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Allwinner Kernel Upstreaming Experiences

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Chen-Yu Tsai
Chen-Yu Tsai

This document discusses experiences with upstreaming kernel code for Allwinner SoCs. It provides an overview of the current status of various ARM SoC vendors' code in the mainline kernel, including Allwinner, Rockchip, and Mediatek. It offers tips for contributors, such as starting with small and simple code changes, reusing existing drivers when possible, and asking maintainers for feedback. It also outlines some challenges, like lack of documentation, porting outdated vendor code, and dealing with binary-only drivers and potential GPL violations.

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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 2015/8/25 2
Disclaimer I have only done Allwinner/sunxi related stuff. Linux-sunxi is community/volunteer based. Your mileage may vary. 2015/8/25 3
Outline • Current Status • Where to start • Tips • What to expect • New SoC bringup • Supporting new boards • Priorities • Difficulties 2015/8/25 4
Current Status Let’s look at some ARM SoC vendors 2015/8/25 5
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/ 2015/8/25 6
Allwinner • Headless server w/ MMC/SATA/USB storage • I2C & SPI • LCD/HDMI/VGA display w/ simplefb • Audio WIP 2015/8/25 7
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 2015/8/25 8
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 2015/8/25 9
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 2015/8/25 10
Mediatek • Basics (machine, clk, pinctrl , uart) • Drivers for • I2C, SPI, MMC, PMIC, Audio • Not seen in DTS yet 2015/8/25 11
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 2015/8/25 12
Where to start 2015/8/25 13
Getting it to boot • Machine definition (arch/arm/mach-*) • Basic DTS • UART 2015/8/25 14
Infrastructure • Clocks • Pinctrl • Reusable 2015/8/25 15
Peripherals • Storage • MMC • SATA • USB • PMIC • I2C • SPI • … • Common vs custom IP? 2015/8/25 16
Tips The kernel is very big and moves very fast 2015/8/25 17
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 2015/8/25 18
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 2015/8/25 19
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 2015/8/25 20
What to expect Not getting merged is not the end of the world 2015/8/25 21
Code issues • Code style • Indentation • Comments • Error path • Bugs • Resource leak? 2015/8/25 22
NACK • Wrong design • Ex: rfkill dt support • Deprecated functions • Ex: platform_data • Occasional disagreement • “simplefb: add clock handling code” 2015/8/25 23
Maintainers are busy • Sometimes you don’t get feedback • Maintainers often write code as well • Lots of mailing list traffic • Ping or resend 2015/8/25 24
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 (?) 2015/8/25 25
New SoC bringup 2015/8/25 26
New SoC 1. 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! 2015/8/25 27
Supporting new boards 2015/8/25 28
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! 2015/8/25 29
Priorities 2015/8/25 30
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 2015/8/25 31
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 2015/8/25 32
Difficulties 2015/8/25 33
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? 2015/8/25 34
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 2015/8/25 35
Binary drivers • Third-party IP vendors • GPUs • VPUs • … • NDAs 2015/8/25 36
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/ 2015/8/25 37
Questions? 2015/8/25 38

More Related Content

Allwinner Kernel Upstreaming Experiences

  • 2. 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 2015/8/25 2
  • 3. Disclaimer I have only done Allwinner/sunxi related stuff. Linux-sunxi is community/volunteer based. Your mileage may vary. 2015/8/25 3
  • 4. Outline • Current Status • Where to start • Tips • What to expect • New SoC bringup • Supporting new boards • Priorities • Difficulties 2015/8/25 4
  • 5. Current Status Let’s look at some ARM SoC vendors 2015/8/25 5
  • 6. 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/ 2015/8/25 6
  • 7. Allwinner • Headless server w/ MMC/SATA/USB storage • I2C & SPI • LCD/HDMI/VGA display w/ simplefb • Audio WIP 2015/8/25 7
  • 8. 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 2015/8/25 8
  • 9. 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 2015/8/25 9
  • 10. 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 2015/8/25 10
  • 11. Mediatek • Basics (machine, clk, pinctrl , uart) • Drivers for • I2C, SPI, MMC, PMIC, Audio • Not seen in DTS yet 2015/8/25 11
  • 12. 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 2015/8/25 12
  • 14. Getting it to boot • Machine definition (arch/arm/mach-*) • Basic DTS • UART 2015/8/25 14
  • 16. Peripherals • Storage • MMC • SATA • USB • PMIC • I2C • SPI • … • Common vs custom IP? 2015/8/25 16
  • 17. Tips The kernel is very big and moves very fast 2015/8/25 17
  • 18. 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 2015/8/25 18
  • 19. 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 2015/8/25 19
  • 20. 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 2015/8/25 20
  • 21. What to expect Not getting merged is not the end of the world 2015/8/25 21
  • 22. Code issues • Code style • Indentation • Comments • Error path • Bugs • Resource leak? 2015/8/25 22
  • 23. NACK • Wrong design • Ex: rfkill dt support • Deprecated functions • Ex: platform_data • Occasional disagreement • “simplefb: add clock handling code” 2015/8/25 23
  • 24. Maintainers are busy • Sometimes you don’t get feedback • Maintainers often write code as well • Lots of mailing list traffic • Ping or resend 2015/8/25 24
  • 25. 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 (?) 2015/8/25 25
  • 27. New SoC 1. 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! 2015/8/25 27
  • 29. 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! 2015/8/25 29
  • 31. 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 2015/8/25 31
  • 32. 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 2015/8/25 32
  • 34. 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? 2015/8/25 34
  • 35. 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 2015/8/25 35
  • 36. Binary drivers • Third-party IP vendors • GPUs • VPUs • … • NDAs 2015/8/25 36
  • 37. 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/ 2015/8/25 37