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The simplest network driver

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# 1  
Old 11-10-2011
The simplest network driver
Hi, I am trying to write the simplest network driver that would send whatever through cable.

My configuration is:
Linux machine with some Intel network adapter
Another machine with WireShark

I connected Intel network adapter to second machine and want anything to pop up at wireshark.

For now I have compiled that driver:
LXR / The Linux Cross Reference
and want to refactor it to allow bind to Intel network adapter and send 1 packet in a simplest way.

Expected result is that after 'insmod dummy.ko' I will see 1 packet leaving Intel network adapter in WireShark.

Could you point me to some necessary steps I should do to achieve that?
I guess handling 'ping' with my refactored dummy driver is harder task...
Thanks
# 2  
Old 11-10-2011
Humm, you are not going to get anywhere by refactoring the dummy network driver. The necessary code is not in that driver for you to connect to any physical hardware.

I suggest you look at the code for the e1000 or e1000e driver.
# 3  
Old 11-17-2011
Ok, as I said before expected result is to send one packet. Another thing is that I would like to learn linux networking details.

e1000e driver is quite complicated.
For example at the begining of driver life in kernel space it does only:
Code:
init_module {
pci_register_driver(&e1000_driver);
}

No more routines are done.
Well... maybe they are done, but I have no idea where to even look for them.
Last edited by fpmurphy; 11-18-2011 at 07:06 AM..
# 4  
Old 11-17-2011
That's just how device drivers work. It gives the kernel a list of functions when it's loaded, the kernel decides which to call when. It's not like a userspace program where it all starts in main()...

You may need to study the organization of the kernel, not just the source code for one particular driver, to understand it.

Studying the source for some other driver won't tell you how the driver you want works, usually.
# 5  
Old 11-18-2011
May I count on you a bit?
I see, that while performing:

Code:
static int __init e1000_init_module(void)
{
       return  pci_register_driver(&e1000_driver);
}

function pointed by .probe is called.

Code:
/* PCI Device API Driver */
static struct pci_driver e1000_driver = {
       .name     = e1000e_driver_name,
       .id_table = e1000e_pci_tbl,
       .probe    = e1000_probe,
       .remove   = __devexit_p(e1000_remove),
       .shutdown = e1000_shutdown,
};

Is that behaviour described somewhere?
As I remember, exactly at this moment (pci_register_driver) driver announce set of device IDs he support, and kernel looking at this announcement and into PCI config space of every device binds proper driver to every device. Am I correct?
# 6  
Old 11-18-2011
You really should take the time to read the book Linux Device Drivers by Corbet, Rubini and Kroah-Hartman. Chapter 12 describes PCI drivers in considerable detail.
# 7  
Old 11-18-2011
Quote:
Originally Posted by Chrisdot
May I count on you a bit?
I see, that while performing:

Code:
static int __init e1000_init_module(void)
{
       return  pci_register_driver(&e1000_driver);
}

function pointed by .probe is called.

Code:
/* PCI Device API Driver */
static struct pci_driver e1000_driver = {
       .name     = e1000e_driver_name,
       .id_table = e1000e_pci_tbl,
       .probe    = e1000_probe,
       .remove   = __devexit_p(e1000_remove),
       .shutdown = e1000_shutdown,
};

Is that behaviour described somewhere?
I suppose you've checked inside Documentation/PCI/ inside the kernel source code?

My understanding of them is far from perfect, mostly limited to editing in extra PCI ID's when manufacturers do shell games me. I don't perfectly understand the internal details, and if I ever did my knowledge would go obsolete in weeks. ;p But I've done a lot of programming making/using/fixing plugin libraries, which makes a lot of what I see in device drivers look familiar.

Imagine a plugin for a media player -- maybe a software visualizer of some sort. The plugin would look like this:

Code:
// these names don't get exported in the library.  They're local-only.
static int visualizer_open(void) { return(0); }
static int visualizer_close(void) { return(0); }
static int visualizer_handledata(void *data, int len)
{
        fprintf(stderr, "%d bytes at %p\n", len, data);
        show_music_on_screen_in_amusing_manner(data, len);
        return(0);
}

struct function
{
        char *formats_supported[4];
        int (*open)(void);
        int (*close)(void);
        int (*handledata)(void *data, int len);
} functions={
       { "wav", "mp3", "ogg", NULL },
       visualizer_open, visualizer_close, visualizer_handledata
};

// This function IS exported.
struct function *get_callbacks(void)  {        return(&functions); }

To use it, the media player opens the library, looks up get_callbacks and calls it.

Code:
struct function
{
        char *formats_supported[4];
        int (*open)(void);
        int (*close)(void);
        int (*handledata)(void *data, int len);
} *functions;

void *lib=dlopen("crazylinesvisualizer.so", RTLD_NOW);
void *(*getcallback)=dlsym(lib, "get_callbacks");
dlclose(lib);

functions=getcallback();

if(strcmp(functions->formats_supported[0], "wav") != 0)
{
        fprintf(stderr, "visualizer doesn't support wav");
        exit(1);
}

functions->open();
whle(music) functions->handledata(data, len);
functions->close();

Bundling them in a structure like that makes it more efficient -- one load operation instead of one for each function -- and more organized.

Kernel device drivers work just like this. Look in a device driver for a character device and you'll find a big structure containing pointers the driver's own personal open(), close(), read(), write(), ioctl(), mmap(), kitchen_sink(), and other such functions which get called whenever a user calls open, read, write, ioctl, mmap, etc. on that device file in userspace. Even when you build drivers in, they're still in a great big table. That's how the kernel finds them, how the kernel decides what order to initialize them, and how it decides which to use for what PCI id.
Quote:
As I remember, exactly at this moment (pci_register_driver) driver announce set of device IDs he support
I don't think the driver "announces" it, it just keeps a list in that big structure. The kernel checks if it matches any present devices and, if not, doesn't bother initializing it.

Non-PNP things might have to do manual probing to see if a device exists.
Last edited by Corona688; 11-18-2011 at 12:00 PM..
These 2 Users Gave Thanks to Corona688 For This Post:
Chrisdot fpmurphy
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