is it possible to do the serial communication with /dev/bus/usb/<bus>/<device>

4
votes

I'm working on a project in which I've to do some serial communication with whichever device is connected (ttyS0, ttyS1 or ttyUSB0). Luckily I've come across a very useful stackoverflow link: "Simple way to query connected USB devices info in Python?". In this link there is a python code which works perfectly fine and it gives a proper device name and details.

here in the example code: "/dev/bus/usb/005/002" is the device information of "FT232 Serial (UART)". SO, is there a way to find either mapping of /dev/bus/usb/005/002 with ttyS0/ ttyUSB0 or direct access of the UART with the device information and do the serial communication using "/dev/bus/usb/< bus >/< device >" instead of ttyS0 or ttyUSB0.

python code:

import re
import subprocess
device_re = re.compile("Bus\s+(?P<bus>\d+)\s+Device\s+(?P<device>\d+).+ID\s(?P<id>\w+:\w+)\s(?P<tag>.+)$", re.I)
df = subprocess.check_output("lsusb")
devices = []
for i in df.split('\n'):
    if i:
        info = device_re.match(i)
        if info:
            dinfo = info.groupdict()
            dinfo['device'] = '/dev/bus/usb/%s/%s' % (dinfo.pop('bus'), dinfo.pop('device'))
            devices.append(dinfo)
print devices

result: 

{'device': '/dev/bus/usb/001/001', 'tag': 'Linux Foundation 2.0 root hub', 'id': '1d6b:0002'}
{'device': '/dev/bus/usb/005/002', 'tag': 'Future Technology Devices International, Ltd FT232 Serial (UART) IC', 'id': '0403:6001'}
{'device': '/dev/bus/usb/005/001', 'tag': 'Linux Foundation 1.1 root hub', 'id': '1d6b:0001'}
{'device': '/dev/bus/usb/004/003', 'tag': 'Lite-On Technology Corp. ', 'id': '04ca:0061'}
{'device': '/dev/bus/usb/004/002', 'tag': 'Dell Computer Corp. ', 'id': '413c:2107'}
{'device': '/dev/bus/usb/004/001', 'tag': 'Linux Foundation 1.1 root hub', 'id': '1d6b:0001'}
{'device': '/dev/bus/usb/003/001', 'tag': 'Linux Foundation 1.1 root hub', 'id': '1d6b:0001'}
{'device': '/dev/bus/usb/002/001', 'tag': 'Linux Foundation 1.1 root hub', 'id': '1d6b:0001'}

Thanking with regards Aatif shaikh

2
pythonlinuxserial-port
Please consider libusb / OpenUSB / pyUSB. How to enumerate USB devices and read/write to them?, pyusb/docs/tutorial.rst – kunif
What are you trying to accomplish by reading and writing directly to the USB device? Normally, you should use /dev/ttyUSB and associated entries; if you are trying to make sure you always connect to the same USB device you should probably use udev – rm5248
I'm just trying to find the link between 'device': '/dev/bus/usb/005/002', 'tag': 'Future Technology Devices International, Ltd FT232 Serial (UART) IC', 'id': '0403:6001'} and ttyUSB0. so that I can always check what type of device is connected to any serial port before I begin the serial communication. – Aatif Shaikh

2 Answers

3
votes

A useful utilities is udevadm. For example, I have this USB serial adapter:

$ lsusb|grep -i prolific
Bus 001 Device 077: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port

Running udevadm on it yields a bunch of information. Here's the start of it:

$ udevadm info -a /dev/bus/usb/001/077

Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.

  looking at device '/devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.4/1-3.4.4':
    KERNEL=="1-3.4.4"
    SUBSYSTEM=="usb"
    DRIVER=="usb"
    ATTR{authorized}=="1"
    ATTR{avoid_reset_quirk}=="0"
    ATTR{bConfigurationValue}=="1"
    ATTR{bDeviceClass}=="00"
    ATTR{bDeviceProtocol}=="00"
    ATTR{bDeviceSubClass}=="00"
    ATTR{bMaxPacketSize0}=="64"
    ATTR{bMaxPower}=="100mA"
    ATTR{bNumConfigurations}=="1"
    ATTR{bNumInterfaces}==" 1"
    ATTR{bcdDevice}=="0300"
    ATTR{bmAttributes}=="a0"
    ATTR{busnum}=="1"
    ATTR{configuration}==""
    ATTR{devnum}=="77"
    ATTR{devpath}=="3.4.4"
    ATTR{idProduct}=="2303"
    ATTR{idVendor}=="067b"
    ATTR{ltm_capable}=="no"
    ATTR{manufacturer}=="Prolific Technology Inc."
    ATTR{maxchild}=="0"
    ATTR{product}=="USB-Serial Controller"
    ATTR{quirks}=="0x0"
    ATTR{removable}=="unknown"
    ATTR{speed}=="12"
    ATTR{urbnum}=="22"
    ATTR{version}==" 2.00"

You can then look into sysfs for more details:

$ ls -l '/sys//devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.4/1-3.4.4'
total 0
drwxr-xr-x 7 root root     0 Oct 11 11:03 1-3.4.4:1.0
-r--r--r-- 1 root root  4096 Oct 11 11:03 bcdDevice
-rw-r--r-- 1 root root  4096 Oct 11 11:03 bConfigurationValue
-r--r--r-- 1 root root  4096 Oct 11 11:03 bDeviceClass
-r--r--r-- 1 root root  4096 Oct 11 11:03 bDeviceProtocol
-r--r--r-- 1 root root  4096 Oct 11 11:03 bDeviceSubClass
-r--r--r-- 1 root root  4096 Oct 11 11:03 bmAttributes
-r--r--r-- 1 root root  4096 Oct 11 11:03 bMaxPacketSize0
-r--r--r-- 1 root root  4096 Oct 11 11:03 bMaxPower
-r--r--r-- 1 root root  4096 Oct 11 11:03 bNumConfigurations
-r--r--r-- 1 root root  4096 Oct 11 11:03 bNumInterfaces
-r--r--r-- 1 root root  4096 Oct 11 11:03 busnum
-r--r--r-- 1 root root  4096 Oct 11 11:03 configuration
-r--r--r-- 1 root root 65553 Oct 11 11:03 descriptors
-r--r--r-- 1 root root  4096 Oct 11 11:03 dev
-r--r--r-- 1 root root  4096 Oct 11 11:03 devnum
-r--r--r-- 1 root root  4096 Oct 11 11:03 devpath
lrwxrwxrwx 1 root root     0 Oct 11 11:03 driver -> ../../../../../../../bus/usb/drivers/usb
drwxr-xr-x 3 root root     0 Oct 11 11:03 ep_00
-r--r--r-- 1 root root  4096 Oct 11 11:03 idProduct
-r--r--r-- 1 root root  4096 Oct 11 11:03 idVendor
-r--r--r-- 1 root root  4096 Oct 11 11:03 ltm_capable
-r--r--r-- 1 root root  4096 Oct 11 11:03 manufacturer
-r--r--r-- 1 root root  4096 Oct 11 11:03 maxchild
lrwxrwxrwx 1 root root     0 Oct 11 11:03 port -> ../1-3.4:1.0/1-3.4-port4
drwxr-xr-x 2 root root     0 Oct 11 11:03 power
-r--r--r-- 1 root root  4096 Oct 11 11:03 product
-r--r--r-- 1 root root  4096 Oct 11 11:03 quirks
-r--r--r-- 1 root root  4096 Oct 11 11:03 removable
--w------- 1 root root  4096 Oct 11 11:03 remove
-r--r--r-- 1 root root  4096 Oct 11 11:03 speed
lrwxrwxrwx 1 root root     0 Oct 11 11:03 subsystem -> ../../../../../../../bus/usb
-rw-r--r-- 1 root root  4096 Oct 11 11:03 uevent
-r--r--r-- 1 root root  4096 Oct 11 11:03 urbnum
-r--r--r-- 1 root root  4096 Oct 11 11:03 version

You'll notice a sub-directory (1-3.4.4:1.0) for each implemented USB device(interface? function?) within the adapter (in my case just one; I have other 4-port USB serial adapter with 4 sub-directories too). If you look in there, you can eventually find the device node:

$ ls -l '/sys//devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.4/1-3.4.4/1-3.4.4:1.0/'
total 0
-rw-r--r-- 1 root root 4096 Oct 11 11:03 authorized
-r--r--r-- 1 root root 4096 Oct 11 11:03 bAlternateSetting
-r--r--r-- 1 root root 4096 Oct 11 11:03 bInterfaceClass
-r--r--r-- 1 root root 4096 Oct 11 11:03 bInterfaceNumber
-r--r--r-- 1 root root 4096 Oct 11 11:03 bInterfaceProtocol
-r--r--r-- 1 root root 4096 Oct 11 11:03 bInterfaceSubClass
-r--r--r-- 1 root root 4096 Oct 11 11:03 bNumEndpoints
lrwxrwxrwx 1 root root    0 Oct 11 11:03 driver -> ../../../../../../../../bus/usb/drivers/pl2303
drwxr-xr-x 3 root root    0 Oct 11 11:03 ep_02
drwxr-xr-x 3 root root    0 Oct 11 11:03 ep_81
drwxr-xr-x 3 root root    0 Oct 11 11:03 ep_83
-r--r--r-- 1 root root 4096 Oct 11 11:03 modalias
drwxr-xr-x 2 root root    0 Oct 11 11:03 power
lrwxrwxrwx 1 root root    0 Oct 11 11:03 subsystem -> ../../../../../../../../bus/usb
-r--r--r-- 1 root root 4096 Oct 11 11:03 supports_autosuspend
drwxr-xr-x 4 root root    0 Oct 11 11:03 ttyUSB0
-rw-r--r-- 1 root root 4096 Oct 11 11:03 uevent

$ ls -l '/sys//devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.4/1-3.4.4/1-3.4.4:1.0/ttyUSB0'
total 0
lrwxrwxrwx 1 root root    0 Oct 11 11:03 driver -> ../../../../../../../../../bus/usb-serial/drivers/pl2303
-r--r--r-- 1 root root 4096 Oct 11 11:03 port_number
drwxr-xr-x 2 root root    0 Oct 11 11:03 power
lrwxrwxrwx 1 root root    0 Oct 11 11:03 subsystem -> ../../../../../../../../../bus/usb-serial
drwxr-xr-x 3 root root    0 Oct 11 11:03 tty
-rw-r--r-- 1 root root 4096 Oct 11 11:03 uevent

$ ls -l '/sys//devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.4/1-3.4.4/1-3.4.4:1.0/ttyUSB0/tty'
total 0
drwxr-xr-x 3 root root 0 Oct 11 11:11 ttyUSB0

$ ls -l '/sys//devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.4/1-3.4.4/1-3.4.4:1.0/ttyUSB0/tty/ttyUSB0'
total 0
-r--r--r-- 1 root root 4096 Oct 11 11:11 dev
lrwxrwxrwx 1 root root    0 Oct 11 11:11 device -> ../../../ttyUSB0
drwxr-xr-x 2 root root    0 Oct 11 11:11 power
lrwxrwxrwx 1 root root    0 Oct 11 11:11 subsystem -> ../../../../../../../../../../../class/tty
-rw-r--r-- 1 root root 4096 Oct 11 11:11 uevent

$ cat '/sys//devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.4/1-3.4.4/1-3.4.4:1.0/ttyUSB0/tty/ttyUSB0/dev'
188:0

That last line is the device node major/minor number, which you can search for in /dev, or use to create your own device node.

The directory /sys/class/tty/ lists all ttys, and /sys/bus/usb-serial/devices/ lists all USB serial adapters. It might be easier to start from those directories (rather than udevadm) to find all relevant devices, then determine which one of those matches the USB device you care about.

$ ls -l /sys/bus/usb-serial/devices/
total 0
lrwxrwxrwx 1 root root 0 Oct 11 11:16 ttyUSB0 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.4/1-3.4.4/1-3.4.4:1.0/ttyUSB0/
lrwxrwxrwx 1 root root 0 Sep  6 13:48 ttyUSB1 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-4/1-4:1.1/ttyUSB1/
lrwxrwxrwx 1 root root 0 Oct 11 11:16 ttyUSB10 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-8/1-8.4/1-8.4:1.3/ttyUSB10/
lrwxrwxrwx 1 root root 0 Sep  6 13:48 ttyUSB12 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-8/1-8.6/1-8.6:1.0/ttyUSB12/
lrwxrwxrwx 1 root root 0 Sep  6 13:48 ttyUSB13 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-8/1-8.6/1-8.6:1.1/ttyUSB13/
lrwxrwxrwx 1 root root 0 Sep  6 13:48 ttyUSB14 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-8/1-8.6/1-8.6:1.2/ttyUSB14/
lrwxrwxrwx 1 root root 0 Sep  6 13:48 ttyUSB15 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-8/1-8.6/1-8.6:1.3/ttyUSB15/
lrwxrwxrwx 1 root root 0 Sep  6 13:48 ttyUSB2 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-4/1-4:1.2/ttyUSB2/
lrwxrwxrwx 1 root root 0 Sep  6 13:48 ttyUSB3 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-4/1-4:1.3/ttyUSB3/
lrwxrwxrwx 1 root root 0 Oct 11 11:16 ttyUSB4 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-8/1-8.4/1-8.4:1.0/ttyUSB4/
lrwxrwxrwx 1 root root 0 Sep  6 13:48 ttyUSB5 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-7/1-7:1.1/ttyUSB5/
lrwxrwxrwx 1 root root 0 Sep  6 13:48 ttyUSB6 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-7/1-7:1.2/ttyUSB6/
lrwxrwxrwx 1 root root 0 Sep  6 13:48 ttyUSB7 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-7/1-7:1.3/ttyUSB7/
lrwxrwxrwx 1 root root 0 Oct 11 11:16 ttyUSB8 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-8/1-8.4/1-8.4:1.1/ttyUSB8/
lrwxrwxrwx 1 root root 0 Oct 11 11:16 ttyUSB9 -> ../../../devices/pci0000:00/0000:00:14.0/usb1/1-8/1-8.4/1-8.4:1.2/ttyUSB9/

If you assume that you can take those path names (the link values) and go up to ../.. and find the USB device, and find its bus/dev number. I /think/ this is safe, but it depends on whether different USB devices/drivers always lay out their sysfs identically, which I believe they do for a given device type.

$ cat '/sys/devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.4/1-3.4.4/1-3.4.4:1.0/ttyUSB0/../../busnum'
1
$ cat '/sys/devices/pci0000:00/0000:00:14.0/usb1/1-3/1-3.4/1-3.4.4/1-3.4.4:1.0/ttyUSB0/../../devnum' 
77

In general, sysfs contains all the information you want; you just have to work out how to traverse it.

1
votes

Re: your comment on your question of "I'm just trying to find the link between 'device': '/dev/bus/usb/005/002', 'tag': 'Future Technology Devices International, Ltd FT232 Serial (UART) IC', 'id': '0403:6001'} and ttyUSB0. so that I can always check what type of device is connected to any serial port before I begin the serial communication.":

Be careful not to read too much into the device name or USB device/vendor ID or name. Here's why.

Device node name/path: - There are different types (protocols) of USB serial adapters. For example, I have both /dev/ttyUSB0 and /dev/ttyACM0 on my system for different types of adapter. - The user can use udev rules to rename or symlink to the device node. For example, on my system, I have a rule that created /dev/console-nano which is a symlink to some ttyUSB node such as /dev/ttyUSB0 (the exact value depends on the order the devices are enumerated).

Thus, you should only check (via sysfs) whether the device node is a tty and perhaps also whether it's backed by the USB subsystem, or similar subsystems such as ISA/PCI/PCIe serial ports.

USB vendor/device ID:

There are many many vendors of USB serial adapters, even just at the chip level. For example, FTDI (many devices e.g. FT232R, FT4232H, ...), CH430, PL2303, Linux USB device port (common on embedded hardware) acting as a CDC ACM serial device, etc.

Thus, you should detect devices based on their capability (being a tty or serial port), rather than based on their specific USB device/vendor ID.