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Subject: Re: Problem with aacraid driver in 2.5.63-bk-latest
From: Mark Haverkamp <markh@osdl.org>
To: Doug Ledford <dledford@redhat.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, dougg@torque.net,
       Christoffer Hall-Frederiksen <hall@jiffies.dk>,
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On Thu, 2003-03-13 at 15:17, Doug Ledford wrote:
> Mark Haverkamp wrote:
> 
> > Does the cmd_per_lun element of the Scsi_Host_Template structure serve
> > more than one purpose? 
> 
> Only when inappropriately abused by LLDD authors.  The cmd_per_lun value 
> is suppossed to be for untagged devices only!  If you have a tape drive 
> that doesn't support tagged commands but you want to be able to 
> internally have the next command queued up and ready to go when the 
> current command completes (in order to keep it streaming better), then 
> you can set cmd_per_lun to 2 and you will get two outstanding commands 
> for this device at a time.  I used that so that in my interrupt handler 
> I could send the next command to the device before I passed the 
> completed command up to the SCSI layer.
> 
> > In scsi_alloc_sdev it is passed into
> > scsi_adjust_queue_depth.  In the aacraid case this is 512.  Later the
> > aacraid driver (in aac_slave_configure) sets the queue depth to either
> > 128 for tagged or 1 if not.
> 
> That's where you are suppossed to set the queue depth on tagged devices.

Then it sounds like the aacraid driver could set cmd_per_lun to a small
number like one since the real queue depth will be set later in
aac_slave_configure.

Mark.
-- 
Mark Haverkamp <markh@osdl.org>

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