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

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Date:	Thu, 5 Jul 2012 17:40:45 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Matthew Wilcox <matthew@....cx>
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 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.

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.

Alan Stern

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