[QUESTION] mmap() error for UIO Device (uio_pci_generic)

[Alper Yazar <alperyazar@xxxxxxxxx>]

Hi,

I am trying to communicate a custom PCIe device in Linux. I didn't
have experience on programming with using system calls and writing
Linux device drivers.

I loaded uio_pci_generic driver for the device and /dev/uio0 was
generated as character device. Then I wrote the following C program to
dump 128K data from the device to a file. The device has only BAR0
with size 128K.

    #include <sys/stat.h>
    #include <fcntl.h>
    #include <sys/mman.h>
    #include <stdio.h>

    #define MMBLOCK_SIZE (1024*128)
    #define PCI_FILE "/dev/uio0"
    #define BIN_FILE "dump.bin"

    int main() {

        int f;
        FILE * fdumpptr;
        char *ptr;

        printf("PCI FILE: %s\n", PCI_FILE);
        printf("BIN FILE: %s\n", BIN_FILE);

        f = open(PCI_FILE, O_RDWR);

        fdumpptr = fopen("dump.bin","w");

        if(NULL == fdumpptr)
        {
            perror("Can't open BIN FILE");
            return 1;
        }

        if(-1 == f)
        {
            perror("Can't open PCI FILE");
            return 1;
        }
        else
        {
            printf("PCI FILE opened successfully\n");
        }

        ptr = mmap(NULL, MMBLOCK_SIZE, PROT_READ, MAP_SHARED, f, 0);

        if (MAP_FAILED == ptr)
        {
            perror("Can't do memory map on PCI FILE");
            return 1;
        }
        else
        {
            printf("memory map succesfull\n");
        }

        if (MMBLOCK_SIZE == fwrite(ptr, 1, MMBLOCK_SIZE, fdumpptr))
        {
            printf("Content dumpped succesffully\n");
        }
        else
        {
            printf("Can't write dump file\n");
            return 1;
        }

        return 0;
    }

However `mmap()` returned `MAP_FAILED`. If I access to device using
sysFS by changing `#define PCI_FILE
"/sys/bus/pci/devices/0000:08:00.0/resource0"` everything works.

So as far as I understood, I could mmap() the /dev/uio0 device but it
gives an error. errno is 22 after failed mmap() call which corresponds
to "Invalid argument". Is it possible to mmap() character device? What
would be the problem?

Nothing happens in dmesg when I modprobe uio_pci_generic, relate my
major and minor number with the driver, run the working or non working
code.

There is no `maps` directory under `/sys/class/uio/ui0/`. Is this a
problem indicator?

Best,
Alper

PS I posted the same question on stackoverflow:
https://stackoverflow.com/questions/49799839/mmap-error-for-uio-device-uio-pci-generic