From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tejun Heo Subject: Re: [PATCH 14/14] libata: use PIO for misc ATAPI commands Date: Fri, 30 Nov 2007 07:52:04 +0900 Message-ID: <474F4294.9030203@gmail.com> References: <1196346817387-git-send-email-htejun@gmail.com> <11963468202627-git-send-email-htejun@gmail.com> <20071129160555.38ee2edc@the-village.bc.nu> <474EE561.7010207@gmail.com> <474EE5F8.1030409@gmail.com> <474F0B11.7000205@rtr.ca> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: Received: from rv-out-0910.google.com ([209.85.198.188]:18553 "EHLO rv-out-0910.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756974AbXK2WwN (ORCPT ); Thu, 29 Nov 2007 17:52:13 -0500 Received: by rv-out-0910.google.com with SMTP id k20so1722424rvb for ; Thu, 29 Nov 2007 14:52:11 -0800 (PST) In-Reply-To: <474F0B11.7000205@rtr.ca> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Mark Lord Cc: Alan Cox , jeff@garzik.org, linux-ide@vger.kernel.org, albertl@mail.com, jens.axboe@oracle.com Mark Lord wrote: > I'm split on this one. For fast systems (typical notebook/desktop) it's > almost a non-issue either way. > > But a lot of media boxes will be using this code, and they tend to have > very low-power, slow-clockrate CPUs in the 200-800Mhz range, and so the > real-time hit there (from PIO) will have a much more significant impact. > > Using DMA as much as possible on those slower platforms is definitely > a plus towards avoiding non-jerky, skipped frames, or start-stoppy DVD > recording. > > Now the most intensive commands are still DMA under the proposed scheme, > so it's not those that one would be concerned with. But dropping to PIO > even for a few uncommon commands will still peg a real-time hit or two > on a slower media processor. > > So.. ???? One thing I don't understand about this argument is that PIO cycle time is not determined by CPU power. It's bound by PCI and ATA bus speed. If you put a faster CPU on the job, it just ends up wasting the same amount of time burning up more CPU cycles or am I misjudging power of those embedded CPUs? -- tejun