megaraid_sas: Optionally add SCSI devices for individual disks in a raidset

[Joe Malicki <jmalicki@metacarta.com>]

This is very dangerous, and you shouldn't use it/apply it unless you're feeling adventurous.

This lets one, optionally, add /dev/sg* devices, and optionally also /dev/sd* devices, for
disks in a RAIDset.  It is modeled after similar functionality in the aacraid driver.

I've been successfully using this to upgrade individual disk firmwares behind a Dell PERC 
RAID controller with sgdskfl from scsirastools - AFTER the RAID array has been stopped
with other tools.

Use the kernel option "megaraid_sas.export_disks=-1" to get /dev/sg* devices.  Unless a
specific option has been given, the patch doesn't modify behavior.

Note that the driver currently says (and has said for some time):

         * FIXME: Currently we don't export them to the midlayer at all.
	 * 	  That will be fixed once LSI engineers have audited the
	 * 	  firmware for possible issues.

So using this is certainly high risk and one should be careful.
(only signing off so others may feel free to use it, not encouraging this
to go anywhere unless LSI feels it's safe)

Signed-off-by: jmalicki@metacarta.com

------------
--- linux-2.6.26.2-orig/drivers/scsi/megaraid/megaraid_sas.c    2008-08-06 12:19:01.000000000 -0400
+++ linux-2.6.26.2/drivers/scsi/megaraid/megaraid_sas.c 2008-08-19 17:12:02.000000000 -0400
@@ -46,6 +46,12 @@
 #include <scsi/scsi_host.h>
 #include "megaraid_sas.h"

+static int export_disks = 0;
+module_param(export_disks, int, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(export_disks, "Export physical disks to the SCSI midlayer (DANGEROUS)"
+       " -1=protect 0=off 1=on");
+
+
 /*
  * poll_mode_io:1- schedule complete completion from q cmd
  */
@@ -905,8 +911,12 @@
         *        That will be fixed once LSI engineers have audited the
         *        firmware for possible issues.
         */
-       if (sdev->channel < MEGASAS_MAX_PD_CHANNELS && sdev->type == TYPE_DISK)
-               return -ENXIO;
+       if (sdev->channel < MEGASAS_MAX_PD_CHANNELS && sdev->type == TYPE_DISK)
{
+               if (export_disks < 0)
+                       sdev->no_uld_attach = 1;
+               else if (export_disks == 0)
+                       return -ENXIO;
+       }

        /*
         * The RAID firmware may require extended timeouts.
--------