Documentation/usb/usbmon.txt

   1 * Introduction
   2
   3 The name "usbmon" in lowercase refers to a facility in kernel which is
   4 used to collect traces of I/O on the USB bus. This function is analogous
   5 to a packet socket used by network monitoring tools such as tcpdump(1)
   6 or Ethereal. Similarly, it is expected that a tool such as usbdump or
   7 USBMon (with uppercase letters) is used to examine raw traces produced
   8 by usbmon.
   9
  10 The usbmon reports requests made by peripheral-specific drivers to Host
  11 Controller Drivers (HCD). So, if HCD is buggy, the traces reported by
  12 usbmon may not correspond to bus transactions precisely. This is the same
  13 situation as with tcpdump.
  14
  15 * How to use usbmon to collect raw text traces
  16
  17 Unlike the packet socket, usbmon has an interface which provides traces
  18 in a text format. This is used for two purposes. First, it serves as a
  19 common trace exchange format for tools while most sophisticated formats
  20 are finalized. Second, humans can read it in case tools are not available.
  21
  22 To collect a raw text trace, execute following steps.
  23
  24 1. Prepare
  25
  26 Mount debugfs (it has to be enabled in your kernel configuration), and
  27 load the usbmon module (if built as module). The second step is skipped
  28 if usbmon is built into the kernel.
  29
  30 # mount -t debugfs none_debugs /sys/kernel/debug
  31 # modprobe usbmon
  32 #
  33
  34 Verify that bus sockets are present.
  35
  36 # ls /sys/kernel/debug/usbmon
  37 1s  1t  2s  2t  3s  3t  4s  4t
  38 #
  39
  40 2. Find which bus connects to the desired device
  41
  42 Run "cat /proc/bus/usb/devices", and find the T-line which corresponds to
  43 the device. Usually you do it by looking for the vendor string. If you have
  44 many similar devices, unplug one and compare two /proc/bus/usb/devices outputs.
  45 The T-line will have a bus number. Example:
  46
  47 T:  Bus=03 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12  MxCh= 0
  48 D:  Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
  49 P:  Vendor=0557 ProdID=2004 Rev= 1.00
  50 S:  Manufacturer=ATEN
  51 S:  Product=UC100KM V2.00
  52
  53 Bus=03 means it's bus 3.
  54
  55 3. Start 'cat'
  56
  57 # cat /sys/kernel/debug/usbmon/3t > /tmp/1.mon.out
  58
  59 This process will be reading until killed. Naturally, the output can be
  60 redirected to a desirable location. This is preferred, because it is going
  61 to be quite long.
  62
  63 4. Perform the desired operation on the USB bus
  64
  65 This is where you do something that creates the traffic: plug in a flash key,
  66 copy files, control a webcam, etc.
  67
  68 5. Kill cat
  69
  70 Usually it's done with a keyboard interrupt (Control-C).
  71
  72 At this point the output file (/tmp/1.mon.out in this example) can be saved,
  73 sent by e-mail, or inspected with a text editor. In the last case make sure
  74 that the file size is not excessive for your favourite editor.
  75
  76 * Raw text data format
  77
  78 The '1t' type data consists of a stream of events, such as URB submission,
  79 URB callback, submission error. Every event is a text line, which consists
  80 of whitespace separated words. The number of position of words may depend
  81 on the event type, but there is a set of words, common for all types.
  82
  83 Here is the list of words, from left to right:
  84 - URB Tag. This is used to identify URBs is normally a kernel mode address
  85  of the URB structure in hexadecimal.
  86 - Timestamp in microseconds, a decimal number. The timestamp's resolution
  87   depends on available clock, and so it can be much worse than a microsecond
  88   (if the implementation uses jiffies, for example).
  89 - Event Type. This type refers to the format of the event, not URB type.
  90   Available types are: S - submission, C - callback, E - submission error.
  91 - "Pipe". The pipe concept is deprecated. This is a composite word, used to
  92   be derived from information in pipes. It consists of three fields, separated
  93   by colons: URB type and direction, Device address, Endpoint number.
  94   Type and direction are encoded with two bytes in the following manner:
  95     Ci Co   Control input and output
  96     Zi Zo   Isochronous input and output
  97     Ii Io   Interrupt input and output
  98     Bi Bo   Bulk input and output
  99   Device address and Endpoint number are 3-digit and 2-digit (respectively)
 100   decimal numbers, with leading zeroes.
 101 - URB Status. In most cases, this field contains a number, sometimes negative,
 102   which represents a "status" field of the URB. This field makes no sense for
 103   submissions, but is present anyway to help scripts with parsing. When an
 104   error occurs, the field contains the error code. In case of a submission of
 105   a Control packet, this field contains a Setup Tag instead of an error code.
 106   It is easy to tell whether the Setup Tag is present because it is never a
 107   number. Thus if scripts find a number in this field, they proceed to read
 108   Data Length. If they find something else, like a letter, they read the setup
 109   packet before reading the Data Length.
 110 - Setup packet, if present, consists of 5 words: one of each for bmRequestType,
 111   bRequest, wValue, wIndex, wLength, as specified by the USB Specification 2.0.
 112   These words are safe to decode if Setup Tag was 's'. Otherwise, the setup
 113   packet was present, but not captured, and the fields contain filler.
 114 - Data Length. For submissions, this is the requested length. For callbacks,
 115   this is the actual length.
 116 - Data tag. The usbmon may not always capture data, even if length is nonzero.
 117   The data words are present only if this tag is '='.
 118 - Data words follow, in big endian hexadecimal format. Notice that they are
 119   not machine words, but really just a byte stream split into words to make
 120   it easier to read. Thus, the last word may contain from one to four bytes.
 121   The length of collected data is limited and can be less than the data length
 122   report in Data Length word.
 123
 124 Here is an example of code to read the data stream in a well known programming
 125 language:
 126
 127 class ParsedLine {
 128         int data_len;           /* Available length of data */
 129         byte data[];
 130
 131         void parseData(StringTokenizer st) {
 132                 int availwords = st.countTokens();
 133                 data = new byte[availwords * 4];
 134                 data_len = 0;
 135                 while (st.hasMoreTokens()) {
 136                         String data_str = st.nextToken();
 137                         int len = data_str.length() / 2;
 138                         int i;
 139                         int b;  // byte is signed, apparently?! XXX
 140                         for (i = 0; i < len; i++) {
 141                                 // data[data_len] = Byte.parseByte(
 142                                 //     data_str.substring(i*2, i*2 + 2),
 143                                 //     16);
 144                                 b = Integer.parseInt(
 145                                      data_str.substring(i*2, i*2 + 2),
 146                                      16);
 147                                 if (b >= 128)
 148                                         b *= -1;
 149                                 data[data_len] = (byte) b;
 150                                 data_len++;
 151                         }
 152                 }
 153         }
 154 }
 155
 156 This format may be changed in the future.
 157
 158 Examples:
 159
 160 An input control transfer to get a port status.
 161
 162 d5ea89a0 3575914555 S Ci:001:00 s a3 00 0000 0003 0004 4 <
 163 d5ea89a0 3575914560 C Ci:001:00 0 4 = 01050000
 164
 165 An output bulk transfer to send a SCSI command 0x5E in a 31-byte Bulk wrapper
 166 to a storage device at address 5:
 167
 168 dd65f0e8 4128379752 S Bo:005:02 -115 31 = 55534243 5e000000 00000000 00000600 00000000 00000000 00000000 000000
 169 dd65f0e8 4128379808 C Bo:005:02 0 31 >
 170
 171 * Raw binary format and API
 172
 173 TBD