linux-kernel - Re: [ 38/48] SCSI & usb-storage: add try_rc_10

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Date:	Thu, 5 Jul 2012 21:05:26 -0600
From:	Matthew Wilcox <matthew@....cx>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hans de Goede <hdegoede@...hat.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"alan@...rguk.ukuu.org.uk" <alan@...rguk.ukuu.org.uk>,
	Matthew Dharm <mdharm-usb@...-eyed-alien.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: [ 38/48] SCSI & usb-storage: add try_rc_10_first flag

On Thu, Jul 05, 2012 at 05:40:45PM -0400, Alan Stern wrote:
> On Tue, 3 Jul 2012, Matthew Wilcox wrote:
> 
> > I think the necessary algorithm is simpler than that:
> > 
> > Send RC10 (unless the device supports PI, in which case it's probably
> > 	enterprisey and well-tested)
> > Send RC16
> > If RC10 capacity agrees with RC16 capacity, use extra RC16 data.
> > (for values of "agrees with" that include the "-1 to use RC16" indicator)
> > 
> > Sure, it's one extra command, but really, who cares?
> 
> Well, some USB devices might care.  Right now we don't send RC16 to 
> them at all because they get RC10 first and it indicates a capacity 
> below 2 TB.

Mmm.  I think I forgot a step:

Send RC10 (unless the device supports PI, in which case it's probably
	enterprisey and well-tested)
If the device indicates a level < SPC_2, stop here
Send RC16
If RC10 capacity agrees with RC16 capacity, use extra RC16 data.
 (for values of "agrees with" that include the "-1 to use RC16" indicator)

> In the end, usb-storage may be forced to use the NO_READ_CAPACITY_16 
> flag with all devices except those we can positively identify as a 
> USB-(S)ATA bridge.

Is that really an improvement over nobbling the IDENTIFY results to
force SCSI_2?

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."
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