Greybus is the name for a new application layer protocol on top of Unipro that controls the Ara Phone from Google. This protocol turns a phone into a modular device, allowing any part of the system to be hotplugged while the phone is running. This talk will describe what this protocol is, why it was designed, and give the basics for how it works. It will discuss how this is implemented in the Linux kernel, and how it easily bridges existing hardware like USB, I2C, GPIO and others with little to no changes needed to existing kernel drivers. Greg KH, Linux Foundation

1. Greybus
Greg Kroah-Hartman
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2. Linaro
Leaflabs
BayLibre
Google
Linux Foundation
Acknowledgments
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3. Familiar high-level semantics from Internet and other networks...
● Send and receive messages of arbitrary size
● Automatic in order delivery
… but sophisticated underpinnings
● Mature and robust technology stack
● Leverages existing high-speed PHY
● Low-power modes
● Low latency
● QoS features
Mobile-friendly basis for other peripheral communication
● UniPro-based camera standard: CSI-3
● UniPro-based flash storage standard: UFS
UniPro is easy to use
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4. UniPro provides bidirectional connections for data transfer
UniPro Switch
Module C
Module A Module B
UniPro connections are established
between modules.
Multiple connections between
modules are possible.
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5. UniPro Switch
Module C
Module A Module B
Each UniPro port on a module establishes a
link with a corresponding port on a UniPro
switch in the Endo. A “Device ID” (like an IP
address) in each packet identifies the
destination module.
Each UniPro connection (red, yellow, and
purple) has two “CPort IDs” (like port
numbers), one for each module.
UniPro packets contain destination CPort
IDs, which identify the packet’s
associated connection.
UniPro provides bidirectional connections for data transfer
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6. UniPro key features and non-features
Features
● Multiple independent bidirectional
connections between endpoints
● Reliable, in order transmission and
reception of datagrams
● Error handling
● Credit-based flow control
● Traffic prioritization
Non-Features
● Stream transmission and reception
(datagrams only)
● Multicast (endpoint-to-endpoint only)
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7. UniPro strictly adheres to OSI layering
DME
Transport
Network
L1
Link
PHY Adapter
L4
L3
L2
L1.5
Physical
Bidirectional datagram connections, L4 flow control
Device and network routing
Framing, error handling, flow control, traffic prioritization
Link initialization, control, and management, other functions
Physical medium
L5+: Application specific protocols. Not part of UniPro; we are defining these for Project Ara.
7

8. Greybus
An application layer for Unipro
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9. Greybus protocol
● Device discovery and description
● Network routing and housekeeping
● Class protocols
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10. Device description
A Manifest describes the device to the system.
Examples of this information:
● Vendor and Product ID
● Serial Number
● “Bundles”
● Protocols used
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11. Module X
Modules Interfaces CPorts Bundles
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12. Interface 1
Modules Interfaces CPorts Bundles
Interface 0
Module X
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13. Interface 1
Module X
Modules Interfaces CPorts Bundles
Interface 0 CPort 0 CPort 0
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14. Interface 1
Module X
Interface 0
Modules Interfaces CPorts Bundles
CPort 1
CPort 2
CPort 3
CPort 4
Bundle 0 Bundle 1 Bundle 0
CPort 1CPort 5
CPort 0CPort 0
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15. Operations
● RPC-like way to talk to an interface
● Request / Response communication
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16. Operations
● RPC-like way to talk to an interface
● Request / Response communication
int get_version(char *major, char *minor);
int vibrator_on(short timeout_ms);
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17. Protocol classes
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18. Protocol classes
Implemented
● Battery
● Vibrator
● NFC
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19. Protocol classes
Implemented
● Battery
● Vibrator
● NFC
In progress
● Audio
● Input (HID)
● Sensors
● Camera
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20. Protocol classes
Implemented
● Battery
● Vibrator
● NFC
In progress
● Audio
● Input (HID)
● Sensors
● Camera
Later
● Wifi
● Bluetooth
● Cellular modem
● GPS
● Lights
● Display
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21. Bridged PHY protocols
● SPI
● SDIO
● UART
● PWM
● USB
● I2
C
● I2
S
● GPIO
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22. “Tunneled” protocols
● CSI
● DSI
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23. github.com/projectara/greybus
Linux kernel implementation
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24. Supervisor control, Module Manifest, Power Management,
EPMs, RF Bus, Driver Core Integration, Sysfs
Greybus Subsystem
Kernel
Greybus Core
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25. Supervisor control, Module Manifest, Power Management,
EPMs, RF Bus, Driver Core Integration, Sysfs
AP Bridge
host controllers
USB Subsystem
Greybus Subsystem
Kernel
Greybus Core
Native UniPro
host controllers
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26. Sensors
Supervisor control, Module Manifest, Power Management,
EPMs, RF Bus, Driver Core Integration, Sysfs
Native UniPro
host controllers
AP Bridge
host controllers
USB Subsystem
Greybus Subsystem
Kernel
Greybus Core
Camera
(CSI-3)
V4L2 iio
Power
Subsystem
Battery
...
Class
Drivers
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27. Sensors
Supervisor control, Module Manifest, Power Management,
EPMs, RF Bus, Driver Core Integration, Sysfs
Native UniPro
host controllers
AP Bridge
host controllers
USB Subsystem
Greybus Subsystem
Kernel
Greybus Core
Camera
(CSI-3)
V4L2 iio
Power
Subsystem
Battery
...
USBBridged
PHY Driver
USB
Subsystem
I2C
Subsystem
I2C
...
Class
Drivers
27

28. Linux kernel - Bridged-PHY support
● USB devices “just work”
● UART devices “just work”
● Userspace access provided for rest
● Existing kernel drivers need 20 lines added
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