Re: Memory mapped IO vs Port IO

From: Jamie Lokier <jamie@shareable.org>
To: Andi Kleen <ak@suse.de>
Cc: Matthew Wilcox <willy@debian.org>, linux-kernel@vger.kernel.org
Subject: Re: Memory mapped IO vs Port IO
Date: Thu, 11 Sep 2003 18:13:16 +0100	[thread overview]
Message-ID: <20030911171316.GI29532@mail.jlokier.co.uk> (raw)
In-Reply-To: <p73oexri9kx.fsf@oldwotan.suse.de>

Andi Kleen wrote:
> My gut feeling is to just fix the drivers to make this runtime switchable
> and get rid of the compile time options.
> 
> This would help distributions (who normally want to build conservative
> by default, but still allow the users easy tuning without recompilation) 
> For that it would be nice if a standard module parameter or maybe 
> sysfs option existed.

Another way to help distributions is to compile those drivers twice,
once for each access type.  There aren't all that many drivers that
need it.

> The overhead of checking for PIO vs mmio at runtime in the drivers
> should be completely in the noise on any non ancient CPU (both MMIO
> and PIO typically take hundreds or thousands of CPU cycles for the bus
> access, having an dynamic function call or an if before that is makes
> no difference at all)

Ah, but what about the ancient CPUs?

-- Jamie