Subject: Re: Inquiry data and emulated SG devices
From: James Bottomley <James.Bottomley@SteelEye.com>
To: Mathieu Fluhr <mfluhr@nero.com>
Cc: Jeff Garzik <jeff@garzik.org>, Robert Hancock <hancockr@shaw.ca>,
       LKML <linux-kernel@vger.kernel.org>, ide <linux-ide@vger.kernel.org>,
       linux-scsi <linux-scsi@vger.kernel.org>
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Date: Thu, 18 Oct 2007 08:13:22 -0400
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On Thu, 2007-10-18 at 13:57 +0200, Mathieu Fluhr wrote:
> On Thu, 2007-10-18 at 06:06 -0400, Jeff Garzik wrote:
> 
> > 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.
> 
> If I fully understand your point, if this faking code wasn't present,
> and if the inquiry data was left as it is, all modern ATAPI devices
> would be considered as really old SCSI devices. Am I right?

Not really.  Firstly, a lot of ATAPI devices report the correct
compliance level and secondly, the consequence of reporting no
compliance (level 0) is that the mid layer will be very careful to use
the most minimal command set it can (the usual consequence of sending a
USB device a command it doesn't understand is to have it crash).

However, this will have no effect for at least CDs:  The sr driver is
only interested in the MMC compliance level not the SCSI compliance
level.

> And then why external (FireWire and USB) devices reports the inquiry
> data correctly? After all they are also considered to be SCSI devices...
>  -> But apparently, from what /proc/scsi/scsi outputs, the ANSI SCSI   
>     revision is set to 0 (for both ieee1394 and USB devices). This 
>     seems to be coherent with what the inquiry data buffer outputs.
> 
> 
> Also another question, more from the developer point of view, you have
> to know which real interface is behind a device. You might get
> performance loss if the real type is not known.
> 
> For example, I send a BLANK cdb to fully blank a disc, with the 'immed'
> flag not set, so that the requested operation is processed to completion
> prior to returning status.
> - for real SCSI devices, this is not harmfull, as the bus is not blocked
> - for IDE devices, AFAIK, the bus will be blocked until the command
> termination... so for a CD erased a 1x, the bus might be blocked for
> 74min.
> 
> 
> Also the SCSI commands that are sent to the device depends from its
> hardware bus type. Not only for the CDB length, but also for MODE
> SENSE/MODE SELECT CDBs in which for example a MODE SENSE (6) would fail
> on an IDE device... even if it is described in the SPC-3 standard.

Er, I think you might have a slight misunderstanding of what the mid
layer does with the scsi level.  It doesn't police incoming commands
that are user generated (the user is free to crash the device or set
their house on fire) according to the compliance level.  All it does is
try to make sure that the mid-layer transactions (which are pretty much
inquiry, mode sense and test unit ready) are all as minimal as possible.

> As far as I saw in libata-scsi.c there a SCSI-to-ATAPI and ATAPI-to-SCSI
> translator that automatically transform the command sent... Am I also
> right on this point?

The former only, I believe.

James


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