docs: usb: rename files to .rst and add them to drivers-api

While there are a mix of things here, most of the stuff
were written from Kernel developer's PoV. So, add them to
the driver-api book.

A follow up for this patch would be to move documents from
there that are specific to sysadmins, adding them to the
admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 934 insertions, 0 deletions

diff --git a/Documentation/usb/gadget-testing.rst b/Documentation/usb/gadget-testing.rst
new file mode 100644
index 000000000000..2eeb3e9299e4
--- /dev/null
+++ b/Documentation/usb/gadget-testing.rst
@@ -0,0 +1,934 @@
+==============
+Gadget Testing
+==============
+
+This file summarizes information on basic testing of USB functions
+provided by gadgets.
+
+.. contents
+
+   1. ACM function
+   2. ECM function
+   3. ECM subset function
+   4. EEM function
+   5. FFS function
+   6. HID function
+   7. LOOPBACK function
+   8. MASS STORAGE function
+   9. MIDI function
+   10. NCM function
+   11. OBEX function
+   12. PHONET function
+   13. RNDIS function
+   14. SERIAL function
+   15. SOURCESINK function
+   16. UAC1 function (legacy implementation)
+   17. UAC2 function
+   18. UVC function
+   19. PRINTER function
+   20. UAC1 function (new API)
+
+
+1. ACM function
+===============
+
+The function is provided by usb_f_acm.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "acm".
+The ACM function provides just one attribute in its function directory:
+
+	port_num
+
+The attribute is read-only.
+
+There can be at most 4 ACM/generic serial/OBEX ports in the system.
+
+
+Testing the ACM function
+------------------------
+
+On the host::
+
+	cat > /dev/ttyACM<X>
+
+On the device::
+
+	cat /dev/ttyGS<Y>
+
+then the other way round
+
+On the device::
+
+	cat > /dev/ttyGS<Y>
+
+On the host::
+
+	cat /dev/ttyACM<X>
+
+2. ECM function
+===============
+
+The function is provided by usb_f_ecm.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "ecm".
+The ECM function provides these attributes in its function directory:
+
+	=============== ==================================================
+	ifname		network device interface name associated with this
+			function instance
+	qmult		queue length multiplier for high and super speed
+	host_addr	MAC address of host's end of this
+			Ethernet over USB link
+	dev_addr	MAC address of device's end of this
+			Ethernet over USB link
+	=============== ==================================================
+
+and after creating the functions/ecm.<instance name> they contain default
+values: qmult is 5, dev_addr and host_addr are randomly selected.
+Except for ifname they can be written to until the function is linked to a
+configuration. The ifname is read-only and contains the name of the interface
+which was assigned by the net core, e. g. usb0.
+
+Testing the ECM function
+------------------------
+
+Configure IP addresses of the device and the host. Then:
+
+On the device::
+
+	ping <host's IP>
+
+On the host::
+
+	ping <device's IP>
+
+3. ECM subset function
+======================
+
+The function is provided by usb_f_ecm_subset.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "geth".
+The ECM subset function provides these attributes in its function directory:
+
+	=============== ==================================================
+	ifname		network device interface name associated with this
+			function instance
+	qmult		queue length multiplier for high and super speed
+	host_addr	MAC address of host's end of this
+			Ethernet over USB link
+	dev_addr	MAC address of device's end of this
+			Ethernet over USB link
+	=============== ==================================================
+
+and after creating the functions/ecm.<instance name> they contain default
+values: qmult is 5, dev_addr and host_addr are randomly selected.
+Except for ifname they can be written to until the function is linked to a
+configuration. The ifname is read-only and contains the name of the interface
+which was assigned by the net core, e. g. usb0.
+
+Testing the ECM subset function
+-------------------------------
+
+Configure IP addresses of the device and the host. Then:
+
+On the device::
+
+	ping <host's IP>
+
+On the host::
+
+	ping <device's IP>
+
+4. EEM function
+===============
+
+The function is provided by usb_f_eem.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "eem".
+The EEM function provides these attributes in its function directory:
+
+	=============== ==================================================
+	ifname		network device interface name associated with this
+			function instance
+	qmult		queue length multiplier for high and super speed
+	host_addr	MAC address of host's end of this
+			Ethernet over USB link
+	dev_addr	MAC address of device's end of this
+			Ethernet over USB link
+	=============== ==================================================
+
+and after creating the functions/eem.<instance name> they contain default
+values: qmult is 5, dev_addr and host_addr are randomly selected.
+Except for ifname they can be written to until the function is linked to a
+configuration. The ifname is read-only and contains the name of the interface
+which was assigned by the net core, e. g. usb0.
+
+Testing the EEM function
+------------------------
+
+Configure IP addresses of the device and the host. Then:
+
+On the device::
+
+	ping <host's IP>
+
+On the host::
+
+	ping <device's IP>
+
+5. FFS function
+===============
+
+The function is provided by usb_f_fs.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "ffs".
+The function directory is intentionally empty and not modifiable.
+
+After creating the directory there is a new instance (a "device") of FunctionFS
+available in the system. Once a "device" is available, the user should follow
+the standard procedure for using FunctionFS (mount it, run the userspace
+process which implements the function proper). The gadget should be enabled
+by writing a suitable string to usb_gadget/<gadget>/UDC.
+
+Testing the FFS function
+------------------------
+
+On the device: start the function's userspace daemon, enable the gadget
+
+On the host: use the USB function provided by the device
+
+6. HID function
+===============
+
+The function is provided by usb_f_hid.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "hid".
+The HID function provides these attributes in its function directory:
+
+	=============== ===========================================
+	protocol	HID protocol to use
+	report_desc	data to be used in HID reports, except data
+			passed with /dev/hidg<X>
+	report_length	HID report length
+	subclass	HID subclass to use
+	=============== ===========================================
+
+For a keyboard the protocol and the subclass are 1, the report_length is 8,
+while the report_desc is::
+
+  $ hd my_report_desc
+  00000000  05 01 09 06 a1 01 05 07  19 e0 29 e7 15 00 25 01  |..........)...%.|
+  00000010  75 01 95 08 81 02 95 01  75 08 81 03 95 05 75 01  |u.......u.....u.|
+  00000020  05 08 19 01 29 05 91 02  95 01 75 03 91 03 95 06  |....).....u.....|
+  00000030  75 08 15 00 25 65 05 07  19 00 29 65 81 00 c0     |u...%e....)e...|
+  0000003f
+
+Such a sequence of bytes can be stored to the attribute with echo::
+
+  $ echo -ne \\x05\\x01\\x09\\x06\\xa1.....
+
+Testing the HID function
+------------------------
+
+Device:
+
+- create the gadget
+- connect the gadget to a host, preferably not the one used
+  to control the gadget
+- run a program which writes to /dev/hidg<N>, e.g.
+  a userspace program found in Documentation/usb/gadget_hid.rst::
+
+	$ ./hid_gadget_test /dev/hidg0 keyboard
+
+Host:
+
+- observe the keystrokes from the gadget
+
+7. LOOPBACK function
+====================
+
+The function is provided by usb_f_ss_lb.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "Loopback".
+The LOOPBACK function provides these attributes in its function directory:
+
+	=============== =======================
+	qlen		depth of loopback queue
+	bulk_buflen	buffer length
+	=============== =======================
+
+Testing the LOOPBACK function
+-----------------------------
+
+device: run the gadget
+
+host: test-usb (tools/usb/testusb.c)
+
+8. MASS STORAGE function
+========================
+
+The function is provided by usb_f_mass_storage.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "mass_storage".
+The MASS STORAGE function provides these attributes in its directory:
+files:
+
+	=============== ==============================================
+	stall		Set to permit function to halt bulk endpoints.
+			Disabled on some USB devices known not to work
+			correctly. You should set it to true.
+	num_buffers	Number of pipeline buffers. Valid numbers
+			are 2..4. Available only if
+			CONFIG_USB_GADGET_DEBUG_FILES is set.
+	=============== ==============================================
+
+and a default lun.0 directory corresponding to SCSI LUN #0.
+
+A new lun can be added with mkdir::
+
+	$ mkdir functions/mass_storage.0/partition.5
+
+Lun numbering does not have to be continuous, except for lun #0 which is
+created by default. A maximum of 8 luns can be specified and they all must be
+named following the <name>.<number> scheme. The numbers can be 0..8.
+Probably a good convention is to name the luns "lun.<number>",
+although it is not mandatory.
+
+In each lun directory there are the following attribute files:
+
+	=============== ==============================================
+	file		The path to the backing file for the LUN.
+			Required if LUN is not marked as removable.
+	ro		Flag specifying access to the LUN shall be
+			read-only. This is implied if CD-ROM emulation
+			is enabled as well as when it was impossible
+			to open "filename" in R/W mode.
+	removable	Flag specifying that LUN shall be indicated as
+			being removable.
+	cdrom		Flag specifying that LUN shall be reported as
+			being a CD-ROM.
+	nofua		Flag specifying that FUA flag
+			in SCSI WRITE(10,12)
+	=============== ==============================================
+
+Testing the MASS STORAGE function
+---------------------------------
+
+device: connect the gadget, enable it
+host: dmesg, see the USB drives appear (if system configured to automatically
+mount)
+
+9. MIDI function
+================
+
+The function is provided by usb_f_midi.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "midi".
+The MIDI function provides these attributes in its function directory:
+
+	=============== ====================================
+	buflen		MIDI buffer length
+	id		ID string for the USB MIDI adapter
+	in_ports	number of MIDI input ports
+	index		index value for the USB MIDI adapter
+	out_ports	number of MIDI output ports
+	qlen		USB read request queue length
+	=============== ====================================
+
+Testing the MIDI function
+-------------------------
+
+There are two cases: playing a mid from the gadget to
+the host and playing a mid from the host to the gadget.
+
+1) Playing a mid from the gadget to the host:
+
+host::
+
+  $ arecordmidi -l
+   Port    Client name                      Port name
+   14:0    Midi Through                     Midi Through Port-0
+   24:0    MIDI Gadget                      MIDI Gadget MIDI 1
+  $ arecordmidi -p 24:0 from_gadget.mid
+
+gadget::
+
+  $ aplaymidi -l
+   Port    Client name                      Port name
+   20:0    f_midi                           f_midi
+
+  $ aplaymidi -p 20:0 to_host.mid
+
+2) Playing a mid from the host to the gadget
+
+gadget::
+
+  $ arecordmidi -l
+   Port    Client name                      Port name
+   20:0    f_midi                           f_midi
+
+  $ arecordmidi -p 20:0 from_host.mid
+
+host::
+
+  $ aplaymidi -l
+   Port    Client name                      Port name
+   14:0    Midi Through                     Midi Through Port-0
+   24:0    MIDI Gadget                      MIDI Gadget MIDI 1
+
+  $ aplaymidi -p24:0 to_gadget.mid
+
+The from_gadget.mid should sound identical to the to_host.mid.
+
+The from_host.id should sound identical to the to_gadget.mid.
+
+MIDI files can be played to speakers/headphones with e.g. timidity installed::
+
+  $ aplaymidi -l
+   Port    Client name                      Port name
+   14:0    Midi Through                     Midi Through Port-0
+   24:0    MIDI Gadget                      MIDI Gadget MIDI 1
+  128:0    TiMidity                         TiMidity port 0
+  128:1    TiMidity                         TiMidity port 1
+  128:2    TiMidity                         TiMidity port 2
+  128:3    TiMidity                         TiMidity port 3
+
+  $ aplaymidi -p 128:0 file.mid
+
+MIDI ports can be logically connected using the aconnect utility, e.g.::
+
+  $ aconnect 24:0 128:0 # try it on the host
+
+After the gadget's MIDI port is connected to timidity's MIDI port,
+whatever is played at the gadget side with aplaymidi -l is audible
+in host's speakers/headphones.
+
+10. NCM function
+================
+
+The function is provided by usb_f_ncm.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "ncm".
+The NCM function provides these attributes in its function directory:
+
+	=============== ==================================================
+	ifname		network device interface name associated with this
+			function instance
+	qmult		queue length multiplier for high and super speed
+	host_addr	MAC address of host's end of this
+			Ethernet over USB link
+	dev_addr	MAC address of device's end of this
+			Ethernet over USB link
+	=============== ==================================================
+
+and after creating the functions/ncm.<instance name> they contain default
+values: qmult is 5, dev_addr and host_addr are randomly selected.
+Except for ifname they can be written to until the function is linked to a
+configuration. The ifname is read-only and contains the name of the interface
+which was assigned by the net core, e. g. usb0.
+
+Testing the NCM function
+------------------------
+
+Configure IP addresses of the device and the host. Then:
+
+On the device::
+
+	ping <host's IP>
+
+On the host::
+
+	ping <device's IP>
+
+11. OBEX function
+=================
+
+The function is provided by usb_f_obex.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "obex".
+The OBEX function provides just one attribute in its function directory:
+
+	port_num
+
+The attribute is read-only.
+
+There can be at most 4 ACM/generic serial/OBEX ports in the system.
+
+Testing the OBEX function
+-------------------------
+
+On device::
+
+	seriald -f /dev/ttyGS<Y> -s 1024
+
+On host::
+
+	serialc -v <vendorID> -p <productID> -i<interface#> -a1 -s1024 \
+                -t<out endpoint addr> -r<in endpoint addr>
+
+where seriald and serialc are Felipe's utilities found here:
+
+	https://github.com/felipebalbi/usb-tools.git master
+
+12. PHONET function
+===================
+
+The function is provided by usb_f_phonet.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "phonet".
+The PHONET function provides just one attribute in its function directory:
+
+	=============== ==================================================
+	ifname		network device interface name associated with this
+			function instance
+	=============== ==================================================
+
+Testing the PHONET function
+---------------------------
+
+It is not possible to test the SOCK_STREAM protocol without a specific piece
+of hardware, so only SOCK_DGRAM has been tested. For the latter to work,
+in the past I had to apply the patch mentioned here:
+
+http://www.spinics.net/lists/linux-usb/msg85689.html
+
+These tools are required:
+
+git://git.gitorious.org/meego-cellular/phonet-utils.git
+
+On the host::
+
+	$ ./phonet -a 0x10 -i usbpn0
+	$ ./pnroute add 0x6c usbpn0
+	$./pnroute add 0x10 usbpn0
+	$ ifconfig usbpn0 up
+
+On the device::
+
+	$ ./phonet -a 0x6c -i upnlink0
+	$ ./pnroute add 0x10 upnlink0
+	$ ifconfig upnlink0 up
+
+Then a test program can be used::
+
+	http://www.spinics.net/lists/linux-usb/msg85690.html
+
+On the device::
+
+	$ ./pnxmit -a 0x6c -r
+
+On the host::
+
+	$ ./pnxmit -a 0x10 -s 0x6c
+
+As a result some data should be sent from host to device.
+Then the other way round:
+
+On the host::
+
+	$ ./pnxmit -a 0x10 -r
+
+On the device::
+
+	$ ./pnxmit -a 0x6c -s 0x10
+
+13. RNDIS function
+==================
+
+The function is provided by usb_f_rndis.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "rndis".
+The RNDIS function provides these attributes in its function directory:
+
+	=============== ==================================================
+	ifname		network device interface name associated with this
+			function instance
+	qmult		queue length multiplier for high and super speed
+	host_addr	MAC address of host's end of this
+			Ethernet over USB link
+	dev_addr	MAC address of device's end of this
+			Ethernet over USB link
+	=============== ==================================================
+
+and after creating the functions/rndis.<instance name> they contain default
+values: qmult is 5, dev_addr and host_addr are randomly selected.
+Except for ifname they can be written to until the function is linked to a
+configuration. The ifname is read-only and contains the name of the interface
+which was assigned by the net core, e. g. usb0.
+
+Testing the RNDIS function
+--------------------------
+
+Configure IP addresses of the device and the host. Then:
+
+On the device::
+
+	ping <host's IP>
+
+On the host::
+
+	ping <device's IP>
+
+14. SERIAL function
+===================
+
+The function is provided by usb_f_gser.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "gser".
+The SERIAL function provides just one attribute in its function directory:
+
+	port_num
+
+The attribute is read-only.
+
+There can be at most 4 ACM/generic serial/OBEX ports in the system.
+
+Testing the SERIAL function
+---------------------------
+
+On host::
+
+	insmod usbserial
+	echo VID PID >/sys/bus/usb-serial/drivers/generic/new_id
+
+On host::
+
+	cat > /dev/ttyUSB<X>
+
+On target::
+
+	cat /dev/ttyGS<Y>
+
+then the other way round
+
+On target::
+
+	cat > /dev/ttyGS<Y>
+
+On host::
+
+	cat /dev/ttyUSB<X>
+
+15. SOURCESINK function
+=======================
+
+The function is provided by usb_f_ss_lb.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "SourceSink".
+The SOURCESINK function provides these attributes in its function directory:
+
+	=============== ==================================
+	pattern		0 (all zeros), 1 (mod63), 2 (none)
+	isoc_interval	1..16
+	isoc_maxpacket	0 - 1023 (fs), 0 - 1024 (hs/ss)
+	isoc_mult	0..2 (hs/ss only)
+	isoc_maxburst	0..15 (ss only)
+	bulk_buflen	buffer length
+	bulk_qlen	depth of queue for bulk
+	iso_qlen	depth of queue for iso
+	=============== ==================================
+
+Testing the SOURCESINK function
+-------------------------------
+
+device: run the gadget
+
+host: test-usb (tools/usb/testusb.c)
+
+
+16. UAC1 function (legacy implementation)
+=========================================
+
+The function is provided by usb_f_uac1_legacy.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory
+is "uac1_legacy".
+The uac1 function provides these attributes in its function directory:
+
+	=============== ====================================
+	audio_buf_size	audio buffer size
+	fn_cap		capture pcm device file name
+	fn_cntl		control device file name
+	fn_play		playback pcm device file name
+	req_buf_size	ISO OUT endpoint request buffer size
+	req_count	ISO OUT endpoint request count
+	=============== ====================================
+
+The attributes have sane default values.
+
+Testing the UAC1 function
+-------------------------
+
+device: run the gadget
+
+host::
+
+	aplay -l # should list our USB Audio Gadget
+
+17. UAC2 function
+=================
+
+The function is provided by usb_f_uac2.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "uac2".
+The uac2 function provides these attributes in its function directory:
+
+	=============== ====================================================
+	c_chmask	capture channel mask
+	c_srate		capture sampling rate
+	c_ssize		capture sample size (bytes)
+	p_chmask	playback channel mask
+	p_srate		playback sampling rate
+	p_ssize		playback sample size (bytes)
+	req_number	the number of pre-allocated request for both capture
+			and playback
+	=============== ====================================================
+
+The attributes have sane default values.
+
+Testing the UAC2 function
+-------------------------
+
+device: run the gadget
+host: aplay -l # should list our USB Audio Gadget
+
+This function does not require real hardware support, it just
+sends a stream of audio data to/from the host. In order to
+actually hear something at the device side, a command similar
+to this must be used at the device side::
+
+	$ arecord -f dat -t wav -D hw:2,0 | aplay -D hw:0,0 &
+
+e.g.::
+
+	$ arecord -f dat -t wav -D hw:CARD=UAC2Gadget,DEV=0 | \
+	  aplay -D default:CARD=OdroidU3
+
+18. UVC function
+================
+
+The function is provided by usb_f_uvc.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "uvc".
+The uvc function provides these attributes in its function directory:
+
+	=================== ================================================
+	streaming_interval  interval for polling endpoint for data transfers
+	streaming_maxburst  bMaxBurst for super speed companion descriptor
+	streaming_maxpacket maximum packet size this endpoint is capable of
+			    sending or receiving when this configuration is
+			    selected
+	=================== ================================================
+
+There are also "control" and "streaming" subdirectories, each of which contain
+a number of their subdirectories. There are some sane defaults provided, but
+the user must provide the following:
+
+	================== ====================================================
+	control header     create in control/header, link from control/class/fs
+			   and/or control/class/ss
+	streaming header   create in streaming/header, link from
+			   streaming/class/fs and/or streaming/class/hs and/or
+			   streaming/class/ss
+	format description create in streaming/mjpeg and/or
+			   streaming/uncompressed
+	frame description  create in streaming/mjpeg/<format> and/or in
+			   streaming/uncompressed/<format>
+	================== ====================================================
+
+Each frame description contains frame interval specification, and each
+such specification consists of a number of lines with an inverval value
+in each line. The rules stated above are best illustrated with an example::
+
+  # mkdir functions/uvc.usb0/control/header/h
+  # cd functions/uvc.usb0/control/
+  # ln -s header/h class/fs
+  # ln -s header/h class/ss
+  # mkdir -p functions/uvc.usb0/streaming/uncompressed/u/360p
+  # cat <<EOF > functions/uvc.usb0/streaming/uncompressed/u/360p/dwFrameInterval
+  666666
+  1000000
+  5000000
+  EOF
+  # cd $GADGET_CONFIGFS_ROOT
+  # mkdir functions/uvc.usb0/streaming/header/h
+  # cd functions/uvc.usb0/streaming/header/h
+  # ln -s ../../uncompressed/u
+  # cd ../../class/fs
+  # ln -s ../../header/h
+  # cd ../../class/hs
+  # ln -s ../../header/h
+  # cd ../../class/ss
+  # ln -s ../../header/h
+
+
+Testing the UVC function
+------------------------
+
+device: run the gadget, modprobe vivid::
+
+  # uvc-gadget -u /dev/video<uvc video node #> -v /dev/video<vivid video node #>
+
+where uvc-gadget is this program:
+	http://git.ideasonboard.org/uvc-gadget.git
+
+with these patches:
+
+	http://www.spinics.net/lists/linux-usb/msg99220.html
+
+host::
+
+	luvcview -f yuv
+
+19. PRINTER function
+====================
+
+The function is provided by usb_f_printer.ko module.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "printer".
+The printer function provides these attributes in its function directory:
+
+	==========	===========================================
+	pnp_string	Data to be passed to the host in pnp string
+	q_len		Number of requests per endpoint
+	==========	===========================================
+
+Testing the PRINTER function
+----------------------------
+
+The most basic testing:
+
+device: run the gadget::
+
+	# ls -l /devices/virtual/usb_printer_gadget/
+
+should show g_printer<number>.
+
+If udev is active, then /dev/g_printer<number> should appear automatically.
+
+host:
+
+If udev is active, then e.g. /dev/usb/lp0 should appear.
+
+host->device transmission:
+
+device::
+
+	# cat /dev/g_printer<number>
+
+host::
+
+	# cat > /dev/usb/lp0
+
+device->host transmission::
+
+	# cat > /dev/g_printer<number>
+
+host::
+
+	# cat /dev/usb/lp0
+
+More advanced testing can be done with the prn_example
+described in Documentation/usb/gadget_printer.rst.
+
+
+20. UAC1 function (virtual ALSA card, using u_audio API)
+========================================================
+
+The function is provided by usb_f_uac1.ko module.
+It will create a virtual ALSA card and the audio streams are simply
+sinked to and sourced from it.
+
+Function-specific configfs interface
+------------------------------------
+
+The function name to use when creating the function directory is "uac1".
+The uac1 function provides these attributes in its function directory:
+
+	========== ====================================================
+	c_chmask   capture channel mask
+	c_srate    capture sampling rate
+	c_ssize    capture sample size (bytes)
+	p_chmask   playback channel mask
+	p_srate    playback sampling rate
+	p_ssize    playback sample size (bytes)
+	req_number the number of pre-allocated request for both capture
+		   and playback
+	========== ====================================================
+
+The attributes have sane default values.
+
+Testing the UAC1 function
+-------------------------
+
+device: run the gadget
+host: aplay -l # should list our USB Audio Gadget
+
+This function does not require real hardware support, it just
+sends a stream of audio data to/from the host. In order to
+actually hear something at the device side, a command similar
+to this must be used at the device side::
+
+	$ arecord -f dat -t wav -D hw:2,0 | aplay -D hw:0,0 &
+
+e.g.::
+
+	$ arecord -f dat -t wav -D hw:CARD=UAC1Gadget,DEV=0 | \
+	  aplay -D default:CARD=OdroidU3