Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752315AbZI2WgJ (ORCPT ); Tue, 29 Sep 2009 18:36:09 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751999AbZI2WgJ (ORCPT ); Tue, 29 Sep 2009 18:36:09 -0400 Received: from earthlight.etchedpixels.co.uk ([81.2.110.250]:50125 "EHLO www.etchedpixels.co.uk" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751857AbZI2WgI (ORCPT ); Tue, 29 Sep 2009 18:36:08 -0400 Date: Tue, 29 Sep 2009 23:35:05 +0100 From: Alan Cox To: Krzysztof Halasa Cc: Arjan van de Ven , "Lennart Baruschka" , linux-kernel@vger.kernel.org Subject: Re: Disabling DMA with ICH10? Message-ID: <20090929233505.7ff11cad@lxorguk.ukuu.org.uk> In-Reply-To: References: <1254173481.4454.32.camel@goodbyte.homelinux.com> <20090928234025.6dc4e3f7@lxorguk.ukuu.org.uk> <20090929075416.77960@gmx.net> <20090929101114.1712314b@infradead.org> <20090929180555.3c4a7259@lxorguk.ukuu.org.uk> X-Mailer: Claws Mail 3.7.2 (GTK+ 2.14.7; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1516 Lines: 43 On Tue, 29 Sep 2009 22:35:54 +0200 Krzysztof Halasa wrote: > Alan Cox writes: > > >> Not that long, there is no emulation there (except for port #80 (hex?) > >> and similar). Normal 33 MHz access. > > > > ATA accesses go across the cable and while some of them are snooped, > > cached and other magic is done there are cases it turns into a > > transaction back and forth with the drive - that *is* slow. PIO 0 in fact > > is ISA speed > > Sure, PATA inserts wait states, the 4 bus cycles per access (or > something like that) is the minimum for PIO on PCI. But it's still > normal 33 MHz access (with as many wait states as the PATA mode needs), > not ISA 8 MHz emulation (fastest PIO would be faster than ISA) You are missing the point. The sequence is INB foo CPU -> PCI device read this register PCI device -> Disk trundle whirr whirrr clunk clunk thud Disk -> PCI device PCI device -> CPU... "5" INB completes The INB is not interruptible mid instruction and stalls the CPU for the full period of the message passing back and forth across the bus. So PIO 0 on PCI stalls the bus for the equivalent of an ISA access, and PIO4 while a good deal faster is still a very long stall in hard real time terms. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/