The Linux I 2 C Subsystem Jean Delvare Suse L3 Support

© Novell, Inc. All rights reserved. 2 What is I 2 C? I 2 C (Inter-Integrated Circuit) is a multi-master, serial, single-ended computer bus invented by Philips, that is used to attach low-speed peripherals to a motherboard, embedded system, or cellphone. SMBus (System Management Bus) is a subset of I 2 C that defines stricter electrical and protocol conventions. TWI (Two-Wire Interface), ACCESS.bus and DDC (Display Data Channel) are basically the same as I 2 C.

© Novell, Inc. All rights reserved. 3 Usages of I 2 C in your computer On the motherboard (SMBus) – SPD EEPROM on the memory modules – Hardware monitoring – Clock chips, voltage regulators, etc. On the graphics adapter (DDC and I 2 C) – Monitor information and control over VGA/DVI – Thermal sensors On the TV adapter (I 2 C) – Tuner, video encoder, video decoder, audio decoder, etc.

© Novell, Inc. All rights reserved. 4 SMBus in your computer (example) Host controller EEPROM (memory modules) Sensor chip 0x5 0 0x5 1 0x5 2 0x2 d

© Novell, Inc. All rights reserved. 5 History of the Linux i2c subsystem April 1998: Implementation by Gerd Knorr (kernel v2.1.94) December 1998: Out-of-tree implementation by Simon G. Vogl, part of lm-sensors 2 December 1999: Merged into the kernel tree (v2.3.34), still part of the lm-sensors project Gerd Knorr's implementation finally dropped (v2.5.29) March to May 2003: Migrated to the Linux 2.5 device driver model by Greg KH (v to v2.5.70) May 2007 to April 2008: Properly integrated into the standard device driver model by David Brownell and Jean Delvare (v to v2.6.25)

© Novell, Inc. All rights reserved. 6 Why I 2 C sucks I 2 C lacks device enumeration I 2 C lacks a standard method for device presence detection I 2 C lacks standard identification registers I 2 C lacks clear semantics of what's a read and what's a write! We can't count on automatic device instantiation. How do we instantiate devices?

© Novell, Inc. All rights reserved. 7 Why SMBus sucks less (but still) SMBus has an ARP mechanism for both device enumeration and identification, – but it's optional. SMBus has a standard subset of bus transactions, with mostly clear semantics, – but not all chips follow the SMBus standard, – and there is still room for confusion

© Novell, Inc. All rights reserved. 8 Binding model migration Legacy model: – Each driver detects and instantiates its own devices – Instantiation from user-space handled by each the driver – No alternative! Standard model: – Devices are instantiated outside drivers – Instantiation from user-space is centralized – For backwards compatibility, drivers can detect their devices and ask i2c-core to instantiate them – Various ways to instantiate devices

Future developments

© Novell, Inc. All rights reserved Remaining issues The i2c-dev driver (user-space access to I 2 C devices) is still not migrated to the new binding model – Full of race conditions – Needs a complete redesign – Missing features (e.g. exclusive bus access) Legacy attributes of i2c adapters (retries, timeout) are unclear – Need to be clarified – Do we care about backwards compatibility? – Move handling to i2c-core

© Novell, Inc. All rights reserved. 1 Missing features SMBus Alert – Done! ☺ by David Brownell and Jean Delvare, merged in kernel Bus multiplexing – Work in progress by Rodolfo Giometti, Michael Lawnick and Jean Delvare, ready for kernel SMBus ARP (Address Resolution Protocol) – Work in progress by Corentin Labbe, long way to go

© Novell, Inc. All rights reserved I 2 C multiplexing, why? Host controller LM63 sensor chip (for CPU0) 0x4 c LM63 sensor chip (for CPU1) Address collision! bus_id = 0-004c Switch i2c-0

© Novell, Inc. All rights reserved I 2 C Multiplexing, how? Problem: the i2c subsystem's bus_id format assumes unique device addresses on every single i2c_adapter. Solution #1: just change the bus_id format! – Not possible: user-space relies on it. Solution #2: cheat on addresses! – Not possible: only 16 bits in format for the address, needed for 10-bit addressing extension; and wouldn't cover the case of cascaded switches. Solution #3: cheat on bus numbers! – This works: just claim that struct i2c_adapter now represents an I 2 C bus segment rather than an I 2 C bus controller.

© Novell, Inc. All rights reserved I 2 C multiplexing implementation Host controller LM63 sensor chip (for CPU0) 0x4c, bus_id = c 0x4c, bus_id = c LM63 sensor chip (for CPU1) Switch i2c-0 i2c-1 i2c-2

© Novell, Inc. All rights reserved Questions?

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