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Linus Torvalds1da177e2005-04-16 15:20:36 -07001* Introduction
2
3The name "usbmon" in lowercase refers to a facility in kernel which is
4used to collect traces of I/O on the USB bus. This function is analogous
5to a packet socket used by network monitoring tools such as tcpdump(1)
6or Ethereal. Similarly, it is expected that a tool such as usbdump or
7USBMon (with uppercase letters) is used to examine raw traces produced
8by usbmon.
9
10The usbmon reports requests made by peripheral-specific drivers to Host
11Controller Drivers (HCD). So, if HCD is buggy, the traces reported by
12usbmon may not correspond to bus transactions precisely. This is the same
13situation as with tcpdump.
14
15* How to use usbmon to collect raw text traces
16
17Unlike the packet socket, usbmon has an interface which provides traces
18in a text format. This is used for two purposes. First, it serves as a
19common trace exchange format for tools while most sophisticated formats
20are finalized. Second, humans can read it in case tools are not available.
21
22To collect a raw text trace, execute following steps.
23
241. Prepare
25
26Mount debugfs (it has to be enabled in your kernel configuration), and
27load the usbmon module (if built as module). The second step is skipped
28if usbmon is built into the kernel.
29
30# mount -t debugfs none_debugs /sys/kernel/debug
31# modprobe usbmon
Pete Zaitcevd9ac2cf2006-06-12 20:09:39 -070032#
Linus Torvalds1da177e2005-04-16 15:20:36 -070033
34Verify that bus sockets are present.
35
Pete Zaitcevd9ac2cf2006-06-12 20:09:39 -070036# ls /sys/kernel/debug/usbmon
Linus Torvalds1da177e2005-04-16 15:20:36 -0700371s 1t 2s 2t 3s 3t 4s 4t
Pete Zaitcevd9ac2cf2006-06-12 20:09:39 -070038#
Linus Torvalds1da177e2005-04-16 15:20:36 -070039
402. Find which bus connects to the desired device
41
42Run "cat /proc/bus/usb/devices", and find the T-line which corresponds to
43the device. Usually you do it by looking for the vendor string. If you have
44many similar devices, unplug one and compare two /proc/bus/usb/devices outputs.
45The T-line will have a bus number. Example:
46
47T: Bus=03 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 0
48D: Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
49P: Vendor=0557 ProdID=2004 Rev= 1.00
50S: Manufacturer=ATEN
51S: Product=UC100KM V2.00
52
53Bus=03 means it's bus 3.
54
553. Start 'cat'
56
57# cat /sys/kernel/debug/usbmon/3t > /tmp/1.mon.out
58
59This process will be reading until killed. Naturally, the output can be
60redirected to a desirable location. This is preferred, because it is going
61to be quite long.
62
634. Perform the desired operation on the USB bus
64
65This is where you do something that creates the traffic: plug in a flash key,
66copy files, control a webcam, etc.
67
685. Kill cat
69
70Usually it's done with a keyboard interrupt (Control-C).
71
72At this point the output file (/tmp/1.mon.out in this example) can be saved,
73sent by e-mail, or inspected with a text editor. In the last case make sure
74that the file size is not excessive for your favourite editor.
75
76* Raw text data format
77
Pete Zaitcevd9ac2cf2006-06-12 20:09:39 -070078The '1t' type data consists of a stream of events, such as URB submission,
Linus Torvalds1da177e2005-04-16 15:20:36 -070079URB callback, submission error. Every event is a text line, which consists
Pete Zaitcev6f23ee12006-12-30 22:43:10 -080080of whitespace separated words. The number or position of words may depend
Linus Torvalds1da177e2005-04-16 15:20:36 -070081on the event type, but there is a set of words, common for all types.
82
83Here 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
Pete Zaitcevd9ac2cf2006-06-12 20:09:39 -070099 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.
Pete Zaitcevae0d6cc2005-06-25 14:32:59 -0700110- 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.
Pete Zaitcevd9ac2cf2006-06-12 20:09:39 -0700114- Data Length. For submissions, this is the requested length. For callbacks,
115 this is the actual length.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700116- Data tag. The usbmon may not always capture data, even if length is nonzero.
Pete Zaitcevd9ac2cf2006-06-12 20:09:39 -0700117 The data words are present only if this tag is '='.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700118- 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
124Here is an example of code to read the data stream in a well known programming
125language:
126
127class 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;
Pete Zaitcevae0d6cc2005-06-25 14:32:59 -0700139 int b; // byte is signed, apparently?! XXX
Linus Torvalds1da177e2005-04-16 15:20:36 -0700140 for (i = 0; i < len; i++) {
Pete Zaitcevae0d6cc2005-06-25 14:32:59 -0700141 // 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;
Linus Torvalds1da177e2005-04-16 15:20:36 -0700150 data_len++;
151 }
152 }
153 }
154}
155
Pete Zaitcevae0d6cc2005-06-25 14:32:59 -0700156This format may be changed in the future.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700157
158Examples:
159
Pete Zaitcevae0d6cc2005-06-25 14:32:59 -0700160An input control transfer to get a port status.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700161
Pete Zaitcevae0d6cc2005-06-25 14:32:59 -0700162d5ea89a0 3575914555 S Ci:001:00 s a3 00 0000 0003 0004 4 <
163d5ea89a0 3575914560 C Ci:001:00 0 4 = 01050000
Linus Torvalds1da177e2005-04-16 15:20:36 -0700164
165An output bulk transfer to send a SCSI command 0x5E in a 31-byte Bulk wrapper
166to a storage device at address 5:
167
168dd65f0e8 4128379752 S Bo:005:02 -115 31 = 55534243 5e000000 00000000 00000600 00000000 00000000 00000000 000000
169dd65f0e8 4128379808 C Bo:005:02 0 31 >
170
171* Raw binary format and API
172
Pete Zaitcev6f23ee12006-12-30 22:43:10 -0800173The overall architecture of the API is about the same as the one above,
174only the events are delivered in binary format. Each event is sent in
175the following structure (its name is made up, so that we can refer to it):
176
177struct usbmon_packet {
178 u64 id; /* 0: URB ID - from submission to callback */
179 unsigned char type; /* 8: Same as text; extensible. */
180 unsigned char xfer_type; /* ISO (0), Intr, Control, Bulk (3) */
181 unsigned char epnum; /* Endpoint number and transfer direction */
182 unsigned char devnum; /* Device address */
183 u16 busnum; /* 12: Bus number */
184 char flag_setup; /* 14: Same as text */
185 char flag_data; /* 15: Same as text; Binary zero is OK. */
186 s64 ts_sec; /* 16: gettimeofday */
187 s32 ts_usec; /* 24: gettimeofday */
188 int status; /* 28: */
189 unsigned int length; /* 32: Length of data (submitted or actual) */
190 unsigned int len_cap; /* 36: Delivered length */
191 unsigned char setup[8]; /* 40: Only for Control 'S' */
192}; /* 48 bytes total */
193
194These events can be received from a character device by reading with read(2),
195with an ioctl(2), or by accessing the buffer with mmap.
196
197The character device is usually called /dev/usbmonN, where N is the USB bus
198number. Number zero (/dev/usbmon0) is special and means "all buses".
199However, this feature is not implemented yet. Note that specific naming
200policy is set by your Linux distribution.
201
202If you create /dev/usbmon0 by hand, make sure that it is owned by root
203and has mode 0600. Otherwise, unpriviledged users will be able to snoop
204keyboard traffic.
205
206The following ioctl calls are available, with MON_IOC_MAGIC 0x92:
207
208 MON_IOCQ_URB_LEN, defined as _IO(MON_IOC_MAGIC, 1)
209
210This call returns the length of data in the next event. Note that majority of
211events contain no data, so if this call returns zero, it does not mean that
212no events are available.
213
214 MON_IOCG_STATS, defined as _IOR(MON_IOC_MAGIC, 3, struct mon_bin_stats)
215
216The argument is a pointer to the following structure:
217
218struct mon_bin_stats {
219 u32 queued;
220 u32 dropped;
221};
222
223The member "queued" refers to the number of events currently queued in the
224buffer (and not to the number of events processed since the last reset).
225
226The member "dropped" is the number of events lost since the last call
227to MON_IOCG_STATS.
228
229 MON_IOCT_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 4)
230
231This call sets the buffer size. The argument is the size in bytes.
232The size may be rounded down to the next chunk (or page). If the requested
233size is out of [unspecified] bounds for this kernel, the call fails with
234-EINVAL.
235
236 MON_IOCQ_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 5)
237
238This call returns the current size of the buffer in bytes.
239
240 MON_IOCX_GET, defined as _IOW(MON_IOC_MAGIC, 6, struct mon_get_arg)
241
242This call waits for events to arrive if none were in the kernel buffer,
243then returns the first event. Its argument is a pointer to the following
244structure:
245
246struct mon_get_arg {
247 struct usbmon_packet *hdr;
248 void *data;
249 size_t alloc; /* Length of data (can be zero) */
250};
251
252Before the call, hdr, data, and alloc should be filled. Upon return, the area
253pointed by hdr contains the next event structure, and the data buffer contains
254the data, if any. The event is removed from the kernel buffer.
255
256 MON_IOCX_MFETCH, defined as _IOWR(MON_IOC_MAGIC, 7, struct mon_mfetch_arg)
257
258This ioctl is primarily used when the application accesses the buffer
259with mmap(2). Its argument is a pointer to the following structure:
260
261struct mon_mfetch_arg {
262 uint32_t *offvec; /* Vector of events fetched */
263 uint32_t nfetch; /* Number of events to fetch (out: fetched) */
264 uint32_t nflush; /* Number of events to flush */
265};
266
267The ioctl operates in 3 stages.
268
269First, it removes and discards up to nflush events from the kernel buffer.
270The actual number of events discarded is returned in nflush.
271
272Second, it waits for an event to be present in the buffer, unless the pseudo-
273device is open with O_NONBLOCK.
274
275Third, it extracts up to nfetch offsets into the mmap buffer, and stores
276them into the offvec. The actual number of event offsets is stored into
277the nfetch.
278
279 MON_IOCH_MFLUSH, defined as _IO(MON_IOC_MAGIC, 8)
280
281This call removes a number of events from the kernel buffer. Its argument
282is the number of events to remove. If the buffer contains fewer events
283than requested, all events present are removed, and no error is reported.
284This works when no events are available too.
285
286 FIONBIO
287
288The ioctl FIONBIO may be implemented in the future, if there's a need.
289
290In addition to ioctl(2) and read(2), the special file of binary API can
291be polled with select(2) and poll(2). But lseek(2) does not work.
292
293* Memory-mapped access of the kernel buffer for the binary API
294
295The basic idea is simple:
296
297To prepare, map the buffer by getting the current size, then using mmap(2).
298Then, execute a loop similar to the one written in pseudo-code below:
299
300 struct mon_mfetch_arg fetch;
301 struct usbmon_packet *hdr;
302 int nflush = 0;
303 for (;;) {
304 fetch.offvec = vec; // Has N 32-bit words
305 fetch.nfetch = N; // Or less than N
306 fetch.nflush = nflush;
307 ioctl(fd, MON_IOCX_MFETCH, &fetch); // Process errors, too
308 nflush = fetch.nfetch; // This many packets to flush when done
309 for (i = 0; i < nflush; i++) {
310 hdr = (struct ubsmon_packet *) &mmap_area[vec[i]];
311 if (hdr->type == '@') // Filler packet
312 continue;
313 caddr_t data = &mmap_area[vec[i]] + 64;
314 process_packet(hdr, data);
315 }
316 }
317
318Thus, the main idea is to execute only one ioctl per N events.
319
320Although the buffer is circular, the returned headers and data do not cross
321the end of the buffer, so the above pseudo-code does not need any gathering.