Re: Inquiry data and emulated SG devices

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Mathieu Fluhr wrote:
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.


Quoting to the latest MtFuji draft (Section 17.7.1):
"The ANSI Version field shall contain a non-zero value to comply with this version of the Specification for a SCSI logical unit or zero for
  an ATAPI logical unit."


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 explain a lot... But (Sorry I am not a scsi mid-layer expert) why
faking what the device outputs?

The SCSI midlayer makes a lot of "if scsi version <= 2" choices. In the case of ATAPI, we do not want to force ATAPI down the path of ancient SCSI devices, as this disables some MMC features that modern ATAPI devices support.

	Jeff



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