Allwinner Kernel Upstreaming Experiences

Chen-Yu Tsai (wens)蔡鎮宇

Chen-Yu Tsai 蔡鎮宇• Software Engineer @ CloudMosa

• Puffin Web Browser

• Totally not embedded Linux related

• Linux-sunxi community hobbyist• Contributions to U-boot and Linux

• git log --oneline --author “Chen-Yu Tsai”

• Translations

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DisclaimerI have only done Allwinner/sunxi related stuff.

Linux-sunxi is community/volunteer based.

Your mileage may vary.

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Outline• Current Status

• Where to start

• Tips

• What to expect

• New SoC bringup

• Supporting new boards

• Priorities

• Difficulties

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Current StatusLet’s look at some ARM SoC vendors

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linux-sunxi?• Cheap hardware

• Development boards

• Documents and SDKs available• Given by hardware vendors

• Allwinner now provides them on github• https://github.com/allwinner-zh/

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Allwinner• Headless server w/ MMC/SATA/USB storage

• I2C & SPI

• LCD/HDMI/VGA display w/ simplefb

• Audio WIP

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Allwinner• Community driven

• git lo --no-merges --perl-regexp --grep

'sunxi|sun[0-9]i' | grep -v m68k | wc –l

• 1118

• Extremely active

• Allwinner supplies documents and their source code• Answers questions

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Rockchip• Basically a fully working system

• MMC/USB

• I2C & SPI

• Full KMS display support

• I2S & audio codecs

• Chromebook based on mainline kernel + dts

• IP blocks mostly from ARM or Synopsys

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Rockchip• Hobbyist maintainer

• git lo --author 'Heiko Stuebner' | wc -l

• 158

• filtered out Samsung related

• Official support• git lo --author @rock-chips.com | wc -l

• 169

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Mediatek• Basics (machine, clk, pinctrl , uart)

• Drivers for• I2C, SPI, MMC, PMIC, Audio

• Not seen in DTS yet

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Mediatek• Outside maintainer

• git lo --author 'Matthias Brugger' | wc -l

• 53

• Since 2014/7/22

• Official support• git lo --author @mediatek.com | wc -l

• 122

• Since 2014/11/4

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Where to start

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Getting it to boot• Machine definition (arch/arm/mach-*)

• Basic DTS

• UART

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Infrastructure• Clocks

• Pinctrl

• Reusable

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Peripherals• Storage

• MMC

• SATA

• USB

• PMIC

• I2C

• SPI

• …

• Common vs custom IP?

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TipsThe kernel is very big and moves very fast

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Start small• Is it a known, supported IP block?

• Only standard resources?• MMIO, IRQ, clocks, resets

• Ex: USB host

• Hardware glue? Quirks?• Is the core supported?

• Is it a library?

• Write a glue driver

• Modify / port

• Ex: SATA AHCI, STMMAC

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Start simple• Clocks can be done one at a time

• Driver describes clock controls

• Driver or DTS describes relationship between clocks• Both methods are used

• Pinctrl / GPIO• Transcribe datasheets

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Look around and Ask• Subscribe to mailing lists

• Look at how other contributors write code

• Learn what reviewers ask for

• Look at how other platforms do it• mediatek pinctrl driver is very similar to sunxi

• Documentation is sometimes lacking• When in doubt, ask

• Release often• No one is going to review unsent code

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What to expectNot getting merged is not the end of the world

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Code issues• Code style

• Indentation

• Comments

• Error path

• Bugs• Resource leak?

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NACK• Wrong design

• Ex: rfkill dt support

• Deprecated functions• Ex: platform_data

• Occasional disagreement• “simplefb: add clock handling code”

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Maintainers are busy• Sometimes you don’t get feedback

• Maintainers often write code as well

• Lots of mailing list traffic

• Ping or resend

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Didn’t get your patches in this release?• You can do better

• Write better code

• Respond to reviewers

• Keep working

• Release cycle is predictable

• Sometimes it’s not your fault• Maintainer missed your patch or pull request

• It happens

• Maintainer went on vacation

• Maintainer missing (?)

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New SoC bringup

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New SoC1. Getting it to Boot

• Machine definition (arch/arm/mach-*)• Basic DTS• UART

2. Clocks / Pinctrl

3. Peripherals• IP blocks don’t often change

• Existing drivers should work

• Quite satisfying!

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Supporting new boards

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Supporting new boards• Need schematics or hardware description

• What peripherals are connected to the board?

• What external ICs are there?• PMICs, audio codecs, sensors, storage…

• Extra GPIOs?• LEDs

• Write a .dts file

• Test that everything works

• Fairly easy!

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Priorities

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Paid work• Not really much say

• Getting products shipped is all that matters

• Still• You should try

• What benefits the most platforms / boards?

• Must have vs. want?• MMC vs USB vs audio

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Community hackers• Expertise / prior experience

• Did clocks before, do it again

• Interest• Me: I want WiFi! I want SMP!

• Low hanging fruit• Me: Network is DWMAC based, only need glue code

• Popular demand• DMA engine

• Audio

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Difficulties

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Documents• NDA?

• Incorrect• How do you know it’s incorrect?

• Missing sections

• Completely non-existent

• No board schematics• Should you risk it going up in smoke?

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Porting vendor code• Is often bad

• Because of time to market concerns

• Quite old• Android kernel still at 3.4?

• Custom frameworks• Obsoleted by common frameworks

• Clocks, resets, pinctrl, gpio, PHY, regmap, …

• Binary blobs• No source code

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Binary drivers• Third-party IP vendors

• GPUs

• VPUs

• …

• NDAs

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GPL violations• There shouldn’t be binary-only blobs in the kernel

• http://linux-sunxi.org/GPL_Violations

• http://www.xda-developers.com/have-you-paid-your-linux-kernel-source-license-fee/

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Questions?

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