overhead, it can mean code duplication and a delay before
certain devices get supported on non-mainline architectures.

Something to think about.

						- Ted

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