usb: cdns3: gadget: prev_req->trb is NULL for ep0

commit 95cd7dc upstream.

And there are no multiple TRBs on EP0 and WA1 workaround,
so it doesn't need to change TRB for EP0. It fixes below oops.

configfs-gadget gadget: high-speed config #1: b
android_work: sent uevent USB_STATE=CONFIGURED
Unable to handle kernel read from unreadable memory at virtual address 0000000000000008
Mem abort info:
android_work: sent uevent USB_STATE=DISCONNECTED
  ESR = 0x96000004
  EC = 0x25: DABT (current EL), IL = 32 bits

  SET = 0, FnV = 0
  EA = 0, S1PTW = 0
Data abort info:
  ISV = 0, ISS = 0x00000004
  CM = 0, WnR = 0
user pgtable: 4k pages, 48-bit VAs, pgdp=00000008b5bb7000
[0000000000000008] pgd=0000000000000000
Internal error: Oops: 96000004 [#1] PREEMPT SMP
Modules linked in:
CPU: 2 PID: 430 Comm: HwBinder:401_1 Not tainted 5.4.24-06071-g6fa8921409c1-dirty #77
Hardware name: Freescale i.MX8QXP MEK (DT)
pstate: 60400085 (nZCv daIf +PAN -UAO)
pc : cdns3_gadget_ep_dequeue+0x1d4/0x270
lr : cdns3_gadget_ep_dequeue+0x48/0x270
sp : ffff800012763ba0
x29: ffff800012763ba0 x28: ffff00082c653c00
x27: 0000000000000000 x26: ffff000068fa7b00
x25: ffff0000699b2000 x24: ffff00082c6ac000
x23: ffff000834f0a480 x22: ffff000834e87b9c
x21: 0000000000000000 x20: ffff000834e87800
x19: ffff000069eddc00 x18: 0000000000000000
x17: 0000000000000000 x16: 0000000000000000
x15: 0000000000000000 x14: 0000000000000000
x13: 0000000000000000 x12: 0000000000000001
x11: ffff80001180fbe8 x10: 0000000000000001
x9 : ffff800012101558 x8 : 0000000000000001
x7 : 0000000000000006 x6 : ffff000835d9c668
x5 : ffff000834f0a4c8 x4 : 0000000096000000
x3 : 0000000000001810 x2 : 0000000000000000
x1 : ffff800024bd001c x0 : 0000000000000001
Call trace:
 cdns3_gadget_ep_dequeue+0x1d4/0x270
 usb_ep_dequeue+0x34/0xf8
 composite_dev_cleanup+0x154/0x170
 configfs_composite_unbind+0x6c/0xa8
 usb_gadget_remove_driver+0x44/0x70
 usb_gadget_unregister_driver+0x74/0xe0
 unregister_gadget+0x28/0x58
 gadget_dev_desc_UDC_store+0x80/0x110
 configfs_write_file+0x1e0/0x2a0
 __vfs_write+0x48/0x90
 vfs_write+0xe4/0x1c8
 ksys_write+0x78/0x100
 __arm64_sys_write+0x24/0x30
 el0_svc_common.constprop.0+0x74/0x168
 el0_svc_handler+0x34/0xa0
 el0_svc+0x8/0xc
Code: 52830203 b9407660 f94042e4 11000400 (b9400841)
---[ end trace 1574516e4c1772ca ]---
Kernel panic - not syncing: Fatal exception
SMP: stopping secondary CPUs
Kernel Offset: disabled
CPU features: 0x0002,20002008
Memory Limit: none
Rebooting in 5 seconds..

Fixes: f616c3b ("usb: cdns3: Fix dequeue implementation")
Cc: stable <[email protected]>
Signed-off-by: Peter Chen <[email protected]>
Signed-off-by: Felipe Balbi <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

drivers/usb/cdns3/gadget.c

@@ -2548,7 +2548,7 @@ int cdns3_gadget_ep_dequeue(struct usb_ep *ep,
 	link_trb = priv_req->trb;
 
 	/* Update ring only if removed request is on pending_req_list list */
-	if (req_on_hw_ring) {
+	if (req_on_hw_ring && link_trb) {
 		link_trb->buffer = TRB_BUFFER(priv_ep->trb_pool_dma +
 			((priv_req->end_trb + 1) * TRB_SIZE));
 		link_trb->control = (link_trb->control & TRB_CYCLE) |