linux-kernel - Re: [PATCH libata/for-3.16-fixes] libata: introduce ata_host->n

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Date:	Wed, 23 Jul 2014 18:46:05 +0200
From:	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jesse Brandeburg <jesse.brandeburg@...il.com>,
	Mike Qiu <qiudayu@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-ide@...r.kernel.org, wenxiong@...ux.vnet.ibm.com,
	brking@...ux.vnet.ibm.com, zhenghch@...ibm.com, haokexin@...il.com,
	Peter Hurley <peter@...leysoftware.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Alexey Kardashevskiy <aik@...abs.ru>
Subject: Re: [PATCH libata/for-3.16-fixes] libata: introduce ata_host->n_tags
 to avoid oops on SAS controllers

On Wednesday, July 23, 2014 12:36:01 PM Tejun Heo wrote:
> Hello,
> 
> On Wed, Jul 23, 2014 at 06:31:58PM +0200, Bartlomiej Zolnierkiewicz wrote:
> > Hmmm, wasn't ATA_MAX_QUEUE used before not ATA_MAX_QUEUE - 1?
> > 
> > It seems that after your patch the loop in the ata_qc_new() will use
> > only 30 tags and not 31 ones?
> 
> It was always 31 (ATA_MAX_QUEUE - 1) with the tag 31 reserved for EH
> commands.  The previous patch just used limit value which is one too
> high.  I'm planning to change that but this is the way it has always
> been.

I see, thanks for explaining this.

BTW:
		/* the last tag is reserved for internal command. */
		if (tag == ATA_TAG_INTERNAL)
			continue;

in ata_qc_new()'s loop now becomes a dead code (loop will be done
maximum ATA_MAX_QUEUE - 2 times and ATA_TAG_INTERNAL is defined
as ATA_MAX_QUEUE - 1).  Compiler can probably optimize it away so
as a bonus of your patch we may get one condition check less in
a hot-path. :)

Best regards,
--
Bartlomiej Zolnierkiewicz
Samsung R&D Institute Poland
Samsung Electronics

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