Re: [PATCH] block: add support for shared tag maps

[Ravi Anand <ravi.anand@qlogic.com>]

>On Fri, 01 Sep 2006, Doug Maxey wrote:
> Ravi,
> 
> While working on a patch to add shared tags to qla4xxx was looking at
> the shost->can_queue settings, I see the value is set pretty high:
> 
> qla4xxx_probe()
> ...
> 	host->can_queue = REQUEST_QUEUE_DEPTH + 128;
> 
> where REQUEST_QUEUE_DEPTH works out to be 1024.
> 
> My question:
> what is the relationship between the can_queue and the
> setting in
> qla4xxx_slave_configure()
> 	if (sdev->tagged_supported)
> 		scsi_activate_tcq(sdev, 32);
> 	else
> 		scsi_deactivate_tcq(sdev, 32);
> 
> Does this imply that the firmware can ultimately track more requests
> than we can possibly stuff in it?  Where do the other 1012 requests get
> queued, in the block layer?

My understanding is that the "can_queue" is on a per HBA basis.
In other words maximum no of cmd's a give host adapter can
queue at any given point of time. 

While the setting the slave_configure() is on a per lun basis.
Which basically means max no of simultaneous cmd's that
can be outstanding for any given lun.

> 
> James' shared tag patch for stex has the following
> +stex_slave_alloc(struct scsi_device *sdev)
> +{
> +	/* Cheat: usually extracted from Inquiry data */
> +	sdev->tagged_supported = 1;
> +
> +	scsi_activate_tcq(sdev, sdev->host->can_queue);
> 
> where in this case the can_queue is set to ST_CAN_QUEUE, which works
> out to 32.

Dont know much about stex driver. So cant comment on it. 
James any thought ? 

Ravi






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