Comment # 3 on bug 973748 from Jiri Slaby

Generally, =n can lead to errors like:

genirq: Flags mismatch irq 4. 00000000 (serial) vs. 00000080
(goldfish_pdev_bus)
CPU: 3 PID: 6626 Comm: init Tainted: G        W       4.6.0-rc1+ #338
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 ffffffff87eb26a0 ffff88003da17618 ffffffff82c8ea9f ffffffff86f0c740
 fffffbfff0fd64d4 ffff88003e8a6aa8 ffff88005eb4c4c8 00000000fffffff0
 ffffffff874f6be0 ffff880034ae63a8 ffff88003da17708 ffffffff8149e73c
Call Trace:
 [<     inline     >] __dump_stack lib/dump_stack.c:15
 [<ffffffff82c8ea9f>] dump_stack+0x12e/0x18f lib/dump_stack.c:51
 [<ffffffff8149e73c>] __setup_irq+0x11fc/0x1610 kernel/irq/manage.c:1369
 [<ffffffff8149ef55>] request_threaded_irq+0x275/0x410 kernel/irq/manage.c:1652
 [<     inline     >] request_irq include/linux/interrupt.h:147
 [<     inline     >] serial_link_irq_chain
drivers/tty/serial/8250/8250_core.c:220
 [<ffffffff830cc4bb>] univ8250_setup_irq+0x6fb/0x8a0
drivers/tty/serial/8250/8250_core.c:347
 [<ffffffff830d6ec7>] serial8250_do_startup+0x537/0x18c0
drivers/tty/serial/8250/8250_port.c:2179
 [<ffffffff830d82a9>] serial8250_startup+0x59/0x70
drivers/tty/serial/8250/8250_port.c:2288
 [<     inline     >] uart_port_startup drivers/tty/serial/serial_core.c:163
 [<ffffffff830c274b>] uart_startup.part.17+0x21b/0x640
drivers/tty/serial/serial_core.c:208
 [<     inline     >] uart_startup drivers/tty/serial/serial_core.c:1627
 [<ffffffff830c512e>] uart_open+0x31e/0x3c0
drivers/tty/serial/serial_core.c:1619
 [<ffffffff83051608>] tty_open+0x2c8/0x1140 drivers/tty/tty_io.c:2131
 [<ffffffff81805baa>] chrdev_open+0x22a/0x4c0 fs/char_dev.c:388
 [<ffffffff817f1282>] do_dentry_open+0x6a2/0xcb0 fs/open.c:736
 [<ffffffff817f495b>] vfs_open+0x17b/0x1f0 fs/open.c:853
 [<     inline     >] do_last fs/namei.c:3238
 [<ffffffff8182d1fb>] path_openat+0x51bb/0x5ce0 fs/namei.c:3374
 [<ffffffff8183139e>] do_filp_open+0x18e/0x250 fs/namei.c:3409
 [<ffffffff817f50dc>] do_sys_open+0x1fc/0x420 fs/open.c:1020
 [<     inline     >] SYSC_open fs/open.c:1038
 [<ffffffff817f532d>] SyS_open+0x2d/0x40 fs/open.c:1033
 [<ffffffff867c6280>] entry_SYSCALL_64_fastpath+0x23/0xc1
arch/x86/entry/entry_64.S:207