xhci: Reserve one command for USB3 LPM disable.

We want to do everything we can to ensure that USB 3.0 Link Power
Management (LPM) can be disabled when it is enabled. If LPM can't be
disabled, we can't suspend USB 3.0 devices, or reset them. To make sure
we can submit the command to disable LPM, allocate a command in the
xhci_hcd structure, and reserve one TRB on the command ring.

We only need one command per xHCI driver instance, because LPM is only
disabled or enabled while the USB core is holding the bandwidth_mutex
that is shared between the xHCI USB 2.0 and USB 3.0 roothubs. The
bandwidth_mutex will be held until the command completes, or times out.

Signed-off-by: Sarah Sharp <[e-mail]>

drivers/usb/host/xhci-mem.c

···
 	xhci->event_ring = NULL;
 	xhci_dbg(xhci, "Freed event ring\n");
 
+	if (xhci->lpm_command)
+		xhci_free_command(xhci, xhci->lpm_command);
 	xhci->cmd_ring_reserved_trbs = 0;
 	if (xhci->cmd_ring)
 		xhci_ring_free(xhci, xhci->cmd_ring);
···
 	xhci_dbg(xhci, "// Setting command ring address to 0x%x\n", val);
 	xhci_write_64(xhci, val_64, &xhci->op_regs->cmd_ring);
 	xhci_dbg_cmd_ptrs(xhci);
+
+	xhci->lpm_command = xhci_alloc_command(xhci, true, true, flags);
+	if (!xhci->lpm_command)
+		goto fail;
+
+	/* Reserve one command ring TRB for disabling LPM.
+	 * Since the USB core grabs the shared usb_bus bandwidth mutex before
+	 * disabling LPM, we only need to reserve one TRB for all devices.
+	 */
+	xhci->cmd_ring_reserved_trbs++;
 
 	val = xhci_readl(xhci, &xhci->cap_regs->db_off);
 	val &= DBOFF_MASK;

drivers/usb/host/xhci.h

···
 	/* slot enabling and address device helpers */
 	struct completion	addr_dev;
 	int slot_id;
+	/* For USB 3.0 LPM enable/disable. */
+	struct xhci_command		*lpm_command;
 	/* Internal mirror of the HW's dcbaa */
 	struct xhci_virt_device	*devs[MAX_HC_SLOTS];
 	/* For keeping track of bandwidth domains per roothub. */