QEMU PCI Memory Mapped I/O - usb

Question

Am trying to understand USB PCI MMIO and am using QEMU as a playgroud.
The following devices are on my VM,
00:1d.0 USB controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #1 (rev 03) (prog-if 00 [UHCI])
Subsystem: Red Hat, Inc. QEMU Virtual Machine
Flags: bus master, fast devsel, latency 0, IRQ 16
I/O ports at c0a0 [size=32]
Kernel driver in use: uhci_hcd
00:1d.1 USB controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #2 (rev 03) (prog-if 00 [UHCI])
Subsystem: Red Hat, Inc. QEMU Virtual Machine
Flags: bus master, fast devsel, latency 0, IRQ 17
I/O ports at c0c0 [size=32]
Kernel driver in use: uhci_hcd
00:1d.2 USB controller: Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #3 (rev 03) (prog-if 00 [UHCI])
Subsystem: Red Hat, Inc. QEMU Virtual Machine
Flags: bus master, fast devsel, latency 0, IRQ 18
I/O ports at c0e0 [size=32]
Kernel driver in use: uhci_hcd
00:1d.7 USB controller: Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI])
Subsystem: Red Hat, Inc. QEMU Virtual Machine
Flags: bus master, fast devsel, latency 0, IRQ 19
Memory at fc072000 (32-bit, non-prefetchable) [size=4K]
Kernel driver in use: ehci-pci
And the following code,
int fd = open("/sys/devices/pci0000:00/0000:00:1d.7/resource0", O_RDWR | O_SYNC);
int mmio = mmap(0, 0x100, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
iopl(3);
outl(0, 0xc0a0);
If I do not include outl(0, 0xc0a0); it seems that any writes to the MMIO address space will not trigger anything.
Looking to try and understand what exactly does outl(0, 0xc0a0); achieve.

Answer 1

outl() writes data. In your code no other function writes to I/O except outl()
https://linux.die.net/man/2/outl
If you want to know more internals about outl use gdb and disassemble it https://gist.github.com/jarun/ea47cc31f1b482d5586138472139d090
https://man7.org/linux/man-pages/man2/iopl.2.html