PCIe Driver - How does user space access it?

Question

I am writing a PCIe driver for Linux, currently without DMA, and need to know how to read and write to the PCIe device once it is enabled from user space.

In the driver I do the basics in probe():

pci_enable_device();
pci_request_regions();
pci_iomap();

But then how do I access this memory from user space to read and write? Do I add file operations to my PCIe driver? Does the memory from pci_iomap show up some place where the user space code can call:

open('mapped memory location');
mmap(...);

If so then what is the location?

Note: the PCIe device will not plugging into any Linux subsystems such as audio, Ethernet, etc.

Not sure if it's the recommended way of doing things, but other kernel drivers expose virtual devices like /dev/vboxnetctl, which I assume you can issue commands to. I'm not certain that the Linux kernel would allow any userspace program to read/write to a PCIe device willy-nilly, that sounds like it might be dangerous. — Naftuli Kay
I was looking at that as well. Is there anything I need to do in order for my device to show up there? For example, what API calls do I make in my driver? — user2205930
Perhaps take a look at the VirtualBox driver source code for ideas. — Naftuli Kay
What is pcim_*() exactly and why should I be using it? Sometimes documentation for Linux can be painful to gather so could you please provide a sentence or two on it? — user2205930

antiduh antiduh · Accepted Answer · 2016-02-08T19:46:38

You can register devices using functions like register_chrdev and device_create. Consider the kernel source for /dev/null and /dev/mem:

static int __init chr_dev_init(void)
{
    int minor;

    if (register_chrdev(MEM_MAJOR, "mem", &memory_fops))
        printk("unable to get major %d for memory devs\n", MEM_MAJOR);

    mem_class = class_create(THIS_MODULE, "mem");
    if (IS_ERR(mem_class))
        return PTR_ERR(mem_class);

    mem_class->devnode = mem_devnode;
    for (minor = 1; minor < ARRAY_SIZE(devlist); minor++) {
        if (!devlist[minor].name)
            continue;

        /*
         * Create /dev/port?
         */
        if ((minor == DEVPORT_MINOR) && !arch_has_dev_port())
            continue;

        device_create(mem_class, NULL, MKDEV(MEM_MAJOR, minor),
                  NULL, devlist[minor].name);
    }

    return tty_init();
}

fs_initcall(chr_dev_init);

PCIe Driver - How does user space access it?

2 Answers