linux-kernel - Re: Inquiry data and emulated SG devices

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Message-Id: <1192705055.13229.3.camel@localhost.localdomain>
Date:	Thu, 18 Oct 2007 06:57:35 -0400
From:	James Bottomley <James.Bottomley@...elEye.com>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Robert Hancock <hancockr@...w.ca>, Mathieu Fluhr <mfluhr@...o.com>,
	LKML <linux-kernel@...r.kernel.org>,
	ide <linux-ide@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: Inquiry data and emulated SG devices

On Wed, 2007-10-17 at 21:31 -0400, Jeff Garzik wrote:
> Robert Hancock wrote:
> > This doesn't seem a very reliable way to identify an IDE device, as all
> > that 0 means is that the device does not claim conformance to any
> > standard. I would think it would be legitimate for an IDE device to put
> > a value like 5 in there as well, if it complies with SPC-4..
> 
> Via the this-doesnt-really-matter-but-it-should-be-noted department:
> 
> According to the latest on t10.org, MMC retroactively permitted SCSI 
> version to be zero, for MMC-compliant USB and ATAPI devices.
> 
> 
> > In the case of libata though, that appears to be due to this code in
> > drivers/ata/libata-scsi.c:
> > 
> >     /* ATAPI devices typically report zero for their SCSI version,
> >      * and sometimes deviate from the spec WRT response data
> >      * format.  If SCSI version is reported as zero like normal,
> >      * then we make the following fixups:  1) Fake MMC-5 version,
> >      * to indicate to the Linux scsi midlayer this is a modern
> >      * device.  2) Ensure response data format / ATAPI information
> >      * are always correct.
> >      */
> >             if (buf[2] == 0) {
> >                 buf[2] = 0x5;
> >                 buf[3] = 0x32;
> >             }
> > 
> > This technically seems to go against what the SCSI/ATA Translation (SAT)
> > spec says regarding INQUIRY on ATAPI devices: "the SATL shall use the
> > ATA PACKET Command feature set to pass all INQUIRY commands and
> > parameter data to the ATAPI device without altering the INQUIRY
> > commands or the parameter data." However, it might realistically be
> > needed if the SCSI layer in the kernel has problems with a device
> > indicating it supports no SCSI version..
> 
> The above tweak is entirely software->software communication...  as the 
> comment you quoted notes, it's just a signal to the SCSI midlayer.
> 
> At the moment, the SCSI midlayer assumes any device that reports scsi 
> version as less than 2 is forced to SCSI version 2.  Ultimately that's 
> incorrect behavior for all ATAPI devices (and later MMC revisions).

Actually, no we don't.  SCSI level 0 means "no compliant standard
specified".  We're quite careful if we see this not to do anything that
might upset the device ... SCSI level 0 is a fairly usual thing to see
on USB devices, which is why we process it differently.

> At the time, libata simply worked around this SCSI buglet in its own 
> code, since that was easier than auditing all SCSI code paths to ensure 
> new ATAPI/USB MMC logic does not break ancient devices.
> 
> But if someone is motivated enough to revisit this...

Like I said, it should all be fixed ... if you try it.

James


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