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8/13/13 Device Drivers, Part 12: USB Drivers in Linux Continued - LINUX For You

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Device Drivers, Part 12: USB Drivers inLinux ContinuedByAnil Kumar Pugalia on November 1, 2011 in Coding, Developers 11 Comments

The 1 2th part of the series on

Linux device drivers takes you

further along the path to

writing your first USB driver in

Linux a continuation from theprevious article.

Pugs continued, Lets build upon the

USB device driver coded in our

previous session, using the same

handy JetFlash pen drive from

Transcend, with the vendor ID 0x058f

and product ID 06387. For that, lets

dig further into the USB protocol, and

then convert ourlearning into code.

USB endpoints and theirtypesDepending on the type and attributes of information to be transferred, a USB device may have

one or more endpoints, each belonging to one of the following four categories:

Control to transfer control information. Examples include resetting the device, querying

information about the device, etc. All USB devices always have the default control endpoint

point as zero.

Interrupt for small and fast data transfers, typically of up to 8 bytes. Examples include data

transfer for serial ports, human interface devices (HIDs) like keyboards, mouse, etc.

Bulk for big but comparatively slower data transfers. A typical example is data transfers for

mass-storage devices.

Isochronous for big data transfers with a bandwidth guarantee, though data integrity may

not be guaranteed. Typical practical usage examples include transfers of time-sensitive datalike audio, video, etc.

Additionally, all but control endpoints could be in or out, indicating the direction of data

transfer; in indicates data flow from the USB device to the host machine, and out, the other

way.

Technically, an endpoint is identified using an 8-bit number, the most significant bit (MSB) of

which indicates the direction 0 means out, and 1 means in. Control endpoints are bi-

directional, and the MSB is ignored.

Figure 1 shows a typical snippet of USB device specifications for devices connected on a system.

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Figure 1: USB's proc w indow snippet (click for larger view )

To be specific, the E: lines in the figure show examples of an interrupt endpoint of a UHCI Host

Controller, and two bulk endpoints of the pen drive under consideration. Also, the endpoint

numbers (in hex) are, respectively, 0x81, 0x01 and 0x82 the MSB of the first and third being

1, indicating in endpoints, represented by (I) in the figure; the second is an (O) or out

endpoint. MxPS specifies the maximum packet size, i.e., the data size that can be transferred in a

single go. Again, as expected, for the interrupt endpoint, it is 2 (


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Yes, definitely; thats what I am going to tell you about, next. Do you remember that as soon as a

USB device is plugged into the system, the USB host controller driver populates its information

into the generic USB core layer? To be precise, it puts that into a set of structures embedded

into one another, exactly as per the USB specifications, Pugs replied.

The following are the exact data structures defined in , ordered here in reverse,

for flow clarity:

So, with access to the struct usb_device handle for a specific device, all the USB-specific

information about the device can be decoded, as shown through the /proc window. But how

does one get the device handle?

In fact, the device handle is not available directly in a driver; rather, the per-interface handles

(pointers to struct usb_interface) are available, as USB drivers are written for device

interfaces rather than the device as a whole.

Recall that the probe and disconnect callbacks, which are invoked by the USB core for every

interface of the registered device, have the corresponding interface handle as their first

parameter. Refer to the prototypes below:

So, with the interface pointer, all information about the corresponding interface can be accessed

and to get the container device handle, the following macro comes to the rescue:

Adding this new learning into last months registration-only dr iver gets the following code listing

(pen_info.c):

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structusb_device{ structusb_device_descriptor descriptor;

structusb_host_config *config, *actconfig; };structusb_host_config{ structusb_config_descriptor desc; structusb_interface *interface[USB_MAXINTERFACES]; };structusb_interface{ structusb_host_interface *altsetting /* array */, *cur_altsetting; };structusb_host_interface{ structusb_interface_descriptor desc; structusb_host_endpoint *endpoint /* array */; };

structusb_host_endpoint{ structusb_endpoint_descriptor desc; };

int(*probe)(structusb_interface *interface, conststructusb_device_id *id);void(*disconnect)(structusb_interface *interface);

structusb_device device = interface_to_usbdev(interface);

12

3456789

1011121314151617181920212223

24252627282930313233

#include #include

#include staticstructusb_device *device;staticintpen_probe(structusb_interface *interface, conststructusb_device_id{ structusb_host_interface *iface_desc; structusb_endpoint_descriptor *endpoint; inti;

iface_desc = interface->cur_altsetting; printk(KERN_INFO "Pen i/f %d now probed: (%04X:%04X)\n", iface_desc->desc.bInterfaceNumber, id->idVendor, id->idProduct); printk(KERN_INFO "ID->bNumEndpoints: %02X\n", iface_desc->desc.bNumEndpoints); printk(KERN_INFO "ID->bInterfaceClass: %02X\n", iface_desc->desc.bInterfaceClass);

for(i = 0; i < iface_desc->desc.bNumEndpoints; i++) { endpoint = &iface_desc->endpoint[i].desc;

printk(KERN_INFO "ED[%d]->bEndpointAddress: 0x%02X\n",

i, endpoint->bEndpointAddress); printk(KERN_INFO "ED[%d]->bmAttributes: 0x%02X\n", i, endpoint->bmAttributes); printk(KERN_INFO "ED[%d]->wMaxPacketSize: 0x%04X (%d)\n", i, endpoint->wMaxPacketSize, endpoint->wMaxPacketSize); }

device = interface_to_usbdev(interface);


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Then, the usual steps for any Linux device driver may be repeated, along with the pen drive

steps:

Build the driver (pen_info.ko file) by running make.

Load the driver using insmod pen_info.ko.

Plug in the pen drive (after making sure that the usb-storage driver is not already loaded).

Unplug the pen drive.

Check the output ofdmesg for the logs.

Unload the driver using rmmod pen_info.

Figure 2 shows a snippet of the above steps on Pugs system. Remember to ensure (in the

output ofcat /proc/bus/usb/devices) that the usual usb-storage driver is not the one

associated with the pen drive interface, but rather the pen_info driver.

Figure 2: Output of dmesg

Summing up

Before taking another break, Pugs shared two of the many mechanisms for a driver to specify its

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return0;}staticvoidpen_disconnect(structusb_interface *interface){ printk(KERN_INFO "Pen i/f %d now disconnected\n", interface->cur_altsetting->desc.bInterfaceNumber);}staticstructusb_device_id pen_table[] ={ { USB_DEVICE(0x058F, 0x6387) }, {} /* Terminating entry */};MODULE_DEVICE_TABLE (usb, pen_table);

staticstructusb_driver pen_driver ={ .name = "pen_driver", .probe = pen_probe, .disconnect = pen_disconnect, .id_table = pen_table,};staticint__init pen_init(void){ returnusb_register(&pen_driver);}staticvoid__exit pen_exit(void){ usb_deregister(&pen_driver);}module_init(pen_init);module_exit(pen_exit);

MODULE_LICENSE("GPL");MODULE_AUTHOR("Anil Kumar Pugalia ");MODULE_DESCRIPTION("USB Pen Info Driver");
http://www.linuxforu.com/wp-content/uploads/2011/11/figure_22_dmesg_log.png


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Anil Kumar Pugalia

The author is a freelance trainer in Linux internals, Linux device drivers,

embedded Linux and related topics. Prior to this, he had worked at Intel

and Nvidia. He has been exploring Linux since 1994. A gold medallis t

from the Indian Institute of Science, Linux and knowledge-sharing are two

of his many passions.

Connect with him: Website - Twitter- Facebook - Google+

device to the USB core, using the struct usb_device_id table. The first one is by specifying

the pair using the USB_DEVICE() macro (as done above). The

second one is by specifying the device class/category using the USB_DEVICE_INFO() macro. In

fact, many more macros are available in for various combinations. Moreover,

multiple of these macros could be specified in the usb_device_id table (terminated by a null

entry), for matching with any one of the criteria, enabling to write a single driver for possibly

many devices.

Earlier, you mentioned writing multiple drivers for a single device, as well. Basically, how do we

selectively register or not register a particular interface of a USB device?, queried Shweta.

Sure. Thats next in line of our discussion, along with the ultimate task in any device driver the data-transfer mechanisms, replied Pugs.

Related Posts:

Device Drivers, Part 11: USB Drivers in Linux

Device Drivers, Part 13: Data Transfer to and from USB Devices

Device Drivers, Part 6: Decoding Character Device File Operations

Device Drivers, Part 5: Character Device Files Creation & Operations

Device Drivers, Part 4: Linux Character Drivers

Tags: LFY November 2011, linux device drivers, Linux Device Drivers Series, mass storage devices, serial ports, usb device driver,

usb devices, USB host controller driver, usb protocol

Article written by:

11 comments

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anil kumar

how to know the functions

pen_probe,pen_disconnect (where these functions available and how to find theheadder files)

aadhirai

I have tried the code in Part 11 which was working for me.

Here I am not getting the /dev/pen0 listed on my system (Linux Ubuntu 12.04), even

when I copy paste the code from here....

compiled & followed all your steps accordingly..

Will you please help me in completing this ?

1

David

Its Fantastic ! !

Thank you for your valuable Linux device driver articles.

Could you please start the same for BSP also.

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anil_pugalia >

I doubt, if it would be possible, as currently doing the same for mathematics in

open source.

ME LWIN JOS E

I got an error when i tried to remove the "usb_storage"

melwin-Satellite-L640 USB_2 # rmmod usb_storage

ERROR: Module usb_storage is in use by ums_realtek

Here is the output for "lsusb"

Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

Bus 001 Device 002: ID 8087:0020 Intel Corp. Integrated Rate Matching Hub

Bus 002 Device 002: ID 8087:0020 Intel Corp. Integrated Rate Matching Hub

Bus 002 Device 005: ID 0930:0214 Toshiba Corp.

Bus 002 Device 006: ID 04f2:b1d6 Chicony Electronics Co., Ltd

MELWIN JOSE >

solved :)

ums_realtk is a module that depends in usb_storage.just "rmmod ums_realtk"

see more

MenDuong

Dear, When I pug and unplug my USB driver. I can't see message in dmesg as above.

My dmesg as follow:

SELinux: initialized (dev sdd1, type vfat), uses genfs_contexts

usb 2-1: USB disconnect, address 5

SELinux: initialized (dev sdb4, type fuseblk), uses genfs_contexts

lo: Disabled Privacy Extensions

SELinux: initialized (dev proc, type proc), uses genfs_contexts


SELinux: initialized (dev proc, type proc), uses genfs_contexts


SELinux: initialized dev roc t e roc uses enfs contexts

anil_pugalia >

Because the usb-storage driver is loaded. You need to make sure that is

unloaded, as mentioned in the article.

MenDuong >

Thank you. Please, how to unload usb-storage?

micheal >

go drivers path and then type rmmod usb-storage

anil_pugalia >

If it is listed in lsmod, just do a "rmmod usb-storage". Otherwise,

you'd have to recompile your kernel, by making usb-storage as a

module.

2
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