linux-kernel - Re: vga and 64-bit memcpy's

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Message-ID: <20140912112619.50d3b651@www.etchedpixels.co.uk>
Date:	Fri, 12 Sep 2014 11:26:19 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Dave Airlie <airlied@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: vga and 64-bit memcpy's

On Fri, 12 Sep 2014 13:57:20 +1000
Dave Airlie <airlied@...il.com> wrote:

> Got an bug report from someone using a silicon motion video card in
> VGA mode about corruption that they tracked down to 64-bit memory
> operations not being supported by the video card, it appears that we
> probably shouldn't be using > 32-bit copies on VGA memory.

VGA memory is write combining usually so the answer in general would be
no. You may also find its a combination of bridge and card and one or the
other is somehow out of spec.
 
> The include/linux/vt_buffer.h defaults scr_memcpyw defaults to using
> memcpy, which on 64-bit x86 machines ends up as rep movsq. Is there
> any way to make this use a 32-bit memcpy instead?

Please don't - you'll reduce performance for everyone else with saner
video cards.

Alan
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