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

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Message-ID: <4717302D.4050107@garzik.org>
Date:	Thu, 18 Oct 2007 06:06:37 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Mathieu Fluhr <mfluhr@...o.com>
CC:	Robert Hancock <hancockr@...w.ca>,
	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

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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