ore: Support for partial component table

Users like the objlayout-driver would like to only pass
a partial device table that covers the IO in question.
For example exofs divides the file into raid-group-sized
chunks and only serves group_width number of devices at
a time.

The partiality is communicated by setting
ore_componets->first_dev and the array covers all logical
devices from oc->first_dev upto (oc->first_dev + oc->numdevs)

The ore_comp_dev() API receives a logical device index
and returns the actual present device in the table.
An out-of-range dev_index will BUG.

Logical device index is the theoretical device index as if
all the devices of a file are present. .i.e:
	total_devs = group_width * mirror_p1 * group_count
	0 <= dev_index < total_devs
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>

fs/exofs/exofs.h

@@ -217,6 +217,7 @@ static inline void exofs_init_comps(struct ore_components *oc,
 	one_comp->obj.id = oid;
 	exofs_make_credential(one_comp->cred, &one_comp->obj);
 
+	oc->first_dev = 0;
 	oc->numdevs = sbi->layout.group_width * sbi->layout.mirrors_p1 *
 							sbi->layout.group_count;
 	oc->single_comp = EC_SINGLE_COMP;

fs/exofs/ore.c

@@ -62,6 +62,10 @@ static struct osd_obj_id *_ios_obj(struct ore_io_state *ios, unsigned index)
 
 static struct osd_dev *_ios_od(struct ore_io_state *ios, unsigned index)
 {
+	ORE_DBGMSG2("oc->first_dev=%d oc->numdevs=%d i=%d oc->ods=%p\n",
+		    ios->oc->first_dev, ios->oc->numdevs, index,
+		    ios->oc->ods);
+
 	return ore_comp_dev(ios->oc, index);
 }
 

include/scsi/osd_ore.h

@@ -49,6 +49,7 @@ struct ore_dev {
 };
 
 struct ore_components {
+	unsigned	first_dev;		/* First logical device no    */
 	unsigned	numdevs;		/* Num of devices in array    */
 	/* If @single_comp == EC_SINGLE_COMP, @comps points to a single
 	 * component. else there are @numdevs components
@@ -70,14 +71,14 @@ struct ore_components {
 static inline struct osd_dev *ore_comp_dev(
 	const struct ore_components *oc, unsigned i)
 {
-	BUG_ON(oc->numdevs <= i);
-	return oc->ods[i]->od;
+	BUG_ON((i < oc->first_dev) || (oc->first_dev + oc->numdevs <= i));
+	return oc->ods[i - oc->first_dev]->od;
 }
 
 static inline void ore_comp_set_dev(
 	struct ore_components *oc, unsigned i, struct osd_dev *od)
 {
-	oc->ods[i]->od = od;
+	oc->ods[i - oc->first_dev]->od = od;
 }
 
 struct ore_striping_info {