1.
Linux Device Drivers
Sarah Sharp
Dec 6, 2014

2.
Sarah Sharp
● USB 3.0 host controller driver author
● Linux kernel developer for 9 years
● Worked in Intel's Open Source
Technology Center (OTC) for 7 years
● Driver maintainer for 5 years

3.
How did I get into
open source?
● Didn't program until college
● Portland State University
● My boyfriend introduced me to
– open source, Linux, rockets

4.
Portland State Aerospace Society

5.
Rockets and Open Source

6.
How did I get into
Linux kernel development?
● Professor Bart Massey
introduced me to
– The local Linux kernel community
– Greg Kroah-Hartman
● Greg suggested usbfs2 project
● Bart helped me get:
– College credit for kernel project
– Intel undergraduate research grant

7.
How did I get a
job in open source?
● Bart encouraged me to present at
OSCON on usbfs2
● Kristin Accardi was on
OSCON paper selection
committee
● Through network
(Bart → Greg → Kristen)
I got a job

8.
Sarah Sharp: Linux kernel
USB 3.0 driver developer
● Hired by Intel in July 2007
● USB 3.0 specification draft
● FPGA prototypes
● Driver merged in April 2009

9.
What is it like to be a
Linux kernel developer?
● very tight-knit group, very slow turn over
● 80% of kernel developers are paid
● 20% are volunteers and students
● all work done through mailing lists
● a few IRC channels
● rarely get together in person

10.
Linux kernel subsystems
● Many different parts of the kernel
● Broken into subsystems
● Each has a maintainer
● Most have a mailing list
● MAINTAINERS file
● scripts/get_maintainer.pl

11.
What Do
Subsystem Maintainers Do?
● Experts in their particular field
● Usually 5-20 years of experience
● Approve changes to their area
● Often have 1-4 sub-maintainers
● This is how Linux scales trust

12.
Intel
Linux Kernel Maintainers
● ACPI - Rafael Wysocki, Zhang Rui
● Bluetooth - Marcel Holtmann
● NFC - Samuel Ortiz
● USB 3.0 - Mathias Nyman
● Intel graphics - Daniel Vetter
● EFI - Matt Fleming
● x86 - H. Peter Anvin, Peter Z
● RAS, machine check - Tony Luck
● Code ellision - Andi Kleen
● NVMe - Matthew Wilcox
● IOMMU - Dave Woodhouse
● Temperature - Fenghua Yu
● DMA offload - Vinod Koul
● Wimax - Inaky Perez-Gonzalez
● Intel wifi - Johannes Berg
● Intel idle - Len Brown
● Intel ethernet - Jeff Kirsher, Jesse
Brandeburg, Carolyn Wyborny
● Intel C600 SAS, IOAT - Dan
Williams
● Serial, GMA500/600/3600, misc -
Alan Cox

13.
What is Being a
Linux Kernel Maintainer Like?
● Hundreds of emails a day
● 1:Many relationship with
contributors
● Rapid development cycle
● Looking at long-term health
of the project
● Maintains code for 3 - 20 years
● Reputation is on the line for
each patch

14.
“Meritocracy”
● Maintainers are vested
● Developers will come and go
● Long-standing contributors gain trust
● Easier for them to do complex things
● Harder for newcomers

15.
Community Trust and
Social Engineering
● Build a trust relationship with maintainers
– Review patches
– Ask intelligent questions
– Submit small bug fixes
● Learn community norms
– Email format
– Coding style
– Testing

16.
Tips for
Asking Intelligent Questions
● Always Cc the maintainer
● Do your research
● Document where you looked
● Be concise and precise
● Ask a question you know the
answer to, then one you don't

17.
Tips for
Bug Reporting
● Always Cc the maintainer
● System information is crucial
– What hardware?
– What kernel, what distro
– 32-bit vs 64-bit
– lspci, lsusb, acpidump
● Reproducibility thinking
● Be responsive
● Hold onto hardware

18.
Linux Kernel
Quality Assurance
● Regressions are not tolerated
● Buggy new code is fixed or reverted
● Shifting towards static analysis tools
– sparse, smatch, Coccinelle
● 0-day testing infrastructure
● kselftest

19.
Getting into Linux kernel
development
● Read the code, review other people's patches
● Participate and ask questions on the mailing list
● First patch tutorial
– http://kernelnewbies.org/FirstKernelPatch
● Staging cleanup patches
– http://kernelnewbies.org/CheckpatchTips
● Linux kernel exercises
– http://eudyptula-challenge.org/

20.
Leveling Up in
Linux Kernel Development
● Larger patchsets
● Use complex tools (sparse, Coccinelle)
● Find an area you're passionate about
● Look for todos, bugzilla entries
● Before starting a large task:
– Always ask the maintainer
– build trust with the maintainer

21.
Tips for Code Review
● Keep a list of the feedback
● Address ALL feedback in revisions
● Track changes in your cover letter
● If you disagree with a maintainer,
back yourself up with facts
● Maintainers don't care about your deadline
● Code must be complete and clean
● Duplicate code or drivers won't be accepted

22.
More Linux Kernel Resources
● Linux Device Drivers book:
http://lwn.net/Kernel/LDD3
● Linux Device Driver tutorial:
https://github.com/gregkh/kernel-tutorial
● FOSS Outreach Program for Women (OPW)
(paid open source and Linux kernel internships)
http://gnome.org/opw

23.
Photo Attribution
● Tux:
http://upload.wikimedia.org/wikipedia/commons/thumb/b/b0/NewTux.svg/400px-
NewTux.svg.png
● Jamey and Sarah Sharp: https://www.flickr.com/photos/igalko/7570033604
● Sarah Sharp and PSAS photos CC BY-SA-NC Sarah Sharp
● Bart Massey: https://www.flickr.com/photos/reidab/9326868060
● OSCON sign: https://www.flickr.com/photos/kubina/912778997
● USB 3.0 host PCI card: https://www.flickr.com/photos/hades2k/9069181710
● Logitech webcam: https://www.flickr.com/photos/bshunrichs/162199777
● Greg Kroah-Hartman: https://www.flickr.com/photos/tian2992/6208491794
● David Brownell:
https://www.flickr.com/photos/13825348@N03/sets/72157608741347102/detail/

24.
Photo Attribution
● Computer guts: https://www.flickr.com/photos/orijinal/4832982682
● Kernel Summit: https://www.flickr.com/photos/linuxfoundation/2987611750
● Mail:
https://www.flickr.com/photos/donebythehandsofabrokenartist/1502717818
9
● Nerd merit badge: https://www.flickr.com/photos/momentum/4203541703
● Trust: https://www.flickr.com/photos/dobs/10726756606
● Question: https://www.flickr.com/photos/marcobellucci/3534516458
● Bug: https://www.flickr.com/photos/103499652@N04/14232715868
● Quality: https://www.flickr.com/photos/dieselbug2007/414348333
● Editing: https://www.flickr.com/photos/nics_events/2349631515