linux-kernel - Re: libata passthru: support PIO multi commands

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Date:	Tue, 12 Jun 2007 17:05:19 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	Albert Lee <albertcc@...ibm.com>
Subject: Re: libata passthru: support PIO multi commands

> ata_scsi_pass_thru() is not executed at ioctl submission time (block 
> queue submission time), but rather immediately before it is issued to 
> the drive.  At that point you know the bus is idle, all other commands 
> have finished executing, and dev->multi_count is fresh not stale.  The 
> code path goes from ata_scsi_pass_thru() to ata_qc_issue() without 
> releasing the spinlock, even.

Think up to user space

Poorusersapp			set multicount to 4

Evilproprietarytuningdaemon	set multicount to 8

Poorusersapp			issue I/O

at which point an error is indeed best.

> But the last point is true -- we should error rather than just warn 
> there, AFAICS.

Definitely. We've been asked "please do something stupid" and not even in
a case where the requester may know better.

Alan
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