cam: kern.cam.max_high_power tuneable / sysctl

Create a tunable for the maxinum number of 'high power' commands to
schedule, kern.cam.max_high_power.  Default remains at 4.

Differential Revision:	https://reviews.freebsd.org/D56462

sys/cam/cam_xpt.c

@@ -86,8 +86,8 @@ SDT_PROBE_DEFINE4(cam, , xpt, async__cb, "void *", "uint32_t",
 _Static_assert(XPT_PRINT_LEN <= XPT_PRINT_MAXLEN, "XPT_PRINT_LEN is too large");
 
 /*
- * This is the maximum number of high powered commands (e.g. start unit)
- * that can be outstanding at a particular time.
+ * This sets a default for the the maximum number of high powered commands
+ * (e.g. start unit) that can be outstanding at a particular time.
  */
 #ifndef CAM_MAX_HIGHPOWER
 #define CAM_MAX_HIGHPOWER  4
@@ -168,6 +168,9 @@ SYSCTL_INT(_kern_cam, OID_AUTO, boot_delay, CTLFLAG_RDTUN,
            &xsoftc.boot_delay, 0, "Bus registration wait time");
 SYSCTL_UINT(_kern_cam, OID_AUTO, xpt_generation, CTLFLAG_RD,
 	    &xsoftc.xpt_generation, 0, "CAM peripheral generation count");
+SYSCTL_INT(_kern_cam, OID_AUTO, max_high_power, CTLFLAG_RWTUN,
+           &xsoftc.num_highpower, 0,
+	   "Max number of high power commands to be issued at once");
 
 struct cam_doneq {
 	struct mtx_padalign	cam_doneq_mtx;
@@ -892,7 +895,10 @@ xpt_init(void *dummy)
 	TAILQ_INIT(&xsoftc.xpt_busses);
 	TAILQ_INIT(&xsoftc.ccb_scanq);
 	STAILQ_INIT(&xsoftc.highpowerq);
-	xsoftc.num_highpower = CAM_MAX_HIGHPOWER;
+
+	/* Fall back to a default if the kenv tunable isn't set */
+	if (xsoftc.num_highpower == 0)
+		xsoftc.num_highpower = CAM_MAX_HIGHPOWER;
 
 	mtx_init(&xsoftc.xpt_highpower_lock, "XPT highpower lock", NULL, MTX_DEF);
 	xsoftc.xpt_taskq = taskqueue_create("CAM XPT task", M_WAITOK,