linux-kernel - Re: [PATCH 20/22] atari_scsi: Set a reasonable default for cmd_per

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Date:	Tue, 15 Mar 2016 14:27:29 +1100 (AEDT)
From:	Finn Thain <fthain@...egraphics.com.au>
To:	Hannes Reinecke <hare@...e.de>
cc:	"James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Michael Schmitz <schmitzmic@...il.com>,
	linux-m68k@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Ondrej Zary <linux@...nbow-software.org>,
	Sam Creasey <sammy@...my.net>
Subject: Re: [PATCH 20/22] atari_scsi: Set a reasonable default for
 cmd_per_lun


On Mon, 14 Mar 2016, Hannes Reinecke wrote:

> On 03/14/2016 05:27 AM, Finn Thain wrote:
> > This setting does not need to be conditional on Atari ST or TT.
> > 
> > Without TCQ support, cmd_per_lun == 2 is probably reasonable...
> > 
> > Signed-off-by: Finn Thain <fthain@...egraphics.com.au>
> > 
> > ---
> >  drivers/scsi/atari_scsi.c |    3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > Index: linux/drivers/scsi/atari_scsi.c
> > ===================================================================
> > --- linux.orig/drivers/scsi/atari_scsi.c	2016-03-14 15:26:45.000000000 +1100
> > +++ linux/drivers/scsi/atari_scsi.c	2016-03-14 15:26:55.000000000 +1100
> > @@ -750,6 +750,7 @@ static struct scsi_host_template atari_s
> >  	.eh_abort_handler	= atari_scsi_abort,
> >  	.eh_bus_reset_handler	= atari_scsi_bus_reset,
> >  	.this_id		= 7,
> > +	.cmd_per_lun		= 2,
> >  	.use_clustering		= DISABLE_CLUSTERING,
> >  	.cmd_size		= NCR5380_CMD_SIZE,
> >  };
> _2_ ? Are you being overly cheeky here?
> I sincerely doubt the driver is capable of submitting two
> simultaneous commands ...

Right. The LLD has LU busy flags to prevent a LU from being issued more 
than one command.

> Care to explain?

It seemed harmless and it is consistent with the all of the other 5380 
drivers.

I don't know why it was done that way. Perhaps it was done to create a 
pipeline. That is, to keep a small number of commands in the LLD issue 
queue so that the NCR5380_main() work item does not have to terminate and 
then get requeued needlessly.

-- 

> 
> Cheers,
> 
> Hannes
>