linux-kernel - Re: [PATCH] x86: Split off mem*io functions

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Message-ID: <4EE7E3F5.1020700@zytor.com>
Date:	Tue, 13 Dec 2011 15:47:01 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Brian Gerst <brgerst@...il.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, tim@...ngt.org
Subject: Re: [PATCH] x86: Split off mem*io functions

On 12/13/2011 02:58 PM, Linus Torvalds wrote:
> 
>  - some devices (for example, traditional CGA/VGA) read and write to
> different banks and bit modes, so a "read-mask-write" operation just
> does not work at all. You have to do pure writes when copying to such
> a destination. Afaik, none of our *current* memory copies do this for
> partial words, but it's an example of something that is valid (and
> sometimes done) in memcpy to avoid unaligned stores, for example.
> 

VGA is even weirder: in some modes (which noone in their sane mind use)
you can make it do operations versus an internal buffer.  The catch is
that the internal buffer is only 8 bits wide, so anything other than
byte writes will fail in the extreme in those modes.

	-hpa

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