USB port in linux

yann · 09-01-2009, 03:37 AM

HI, i was wondering how to access USB port in C on a linux computer....i would like just send signals to other computers, or just to run a DC motor.

MK27 · 09-01-2009, 04:19 PM

Pretty sure you will have to write a kernel module. Have a look at this thread from today:

About Device Driver Programming And Socket Programming

And maybe you will find this interesting:

Writing a Simple USB Driver

I do remember getting slightly peeved about that last one since there is some crucial information absent from it* which you will realize after being frustrated by it (the nastiest kind of hole you can leave in a demo, I know!), but it does provide a nice clear acurate (IMO) perspective.

* even worse I can't remember what it was and am doing totally unrelated things right now, so don't have the headspace to figure it out...sorry

Magos · 09-02-2009, 08:30 AM

Technically USB is not a port, it's a bus.

abachler · 09-02-2009, 08:49 AM

Quote:

Originally Posted by Magos

Technically USB is not a port, it's a bus.

Internally its a bus, externally it has a port, electronically its accessed through a logical port.

yann · 09-12-2009, 09:24 AM

Quote:

Originally Posted by MK27

Pretty sure you will have to write a kernel module. Have a look at this thread from today:

About Device Driver Programming And Socket Programming

And maybe you will find this interesting:

Writing a Simple USB Driver

I do remember getting slightly peeved about that last one since there is some crucial information absent from it* which you will realize after being frustrated by it (the nastiest kind of hole you can leave in a demo, I know!), but it does provide a nice clear acurate (IMO) perspective.

* even worse I can't remember what it was and am doing totally unrelated things right now, so don't have the headspace to figure it out...sorry

I know this sound nooblish but, those commands :

Code:

static struct usb_driver led_driver = {
	.owner =	THIS_MODULE,
	.name =		"usbled",
	.probe =	led_probe,
	.disconnect =	led_disconnect,
	.id_table =	id_table,
};

and others

do i execute them in a C program or shell (terminal)?

MK27 · 09-12-2009, 11:42 AM

No, it is a C structure assignment. (You will need to read up about structs in C if you do not know what they are). struct usb_driver is defined somewhere else (in one of the kernel headers). led_driver is a struct usb_driver. Here's a parallel example:

Code:

#include <stdio.h>

struct form {      /* struct form definition */
	const char *name;
};

int main() {
	struct form eg = {   /* struct form eg declaration and assignment */
		.name = "hello world",
	};
	printf("%s\n",eg.name);
	return 0;
}

Make sense? This may be C99 syntax but evidently it is okay with the kernel and linux programming generally.

yann · 09-13-2009, 04:38 AM

Quote:

Originally Posted by MK27

No, it is a C structure assignment. (You will need to read up about structs in C if you do not know what they are). struct usb_driver is defined somewhere else (in one of the kernel headers). led_driver is a struct usb_driver. Here's a parallel example:

Code:

#include <stdio.h>

struct form {      /* struct form definition */
	const char *name;
};

int main() {
	struct form eg = {   /* struct form eg declaration and assignment */
		.name = "hello world",
	};
	printf("%s\n",eg.name);
	return 0;
}

Make sense? This may be C99 syntax but evidently it is okay with the kernel and linux programming generally.

thank you, actually i speak Croatian, not English, I understand "writing ssimple kernel driver" document, but i do not in some way...one thing i dont understansd, is in wich order do these commands go...could you please explain me that(or just post the code) (i know i shouldnt ask you to do that, sry i rely need it please.)):

MK27 · 09-13-2009, 08:08 AM

Quote:

Originally Posted by yann

thank you, actually i speak Croatian, not English, I understand "writing ssimple kernel driver" document, but i do not in some way...one thing i dont understansd, is in wich order do these commands go...

It must be hard to deal with a language barrier for material like that. I am always surprised more people do not translate stuff like this, voluntarily -- I suppose by the time your english is good enough, maybe you cannot be bothered to help anyone else out? Anyway...

That's all one C program. However, kernel programming does not use the standard C libraries and is specially compiled, so there are some significant differences. For example, there is no main().

This is because a module runs as part of the kernel process, and is not really an independent executable. When the module is loaded* -- ie, the program is started -- it registers a number of callback functions and other information with the kernel. A callback is a function which is called in response to some event, eg, in a user interactive program, you could have a callback for when the mouse is moved. Callbacks in modules are called by the kernel itself when, for example, the kernel detects a USB device plugged into the system. So an individual USB module must first register the ID of the device for which it was written, and some functions to deal with generic USB events (that is, basic events common to all USB devices, such as being plugged and probed by the OS). Those functions are a struct usb_driver, the ID is in a struct usb_device_id. These should be global to the scope of the module, eg, they are not assigned in any function.

Notice there is a pointer in the usb_driver struct to a usb_device_id struct. What does go in an actual function is a call to usb_register(), which takes a pointer to the usb_driver struct which is declared globally (and has a pointer to the device_id struct). That function is the module's initialization function, and it is defined as such at compilation by including this line at the end of the module code:

module_init (my_inititialization_function);

I haven't done this stuff in a while. This is the simple part; USB module programming gets quite complex. Don't feel you have to go through with it at this point, it may be "counter productive" for you until you have more experience with the C language -- there is a lot of need for looping thru arrays of nested structs:

Code:

for (i=0;i<dev->actconfig->desc.bNumInterfaces;i++) {
     tmp=dev->actconfig->interface[i];
     printk("usbgetinfo: Configuration %d of %d\n",i+1,dev->actconfig->desc.bNumInterfaces);
     for (j=0;j<tmp->num_altsetting; j++) {
        hinfc=&tmp->altsetting[j];
        printk("\tInterface %d of %d\n",j+1,tmp->num_altsetting);
        for (k=0; k<hinfc->desc.bNumEndpoints; k++) {
            endpoint = &hinfc->endpoint[k]; 
            /* endpoint->desc is a struct usb_endpoint_descriptor */
            printk("\t\tEndpoint: %d",endpoint->desc.bEndpointAddress);
            if (endpoint->desc.bEndpointAddress & USB_DIR_IN) printk(" (in) ");
            if (endpoint->desc.bEndpointAddress & USB_DIR_OUT) printk(" (out) ");
            type = endpoint->desc.bmAttributes & USB_ENDPOINT_XFERTYPE_MASK;
            switch (type) {
                case (USB_ENDPOINT_XFER_BULK): printk("BULK\n"); break;
                case (USB_ENDPOINT_XFER_ISOC): printk("ISOCHRONOUS\n"); break;
                case (USB_ENDPOINT_XFER_INT): printk("INTERRUPT\n"); break;
                default: printk("??\n"); break;
            }   
        }   
    }
}

Notice the use of printk() instead of printf(), since you cannot use stdio.h in a kernel module. Having fun yet?

Here is an online book you may want to look at:
Linux Device Drivers, 2nd Edition: Online Book
Unfortunately, the kernel changes all the time and there were some significant changes to some of the names of usb functions in the past year or so, meaning some material may contain the old function names. But the general idea remains the same.

* the module can also be placed "in tree" with the other modules and added to a database with the depmod shell command; the kernel can then load the module on the basis of the info in the database (the device ID, etc).

09-01-2009, 03:37 AM	#1
yann Registered User Join Date: Sep 2009 Location: Zagreb, Croatia Posts: 179	USB port in linux HI, i was wondering how to access USB port in C on a linux computer....i would like just send signals to other computers, or just to run a DC motor.
yann is offline

09-01-2009, 04:19 PM	#2
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	Pretty sure you will have to write a kernel module. Have a look at this thread from today: About Device Driver Programming And Socket Programming And maybe you will find this interesting: Writing a Simple USB Driver I do remember getting slightly peeved about that last one since there is some crucial information absent from it* which you will realize after being frustrated by it (the nastiest kind of hole you can leave in a demo, I know!), but it does provide a nice clear acurate (IMO) perspective. * even worse I can't remember what it was and am doing totally unrelated things right now, so don't have the headspace to figure it out...sorry __________________ Accuracy and integrity mean nothing if you don't make it past the censors...PYTHAGORAS Last edited by MK27; 09-01-2009 at 04:34 PM.
MK27 is offline

09-02-2009, 08:30 AM	#3
Magos Confused Join Date: Sep 2001 Location: Sweden Posts: 3,122	Technically USB is not a port, it's a bus. __________________ MagosX.com Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.
Magos is offline

09-12-2009, 11:42 AM	#6
MK27 subminimalist Join Date: Jul 2008 Location: NYC Posts: 3,944	No, it is a C structure assignment. (You will need to read up about structs in C if you do not know what they are). struct usb_driver is defined somewhere else (in one of the kernel headers). led_driver is a struct usb_driver. Here's a parallel example: Code: #include <stdio.h> struct form { /* struct form definition / const char name; }; int main() { struct form eg = { /* struct form eg declaration and assignment / .name = "hello world", }; printf("%s\n",eg.name); return 0; } Make sense? This may be C99 syntax but evidently it is okay with the kernel and linux programming generally. __________________ Accuracy and integrity mean nothing if you don't make it past the censors...*PYTHAGORAS
MK27 is offline

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