From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Garzik Subject: Re: [RFC 1/4] libata: cache device select Date: Mon, 01 Mar 2010 15:15:53 -0500 Message-ID: <4B8C2079.7010607@garzik.org> References: <20100217130847.16338.55586.stgit@localhost.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from mail-yw0-f197.google.com ([209.85.211.197]:51850 "EHLO mail-yw0-f197.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751354Ab0CAUPz (ORCPT ); Mon, 1 Mar 2010 15:15:55 -0500 In-Reply-To: <20100217130847.16338.55586.stgit@localhost.localdomain> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Alan Cox Cc: jeff@garzik.org.com, linux-kernel@vger.kernel.org, linux-ide@vger.kernel.org On 02/17/2010 08:10 AM, Alan Cox wrote: > Avoid the device select overhead on every qc_issue (> 10uS) by caching the > currently selected device. This shows up on profiles under load. Best case > this costs us 10uS for the delay, worst case with a dumb interface it's > costing us about *1mS* a command. > > I believe the logic here is sufficient, but would welcome some second reviews > as its not something you want to get wrong ! > > > Signed-off-by: Alan Cox > --- > > drivers/ata/libata-sff.c | 8 ++++++-- > include/linux/libata.h | 1 + > 2 files changed, 7 insertions(+), 2 deletions(-) > > > diff --git a/drivers/ata/libata-sff.c b/drivers/ata/libata-sff.c > index 63d9c6a..cf0332a 100644 > --- a/drivers/ata/libata-sff.c > +++ b/drivers/ata/libata-sff.c > @@ -469,6 +469,7 @@ void ata_sff_dev_select(struct ata_port *ap, unsigned int device) > > iowrite8(tmp, ap->ioaddr.device_addr); > ata_sff_pause(ap); /* needed; also flushes, for mmio */ > + ap->sff_selected = device; > } > EXPORT_SYMBOL_GPL(ata_sff_dev_select); > > @@ -1538,7 +1539,8 @@ unsigned int ata_sff_qc_issue(struct ata_queued_cmd *qc) > } > > /* select the device */ > - ata_dev_select(ap, qc->dev->devno, 1, 0); > + if (qc->dev->devno != ap->sff_selected) > + ata_dev_select(ap, qc->dev->devno, 1, 0); > > /* start the command */ > switch (qc->tf.protocol) { My main worry here is that this logic excises the 150ms wait in ata_dev_select() that has been used effectively to allow ATAPI devices to "collect themselves" after waiting for idle, prior to command issuance. Jeff