Bus snooping/sniffing

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Purpose and relevance to development work -- description coming soon


Snooping Utilities:


Snooping Utilities:

  • usbsnoop - a Windows based utility for sniffing/monitoring communications traffic for a USB device. Note: In case usbsnoop/SniffUSB doesn't work for you, here are a few time limited apps that should work under Vista:
    • USB Monitor - 14-day trial period
    • USBlyzer - fully functional evaluation version for 33 days (only 32 bit)
  • SnoopyPro - Windows based snoop for USB device communications traffic
  • usbsnoop/SniffUSB - Windows based snoop for USB device communications traffic
  • USBPcap - open source USB sniffer for Windows XP, Vista, 7 and 8.
  • usbmon - Linux kernel module which can snoop USB device communications traffic
    • parse_tcpdump_log.pl - a script to directly talk with usbmon, parsing the result into a format feasible for analysis. This is part of the contrib stuff at the v4l-utils tree. It allows both realtime and offline parsing of usbmon data, and its format output is compatible with other parsers like the em28xx log parser.
    • Wireshark - logs usbmon output, via libpcap
    • USBMon - logs usbmon output

Log parsers, format etc

Snooping Procedures:

  • Use a Snopping utility to get the log.
  • Group URB transactions into a shorter log by using a parser, like parse-usbsnoop.php
  • Identify the URB transactions at the control endpoint. URB transactions look like those:

40 02 00 00 ba 00 03 00 >>> 20 11 00

URB fields
Byte Meaning
1 bit 7 = 1 - IN / 0 - OUT

bit 6 = 1 - Vendor Class

2 URB Request
3-4 URB Value in big endian
5-6 URB Index in big endian
7-8 URB message size in big endian

For example, let's analyse the folowing URB's:

control URB examples
URB sequence log (URB setup + URB IN or OUT) Byte 1 Byte 2 Bytes 3-4 Bytes 5-6 Bytes 7-8 Message
40 00 00 00 08 00 01 00 >>> 3d USB OUT, Vendor Class Req = 0x00 Value = 0x0000 Index = 0x0008 Size = 0x0001 { 0x3d }
40 02 00 00 ba 00 03 00 >>> 20 11 00 USB OUT, Vendor Class Req = 0x02 Value = 0x0000 Index = 0x00ba Size = 0x0003 { 0x20, 0x11, 0x00 }
c0 00 00 00 15 00 01 00 <<< 00 USB IN, Vendor Class Req = 0x00 Value = 0x0000 Index = 0x0015 Size = 0x0001 { 0x00 }

After getting the log, you should analyse and understand the meaning of each of URB fields on your device.

For example, in the case of em28xx, Req is used to indicate internal registers or I2C, Value is always 0 and Index indicates what device register is being used.

On em28xx, the em28xx log parser could be used in order to proccess the URBs and the driver dmesg dumps (in the compact format as shown above) and print them into a more human way, generating C like statements that can be added at em28xx source code (with a few adaptations, in the case of i2c messages):

 em28xx_write_reg(dev, EM28XX_R08_GPIO, 0x3d);
 i2c_master_send(0xba>>1, { 20 11 00  }, 0x03);
 em28xx_read_reg(dev, EM28XX_R15_RGAIN);         /* read 0x00 */

If all you want is to check what the em28xx Linux driver is doing, and provided that the usbmon module is loaded (the Kernel should be compiled with CONFIG_USB_MON option), you can just use:

  ./parse_tcpdump_log.pl --pcap |./parse_em28xx.pl

There are some other parsers for some specific devices at the contrib directory of the v4l-tools git tree.

Command Playback Utilities:

  • usb-robot - plays back USB Snoopy capture logs
  • usbreplay - plays back usbsnoop capture logs


Snooping Procedure:

The data transferred by the system over the i2c bus (and smbus) can be snooped using the kernel's FTRACE facility. To use this, the following kernel options need to be enabled:


i2c tracing can then be enabled by:

echo 1 >/sys/kernel/debug/tracing/events/i2c/enable

Note that this will, by default, trace the traffic through all i2c adapters on the system. You can be selective about what you trace by something like:

echo adapter_nr==1 >/sys/kernel/debug/tracing/events/i2c/filter

The trace output can be found in prettified text form by:

cat /sys/kernel/debug/tracing/trace

This will look something like:

... i2c_write: i2c-5 #0 a=044 f=0000 l=2 [02-14]
... i2c_read: i2c-5 #1 a=044 f=0001 l=4
... i2c_reply: i2c-5 #1 a=044 f=0001 l=4 [33-00-00-00]
... i2c_result: i2c-5 n=2 ret=2

Where the components are:

  • i2c-<adapter-nr>
  • #<message-array-index>
  • a=<addr>
  • f=<flags>
  • l=<datalen>
  • n=<message-array-size>
  • ret=<result>
  • [<data-transferred>]

External Links