usb: xhci: do not free an empty cmd ring

It was first found on HUAWEI Kirin 9006C platform with a builtin xhci
controller during stress cycle test(stress-ng, glmark2, x11perf, S4...).

phase one:
[26788.706878] PM: dpm_run_callback(): platform_pm_thaw+0x0/0x68 returns -12
[26788.706878] PM: Device xhci-hcd.1.auto failed to thaw async: error -12
...
phase two:
[28650.583496] [2023:01:19 04:43:29]Unable to handle kernel NULL pointer dereference at virtual address 0000000000000028
...
[28650.583526] user pgtable: 4k pages, 39-bit VAs, pgdp=000000027862a000
[28650.583557] [0000000000000028] pgd=0000000000000000
...
[28650.583587] pc : xhci_suspend+0x154/0x5b0
[28650.583618] lr : xhci_suspend+0x148/0x5b0
[28650.583618] sp : ffffffc01c7ebbd0
[28650.583618] x29: ffffffc01c7ebbd0 x28: ffffffec834d0000
[28650.583618] x27: ffffffc0106a3cc8 x26: ffffffb2c540c848
[28650.583618] x25: 0000000000000000 x24: ffffffec82ee30b0
[28650.583618] x23: ffffffb43b31c2f8 x22: 0000000000000000
[28650.583618] x21: 0000000000000000 x20: ffffffb43b31c000
[28650.583648] x19: ffffffb43b31c2a8 x18: 0000000000000001
[28650.583648] x17: 0000000000000803 x16: 00000000fffffffe
[28650.583648] x15: 0000000000001000 x14: ffffffb150b67e00
[28650.583648] x13: 00000000f0000000 x12: 0000000000000001
[28650.583648] x11: 0000000000000000 x10: 0000000000000a80
[28650.583648] x9 : ffffffc01c7eba00 x8 : ffffffb43ad10ae0
[28650.583648] x7 : ffffffb84cd98dc0 x6 : 0000000cceb6a101
[28650.583679] x5 : 00ffffffffffffff x4 : 0000000000000001
[28650.583679] x3 : 0000000000000011 x2 : 0000000000e2cfa8
[28650.583679] x1 : 00000000823535e1 x0 : 0000000000000000

gdb:
(gdb) l *(xhci_suspend+0x154)
0xffffffc010b6cd44 is in xhci_suspend (/.../drivers/usb/host/xhci.c:854).
849	{
850		struct xhci_ring *ring;
851		struct xhci_segment *seg;
852
853		ring = xhci->cmd_ring;
854		seg = ring->deq_seg;
(gdb) disassemble 0xffffffc010b6cd44
...
0xffffffc010b6cd40 <+336>:	ldr	x22, [x19, torvalds#160]
0xffffffc010b6cd44 <+340>:	ldr	x20, [x22, torvalds#40]
0xffffffc010b6cd48 <+344>:	mov	w1, #0x0                   	// #0

During phase one, platform_pm_thaw called xhci_plat_resume which called
xhci_resume. The rest possible calling routine might be
xhci_resume->xhci_init->xhci_mem_init, and xhci->cmd_ring was cleaned in
xhci_mem_cleanup before xhci_mem_init returned -ENOMEM.

During phase two, systemd was tring to hibernate again and called
xhci_suspend, then xhci_clear_command_ring dereferenced xhci->cmd_ring
which was already NULL.

So if xhci->cmd_ring is NULL, xhci_clear_command_ring just return.

Co-developed-by: sunke <sunke@kylinos.cn>
Signed-off-by: sunke <sunke@kylinos.cn>
Signed-off-by: Hongyu Xie <xiehongyu1@kylinos.cn>

drivers/usb/host/xhci.c

@@ -919,6 +919,10 @@ static void xhci_clear_command_ring(struct xhci_hcd *xhci)
 {
 	struct xhci_ring *ring;
 	struct xhci_segment *seg;
+	if (!xhci->cmd_ring) {
+		xhci_err(xhci, "Empty cmd ring");
+		return;
+	}
 
 	ring = xhci->cmd_ring;
 	seg = ring->deq_seg;