Modern USB Gadget with Custom USB Functions

Modern USB Gadget with Custom USB Functions...

...and its integration with systemd

Andrzej Pietrasiewicz <[email protected]> ELC 2019

2

USB Device user?

A Linux machine connected to a PC

3

Overview

● Composition and components+ your own USB functionality

● Tools

● systemd integration+ your own USB functionality

4

Composition and components

5

Endpoints

● Control

● Bulk

● Interrupt

● Isochronous

USB HOST

USB DEVICE

0 0 1 1 2 2

Data for “1”

6

Interface

● Collection of endpoints for a particular purpose

● Implemented as “USB function” in Linux kernel

7

Configuration

● Collection of 1+ interface(s)

● Device contains 1+ configuration(s)

– Only 1 active at a time

– Rarely > 1

8

FunctionFS

● A USB function

● A filesystem with ep0 and epX….

9

Theoretical # combinations

(201 )+(202 )+(203 )+...+(2020)=ALOT

10

Tools

11

Configfs

● /sys/kernel/config/usb_gadget

● mkdir, echo, ln -s, rmdir

12

mkdir g1cd g1

echo 0x1d6b > idVendorecho 0x104 > idProduct

mkdir strings/0x409 # US English, others rarely seenecho "Collabora" > strings/0x409/manufacturerecho "ECM" > strings/0x409/product

mkdir configs/c.1 # dot and number mandatory

mkdir functions/ecm.usb0

ln -s functions/ecm.usb0/ configs/c.1/

# bind!# ls /sys/class/udc to see available UDCsecho 12480000.hsotg > UDC

13

mkdir g1cd g1







14

mkdir g1cd g1







15

mkdir g1cd g1







16

mkdir g1cd g1







17

mkdir g1cd g1







18

gt

● github.com/kopasiak/gt

● github.com/libusbgx/libusbgx.git

● gadget “schemes”

19

attrs : { idVendor = 0x1D6B; idProduct = 0x104;};strings = ( { lang = 0x409; manufacturer = "Collabora"; product = "ECM"; });functions : { ecm_usb0 : { instance = "usb0"; type = "ecm"; };};

configs = ( { id = 1; name = "c"; functions = ( { name = "ecm.usb0"; function = "ecm_usb0"; } ); } );

20



21



22



23



24



25

systemd integration

26

systemd

● Various units

– service, mount, target, socket, ...

● Tracked dependencies

● High parallelization

● Lazy initialization (socket units)

27

The “glue layer”

● usb-gadget.target

● udev event

28



● udev event

[Unit] Description=Hardware activated USB gadget Documentation=man:systemd.special(7)

29



● udev event

[Unit] Description=Hardware activated USB gadget Documentation=man:systemd.special(7)

SUBSYSTEM=="udc", ACTION=="add", TAG+="systemd", \ENV{SYSTEMD_WANTS}+="usb-gadget.target"

30

Service unit[Unit]Description=Load USB gadget scheme for ecmRequires=sys-kernel-config.mountAfter=sys-kernel-config.mountDefaultDependencies=no

[Service]ExecStart=/bin/gt load ecm.scheme ecmRemainAfterExit=yesExecStop=/bin/gt rm -rf ecmType=simple

[Install]WantedBy=usb-gadget.target

31




32




33




34




35




systemctl [en|dis]able usb-gadget.service

36




systemctl [en|dis]able usb-gadget.service

37

Service unit templatized[Unit]Description=Load USB gadget scheme for %iRequires=sys-kernel-config.mountAfter=sys-kernel-config.mountDefaultDependencies=no

[Service]ExecStart=/bin/gt load %i.scheme %iRemainAfterExit=yesExecStop=/bin/gt rm -rf %iType=simple


38

Service unit templatized[Unit]Description=Load USB gadget scheme for %iRequires=sys-kernel-config.mountAfter=sys-kernel-config.mountDefaultDependencies=no

[Service]ExecStart=/bin/gt load %i.scheme %iRemainAfterExit=yesExecStop=/bin/gt rm -rf %iType=simple


systemctl [en|dis]able [email protected]

39

systemd integrationYour own USB function

40

FunctionFS vs systemd

● Gadget scheme

● Service unit (create gadget, gt load -o)

● Mount unit (FunctionFS instance)

● Socket unit (start its service unit, enable the gadget)

● Service unit (spawn userspace daemon)

41

attrs : { idVendor = 0x1D6B; idProduct = 0x104;};strings = ( { lang = 0x409; manufacturer = "Collabora"; product = "MTP Gadget"; });functions : { ffs_mtp : { instance = "mtp"; type = "ffs"; };};

configs = ( { id = 1; name = "c"; functions = ( { name = "ffs.mtp"; function = "ffs_mtp"; } ); } );

42



43



44

Mount unit[Unit]Description=FunctionFS instance for ffs_mtpRequires=usb-gadget-ffs@ffs_mtp.serviceAfter=usb-gadget-ffs@ffs_mtp.serviceBefore=ffs@ffs_mtp.socket

[Mount]What=mtpWhere=/run/ffs_mtpType=functionfsOptions=defaultsTimeoutSec=5

45



46



47



48

Socket unit[Unit]Description=USB FunctionFS socket for %iRequires=run-%i.mountAfter=run-%i.mountDefaultDependencies=no

[Socket]ListenUSBFunction=/run/%iService=%i.serviceExecStartPost=/bin/gt enable %iTriggerLimitIntervalSec=0


49




50




51




52




53




54

Service unit

[Unit]Description=MTP responderStartLimitIntervalSec=0

[Service]Type=simpleExecStart=/usr/bin/cmtp-responderUSBFunctionDescriptors=/etc/cmtp-responder/descsUSBFunctionStrings=/etc/cmtp-responder/strsKillMode=processRestartSec=3Restart=on-failure

55

Service unit



56

Service unit



57

Service unit



include/uapi/linux/usb/functionfs.h

58

Descriptors and strings

● github.com/kopasiak/gt/pull/8

● Declarative config → blob

– USBFunctionDescriptors, USBFunctionStrings

59

Demo

60

MTP Device

● github.com/cmtp-responder/cmtp-responder.git

● Odroid U2

61

Summary

● Composition and components+ your own USB functionality

● Tools

● systemd integration+ your own USB functionality

62

Thank you!

Modern USB Gadget with Custom USB Functions

Documents