The I2C subsystem is not the shiniest part of the Linux Kernel. For embedded devices, though, it is one of the many puzzle pieces which just have to work. Wolfram Sang has the experience of maintaining this subsystem for nearly 7 years now. This talk gives a short overview of how maintaining works in general and specifically in this subsystem. But mainly, it will highlight noteworthy points in the timeline and lessons learnt from that. It will present trends, not so much regarding I2C but more the Linux Kernel and the embedded ecosystem in general. And of course, there will be plenty of anecdotes and bits from behind the scenes for your entertainment. Wolfram Sang

1. From maintaining I2C to the big (embedded) picture
Wolfram Sang, Consultant / Renesas
24.09.2019, EmbeddedRecipes
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2. Simple SoC Block Diagram
System Controller
Advanced Int. Ctrl.
Power Mgt. Ctrl.
PLL
Osc
RC Osc
Reset Ctrl.
Brownout Detect
Power On Reset
Prog. Int. Timer
Watchdog Timer
Real Time Timer
Debug Unit
PID Ctrl.
JTAG
Scan
Voltage
Regulator
Peripheral
Bridge
MemoryController
EBI
SRAM
Flash
Flash
Programmer
Peripheral
Data Controller
Application-Specific
Logic
CAN
USB
PWM
SD/MMC
Framebuffer/DisplayADC
I2C / TWI
SPI
UART
Ethernet
PIO
ARM
Processor
PIO
PIO
APB
ASB/
AHB
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3. I2C: its simplicity is a problem
What could go wrong?
specs didn’t change much since 1982
definately not rocket science1
drivers are (relatively) simple
So, do an IP core, dump the driver, move on…
1
yet, gory details are everywhere
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4. I2C: its simplicity is a problem
What could go wrong?
specs didn’t change much since 1982
definately not rocket science1
drivers are (relatively) simple
So, do an IP core, dump the driver, move on…
I2C is a good example
a Fly-by subsystem
mostly used by embedded
largely maintained in spare time
1
yet, gory details are everywhere
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5. Growth of I2C bus master drivers
0
20
40
60
80
100
120
140
v3.0 v3.5 v3.10 v3.15 v4.0 v4.5 v4.10 v4.15 v4.20
2011 2012 2013 2014 2015 2016 2017 2018 2018
number of files in drivers/i2c/busses
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6. Bad News
Insane fragmentation!
:(
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7. Good News
Message was received:
People are sending patches!
o/
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8. More Bad News
We now have a scaling problem:
Who reviews all these patches2?
2
especially in fly-by subsystems
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9. Number of unprocessed patches
0
50
100
150
200
250
v3.0 v3.5 v3.10 v3.15 v4.0 v4.5
2011 2012 2013 2014 2015 2016
linux-i2c (72 commits in 4.8)
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10. 1st lesson learned
There will be backlog3
3
Relax and live with it…
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11. 2nd lesson learned
Do only stuff you are personally
interested in
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12. Not maintaining the drivers anymore
commit 95df14f7937482a2a527814061eda9b534adb7eb
Author: Wolfram Sang <wsa@the-dreams.de>
Date: Tue Apr 10 21:49:13 2018 +0200
MAINTAINERS: remove me as maintainer for I2C host drivers
The number of I2C host controller drivers keeps increasing, and although
I had some success acquiring specific driver maintainers, my bandwidth
is by far not enough to act as a fallback for the rest of the drivers.
...
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13. Group maintainership
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14. Group maintainership
Which group???happy it works for DRM, but difficult for fly-bys
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15. Group maintainership
Which group???happy it works for DRM, but difficult for fly-bys
Still, thanks a ton to the I2C mux and I2C ACPI submaintainers!
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16. Not that I tried…
From: Wolfram Sang <wsa@the-dreams.de>
Subject: I2C delay due to maintainer illness
sadly, I have been tied to bed for a few days now, not in a condition to
really work on I2C...
I think it is annoying, sure, but no catastrophe. However, it shows that
I am the single-point-of-failure for I2C patches, what I don't like.
Like I said before, I am open to group maintainership. If you think you
are a reliable candidate, please get in touch with me.
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17. Not that I tried…
From: Wolfram Sang <wsa@the-dreams.de>
Subject: I2C delay due to maintainer illness
sadly, I have been tied to bed for a few days now, not in a condition to
really work on I2C...
I think it is annoying, sure, but no catastrophe. However, it shows that
I am the single-point-of-failure for I2C patches, what I don't like.
Like I said before, I am open to group maintainership. If you think you
are a reliable candidate, please get in touch with me.
“Get well soon!”
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18. 3rd lesson learned
Divide & conquer
(to the extreme4)
4
i.e. to the driver level
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19. Problem: new drivers
From: Khalil Blaiech <kblaiech@mellanox.com>
Subject: [PATCH v5 1/2] i2c: i2c-mlx: I2C SMBus driver for Mellanox BlueField
Added BlueField I2C driver to offer master and slave support for
Mellanox BlueField SoCs. The driver implements an SMBus adapter
and interfaces to multiple busses that can be probed using both
ACPI and Device Tree infrastructures.
Signed-off-by: Khalil Blaiech <kblaiech@mellanox.com>
---
drivers/i2c/busses/Kconfig | 13 +
drivers/i2c/busses/Makefile | 1 +
drivers/i2c/busses/i2c-mlx.c | 2513 ++++++++++++++++++++++++++++++++++++++++++
3 files changed, 2527 insertions(+)
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20. 4th lesson learned
Let escalateI don’t like it, but I don’t scale
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21. See the plateaus?
0
20
40
60
80
100
120
140
v3.0 v3.5 v3.10 v3.15 v4.0 v4.5 v4.10 v4.15 v4.20
2011 2012 2013 2014 2015 2016 2017 2018 2018
number of files in drivers/i2c/busses
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22. Something to get rid off
This picture5
is still around
“The overall picture … is one of a development process that continues to
function like a relatively well-tuned machine. The number of contributors
continues to increase, the patch flow is steady, and there do not appear to
be many process-scalability issues in sight.”
Disclaimer: LWN’s opinion has changed meanwhile to the best of my
knowledge.
5
LWN on May 11th, 2016
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23. News from Linux Plumbers
Dmitry Vyukov gave a talk at LPC.
It sparked some discussions
tooling
workflow
documentation
All nice but…
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24. For my situation: tip of the iceberg
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25. Proposed solution I
Send out the new message
Next level reached
Old message arrived: Submit patches
New message: Take part in QA6
6
review other patches and maintain core code
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26. Proposed solution II
Kernel Infrastructure Initiative?
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27. Proposed solution III
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28. Proposed solution III
Maintainers on strike!
:)
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29. Disclaimer
My bias
embedded
fly-by
(mostly) voluntary
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30. The End
Questions? Comments?
Questions?
Right here, right now…
At the conference
wsa@the-dreams.de
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31. References
ArmSoC diagram from my previous talk “Maintainer’s Diary: We
have a scaling problem” at ELC Europe 2013 in Edinburgh (based on
this version from Wikimedia (by Cburnett))
Iceberg picture from Wikimedia (by Uwe Kils and Wiska Bodo)
List of unprocessed patches in I2C from my previous talk “I Still Think
We Have a Scaling Problem” at LinuxCon Europe 2016 in Berlin
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