PCI Access Without a Driver

At work recently, I had a new PCI device that I needed to experiment with. I was dreading writing a Linux kernel driver to talk to it. It turns out, Linux makes it possible to read and write to a PCI device's memory space without a driver! Woohoo!

Linux provides a sysfs interface to PCI devices. From that interface, the memory space can be mmaped and then read and written. No driver involved.

As a quick example, we can use lspci to get information about a particular device.

$ vendor="10ee" # Use your device ID
$ device="7014" # Use your vendor ID
$ lspci -d $vendor:$device -nvv
04:00.0 1180: 10ee:7014
    ...
    Region 0: Memory at f7300000 (32-bit, non-prefetchable) [size=128K]

Then we can look at the sysfs interface, at /sys/bus/pci/devices/. The first bit of data in the output of lspci gives the location of the device on the bus, that we can use when traversing the sysfs interface.

$ ls -alF /sys/bus/pci/devices/0000\:04\:00.0/
total 0
drwxr-xr-x 3 root root      0 Jul  1 12:42 ./
drwxr-xr-x 8 root root      0 Jul  1 12:42 ../
-rw-r--r-- 1 root root   4096 Jul  9 12:48 broken_parity_status
-r--r--r-- 1 root root   4096 Jul  1 12:42 class
-rw-r--r-- 1 root root   4096 Jul  9 12:44 config
-r--r--r-- 1 root root   4096 Jul  1 12:42 device
...
-r--r--r-- 1 root root   4096 Jul  1 12:43 resource
-rw------- 1 root root 131072 Jul  1 12:43 resource0
...
-r--r--r-- 1 root root   4096 Jul  1 12:42 vendor

This interface has some useful files like vendor and device that confirm that we have the right device. These are also useful for programatically finding the correct device, rather than using lspci.

$ cat /sys/bus/pci/devices/0000\:04\:00.0/vendor
0x10ee
$ cat /sys/bus/pci/devices/0000\:04\:00.0/device
0x7014

Looking back at the lspci output, we can also find memory resources and addresses. These are represented as resource0...resourceN in the sysfs interface. That's what we use to get access to the PCI memory space.

Open the resource0 file (which can be some number other than 0 depending on the device).

int fd = open("/sys/bus/pci/devices/0000:04:00.0/resource0", O_RDWR | O_SYNC);

Then use the memory address and size from the lspci output to mmap the file.

void* base_address = (void*)0xf7300000;
size_t size = 128 * 1024// 128K 
void* void_memory = mmap(base_address,
                         size,
                         PROT_READ | PROT_WRITE,
                         MAP_SHARED,
                         fd,
                         0);
uint16_t* memory = (uint16_t*)void_memory;

Now memory provides direct access to read and write the PCI memory space. We can hack away!

// Read the value of the first register 
uint16_t first_register = memory[0];
 
// Write a value to the third register 
memory[2] = 0x0007;

Now, this isn't the perfect scenario. For one, we need to be root to access this memory space. For two, there's no sign of interrupt handling anywhere.

But for basic poking around on a new device, it works pretty slick. No kernel module development required.