1 =======================
2 Linux UVC Gadget Driver
3 =======================
4
5 Overview
6 --------
7 The UVC Gadget driver is a driver for hardware on the *device* side of a USB
8 connection. It is intended to run on a Linux system that has USB device-side
9 hardware such as boards with an OTG port.
10
11 On the device system, once the driver is bound it appears as a V4L2 device with
12 the output capability.
13
14 On the host side (once connected via USB cable), a device running the UVC Gadget
15 driver *and controlled by an appropriate userspace program* should appear as a UVC
16 specification compliant camera, and function appropriately with any program
17 designed to handle them. The userspace program running on the device system can
18 queue image buffers from a variety of sources to be transmitted via the USB
19 connection. Typically this would mean forwarding the buffers from a camera sensor
20 peripheral, but the source of the buffer is entirely dependent on the userspace
21 companion program.
22
23 Configuring the device kernel
24 -----------------------------
25 The Kconfig options USB_CONFIGFS, USB_LIBCOMPOSITE, USB_CONFIGFS_F_UVC and
26 USB_F_UVC must be selected to enable support for the UVC gadget.
27
28 Configuring the gadget through configfs
29 ---------------------------------------
30 The UVC Gadget expects to be configured through configfs using the UVC function.
31 This allows a significant degree of flexibility, as many of a UVC device's
32 settings can be controlled this way.
33
34 Not all of the available attributes are described here. For a complete enumeration
35 see Documentation/ABI/testing/configfs-usb-gadget-uvc
36
37 Assumptions
38 ~~~~~~~~~~~
39 This section assumes that you have mounted configfs at `/sys/kernel/config` and
40 created a gadget as `/sys/kernel/config/usb_gadget/g1`.
41
42 The UVC Function
43 ~~~~~~~~~~~~~~~~
44
45 The first step is to create the UVC function:
46
47 .. code-block:: bash
48
49 # These variables will be assumed throughout the rest of the document
50 CONFIGFS="/sys/kernel/config"
51 GADGET="$CONFIGFS/usb_gadget/g1"
52 FUNCTION="$GADGET/functions/uvc.0"
53
54 mkdir -p $FUNCTION
55
56 Formats and Frames
57 ~~~~~~~~~~~~~~~~~~
58
59 You must configure the gadget by telling it which formats you support, as well
60 as the frame sizes and frame intervals that are supported for each format. In
61 the current implementation there is no way for the gadget to refuse to set a
62 format that the host instructs it to set, so it is important that this step is
63 completed *accurately* to ensure that the host never asks for a format that
64 can't be provided.
65
66 Formats are created under the streaming/uncompressed and streaming/mjpeg configfs
67 groups, with the framesizes created under the formats in the following
68 structure:
69
70 ::
71
72 uvc.0 +
73 |
74 + streaming +
75 |
76 + mjpeg +
77 | |
78 | + mjpeg +
79 | |
80 | + 720p
81 | |
82 | + 1080p
83 |
84 + uncompressed +
85 |
86 + yuyv +
87 |
88 + 720p
89 |
90 + 1080p
91
92 Each frame can then be configured with a width and height, plus the maximum
93 buffer size required to store a single frame, and finally with the supported
94 frame intervals for that format and framesize. Width and height are enumerated in
95 units of pixels, frame interval in units of 100ns. To create the structure
96 above with 2, 15 and 100 fps frameintervals for each framesize for example you
97 might do:
98
99 .. code-block:: bash
100
101 create_frame() {
102 # Example usage:
103 # create_frame <width> <height> <group> <format name>
104
105 WIDTH=$1
106 HEIGHT=$2
107 FORMAT=$3
108 NAME=$4
109
110 wdir=$FUNCTION/streaming/$FORMAT/$NAME/${HEIGHT}p
111
112 mkdir -p $wdir
113 echo $WIDTH > $wdir/wWidth
114 echo $HEIGHT > $wdir/wHeight
115 echo $(( $WIDTH * $HEIGHT * 2 )) > $wdir/dwMaxVideoFrameBufferSize
116 cat <<EOF > $wdir/dwFrameInterval
117 666666
118 100000
119 5000000
120 EOF
121 }
122
123 create_frame 1280 720 mjpeg mjpeg
124 create_frame 1920 1080 mjpeg mjpeg
125 create_frame 1280 720 uncompressed yuyv
126 create_frame 1920 1080 uncompressed yuyv
127
128 The only uncompressed format currently supported is YUYV, which is detailed at
129 Documentation/userspace-api/media/v4l/pixfmt-packed-yuv.rst.
130
131 Color Matching Descriptors
132 ~~~~~~~~~~~~~~~~~~~~~~~~~~
133 It's possible to specify some colometry information for each format you create.
134 This step is optional, and default information will be included if this step is
135 skipped; those default values follow those defined in the Color Matching Descriptor
136 section of the UVC specification.
137
138 To create a Color Matching Descriptor, create a configfs item and set its three
139 attributes to your desired settings and then link to it from the format you wish
140 it to be associated with:
141
142 .. code-block:: bash
143
144 # Create a new Color Matching Descriptor
145
146 mkdir $FUNCTION/streaming/color_matching/yuyv
147 pushd $FUNCTION/streaming/color_matching/yuyv
148
149 echo 1 > bColorPrimaries
150 echo 1 > bTransferCharacteristics
151 echo 4 > bMatrixCoefficients
152
153 popd
154
155 # Create a symlink to the Color Matching Descriptor from the format's config item
156 ln -s $FUNCTION/streaming/color_matching/yuyv $FUNCTION/streaming/uncompressed/yuyv
157
158 For details about the valid values, consult the UVC specification. Note that a
159 default color matching descriptor exists and is used by any format which does
160 not have a link to a different Color Matching Descriptor. It's possible to
161 change the attribute settings for the default descriptor, so bear in mind that if
162 you do that you are altering the defaults for any format that does not link to
163 a different one.
164
165
166 Header linking
167 ~~~~~~~~~~~~~~
168
169 The UVC specification requires that Format and Frame descriptors be preceded by
170 Headers detailing things such as the number and cumulative size of the different
171 Format descriptors that follow. This and similar operations are achieved in
172 configfs by linking between the configfs item representing the header and the
173 config items representing those other descriptors, in this manner:
174
175 .. code-block:: bash
176
177 mkdir $FUNCTION/streaming/header/h
178
179 # This section links the format descriptors and their associated frames
180 # to the header
181 cd $FUNCTION/streaming/header/h
182 ln -s ../../uncompressed/yuyv
183 ln -s ../../mjpeg/mjpeg
184
185 # This section ensures that the header will be transmitted for each
186 # speed's set of descriptors. If support for a particular speed is not
187 # needed then it can be skipped here.
188 cd ../../class/fs
189 ln -s ../../header/h
190 cd ../../class/hs
191 ln -s ../../header/h
192 cd ../../class/ss
193 ln -s ../../header/h
194 cd ../../../control
195 mkdir header/h
196 ln -s header/h class/fs
197 ln -s header/h class/ss
198
199
200 Extension Unit Support
201 ~~~~~~~~~~~~~~~~~~~~~~
202
203 A UVC Extension Unit (XU) basically provides a distinct unit to which control set
204 and get requests can be addressed. The meaning of those control requests is
205 entirely implementation dependent, but may be used to control settings outside
206 of the UVC specification (for example enabling or disabling video effects). An
207 XU can be inserted into the UVC unit chain or left free-hanging.
208
209 Configuring an extension unit involves creating an entry in the appropriate
210 directory and setting its attributes appropriately, like so:
211
212 .. code-block:: bash
213
214 mkdir $FUNCTION/control/extensions/xu.0
215 pushd $FUNCTION/control/extensions/xu.0
216
217 # Set the bUnitID of the Processing Unit as the source for this
218 # Extension Unit
219 echo 2 > baSourceID
220
221 # Set this XU as the source of the default output terminal. This inserts
222 # the XU into the UVC chain between the PU and OT such that the final
223 # chain is IT > PU > XU.0 > OT
224 cat bUnitID > ../../terminal/output/default/baSourceID
225
226 # Flag some controls as being available for use. The bmControl field is
227 # a bitmap with each bit denoting the availability of a particular
228 # control. For example to flag the 0th, 2nd and 3rd controls available:
229 echo 0x0d > bmControls
230
231 # Set the GUID; this is a vendor-specific code identifying the XU.
232 echo -e -n "\x01\x02\x03\x04\x05\x06\x07\x08\x09\x0a\x0b\x0c\x0d\x0e\x0f\x10" > guidExtensionCode
233
234 popd
235
236 The bmControls attribute and the baSourceID attribute are multi-value attributes.
237 This means that you may write multiple newline separated values to them. For
238 example to flag the 1st, 2nd, 9th and 10th controls as being available you would
239 need to write two values to bmControls, like so:
240
241 .. code-block:: bash
242
243 cat << EOF > bmControls
244 0x03
245 0x03
246 EOF
247
248 The multi-value nature of the baSourceID attribute belies the fact that XUs can
249 be multiple-input, though note that this currently has no significant effect.
250
251 The bControlSize attribute reflects the size of the bmControls attribute, and
252 similarly bNrInPins reflects the size of the baSourceID attributes. Both
253 attributes are automatically increased / decreased as you set bmControls and
254 baSourceID. It is also possible to manually increase or decrease bControlSize
255 which has the effect of truncating entries to the new size, or padding entries
256 out with 0x00, for example:
257
258 ::
259
260 $ cat bmControls
261 0x03
262 0x05
263
264 $ cat bControlSize
265 2
266
267 $ echo 1 > bControlSize
268 $ cat bmControls
269 0x03
270
271 $ echo 2 > bControlSize
272 $ cat bmControls
273 0x03
274 0x00
275
276 bNrInPins and baSourceID function in the same way.
277
278 Configuring Supported Controls for Camera Terminal and Processing Unit
279 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
280
281 The Camera Terminal and Processing Units in the UVC chain also have bmControls
282 attributes which function similarly to the same field in an Extension Unit.
283 Unlike XUs however, the meaning of the bitflag for these units is defined in
284 the UVC specification; you should consult the "Camera Terminal Descriptor" and
285 "Processing Unit Descriptor" sections for an enumeration of the flags.
286
287 .. code-block:: bash
288
289 # Set the Processing Unit's bmControls, flagging Brightness, Contrast
290 # and Hue as available controls:
291 echo 0x05 > $FUNCTION/control/processing/default/bmControls
292
293 # Set the Camera Terminal's bmControls, flagging Focus Absolute and
294 # Focus Relative as available controls:
295 echo 0x60 > $FUNCTION/control/terminal/camera/default/bmControls
296
297 If you do not set these fields then by default the Auto-Exposure Mode control
298 for the Camera Terminal and the Brightness control for the Processing Unit will
299 be flagged as available; if they are not supported you should set the field to
300 0x00.
301
302 Note that the size of the bmControls field for a Camera Terminal or Processing
303 Unit is fixed by the UVC specification, and so the bControlSize attribute is
304 read-only here.
305
306 Custom Strings Support
307 ~~~~~~~~~~~~~~~~~~~~~~
308
309 String descriptors that provide a textual description for various parts of a
310 USB device can be defined in the usual place within USB configfs, and may then
311 be linked to from the UVC function root or from Extension Unit directories to
312 assign those strings as descriptors:
313
314 .. code-block:: bash
315
316 # Create a string descriptor in us-EN and link to it from the function
317 # root. The name of the link is significant here, as it declares this
318 # descriptor to be intended for the Interface Association Descriptor.
319 # Other significant link names at function root are vs0_desc and vs1_desc
320 # For the VideoStreaming Interface 0/1 Descriptors.
321
322 mkdir -p $GADGET/strings/0x409/iad_desc
323 echo -n "Interface Associaton Descriptor" > $GADGET/strings/0x409/iad_desc/s
324 ln -s $GADGET/strings/0x409/iad_desc $FUNCTION/iad_desc
325
326 # Because the link to a String Descriptor from an Extension Unit clearly
327 # associates the two, the name of this link is not significant and may
328 # be set freely.
329
330 mkdir -p $GADGET/strings/0x409/xu.0
331 echo -n "A Very Useful Extension Unit" > $GADGET/strings/0x409/xu.0/s
332 ln -s $GADGET/strings/0x409/xu.0 $FUNCTION/control/extensions/xu.0
333
334 The interrupt endpoint
335 ~~~~~~~~~~~~~~~~~~~~~~
336
337 The VideoControl interface has an optional interrupt endpoint which is by default
338 disabled. This is intended to support delayed response control set requests for
339 UVC (which should respond through the interrupt endpoint rather than tying up
340 endpoint 0). At present support for sending data through this endpoint is missing
341 and so it is left disabled to avoid confusion. If you wish to enable it you can
342 do so through the configfs attribute:
343
344 .. code-block:: bash
345
346 echo 1 > $FUNCTION/control/enable_interrupt_ep
347
348 Bandwidth configuration
349 ~~~~~~~~~~~~~~~~~~~~~~~
350
351 There are three attributes which control the bandwidth of the USB connection.
352 These live in the function root and can be set within limits:
353
354 .. code-block:: bash
355
356 # streaming_interval sets bInterval. Values range from 1..255
357 echo 1 > $FUNCTION/streaming_interval
358
359 # streaming_maxpacket sets wMaxPacketSize. Valid values are 1024/2048/3072
360 echo 3072 > $FUNCTION/streaming_maxpacket
361
362 # streaming_maxburst sets bMaxBurst. Valid values are 1..15
363 echo 1 > $FUNCTION/streaming_maxburst
364
365
366 The values passed here will be clamped to valid values according to the UVC
367 specification (which depend on the speed of the USB connection). To understand
368 how the settings influence bandwidth you should consult the UVC specifications,
369 but a rule of thumb is that increasing the streaming_maxpacket setting will
370 improve bandwidth (and thus the maximum possible framerate), whilst the same is
371 true for streaming_maxburst provided the USB connection is running at SuperSpeed.
372 Increasing streaming_interval will reduce bandwidth and framerate.
373
374 The userspace application
375 -------------------------
376 By itself, the UVC Gadget driver cannot do anything particularly interesting. It
377 must be paired with a userspace program that responds to UVC control requests and
378 fills buffers to be queued to the V4L2 device that the driver creates. How those
379 things are achieved is implementation dependent and beyond the scope of this
380 document, but a reference application can be found at https://gitlab.freedesktop.org/camera/uvc-gadget
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