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

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Date:	Tue, 3 Jul 2012 11:32:31 -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 Tue, Jul 03, 2012 at 11:49:00AM -0400, Alan Stern wrote:
> On Tue, 3 Jul 2012, James Bottomley wrote:
> 
> > >   What happened is that T10
> > > in their infinite wisdom decided to put things like "supports TRIM" and
> > > "is actually a 4k block size but fakes 512 byte blocks" in the Read
> > > Capacity 16 results.  So if we want to support those kinds of things
> > > (and I think we do), then we need to send Read Capacity 16 to devices.
> > 
> > But anyway, we're stuck ... we have to send RC16 first to support these
> > features.  We did protest to T10 at the time, but to no avail.
> 
> Does it have to be sent _first_?
> 
> Or would it be okay to send _both_ commands and believe the RC10
> capacity rather than the RC16 capacity if they differ?

I have no problem with doing that (and believing RC16 over RC10 if RC10
claims 0xffffffff, naturally).  The problem, as I understand it, is that
some devices crash upon receiving RC16 rather than just returning nonsense.

> > I still think a whitelist of USB devices sending proper SCSI level
> > information in the inquiry might be the best way forward.
> 
> I'm doubtful.  It wouldn't be at all surprising for devices to claim 
> they support a particular level when in fact they support some but not 
> all of the required commands.  Then what do you do?  Put them on the 
> whitelist because of the commands they support, or leave them off 
> because of the other ones?  What happens later on when you decide to 
> use more of the required commands?

SCSI has a fairly extensive collection of black/white list flags already
... see scsi_devinfo.[ch].

-- 
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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