Mainlining

Mainlining is the process of replacing the kernel provided by the device vendor (the "downstream" kernel), with a version close to the actively developed version released on kernel.org (the "mainline" kernel). It involves porting (and most importantly: cleaning up) device-specific code from the downstream kernel to the mainline kernel. For some SoCs, common support already exists in mainline, so you can focus on device-specific components.

Why?

Mainlining is a fairly complicated process, especially if you're one of the first to start work on a given SoC. However through community effort there is a growing number of SoCs that have increasingly stable mainline ports (see below). In general it is relatively simple to take your existing Android kernel and boot it without any of the Android blobs, however from that point it can be a lot of working to create a working system and at the end of it you don't receive any security patches or general improvements.

Apart from purely practical insufficiencies, the cold hard truth is that the downstream code is simply not meant to be maintained or get used outside Android. The code quality is often several magnitudes worse, which makes maintenance rather complicated. Often large parts of the drivers are moved into proprietary parts in Android userspace - which are generally impossible to use without emulating the expected Android environment. At the end, the effort to make such drivers work properly on long term is often as much as rewriting them properly for mainline.

Successfully mainlining an SoC or device means that you benefit from every improvement the Linux kernel has to offer, you can run a fairly standard Linux distro (in theory even any Linux distro that has support for your device’s architecture) and benefit from kernel updates and improvements without needing to compile your own kernel or deal with frustrating patches. A notable easy-to-notice benefit of running mainline is minimal-effort-needed GPU support for almost all GPUs (except PowerVR unfortunately), which is needed for using modern phone DEs like Plasma Mobile or Phosh with acceptable performance. Lastly, mainlining is a terrific learning opportunity that you will never regret.

If your device's SoC is in the list below there's a very decent chance that you can get the same functionality working on your device and eventually upstream your patches to the kernel, helping to build a solid base for other devices too!

Overview

Supported SoCs

Some SoCs are already supported quite well and are used for some devices in postmarketOS. Getting started with one of these might be easier for you, because you can get help from others working with the same SoC. The feature matrix below shows which features are already supported for a particular SoC. The pages for each SoC will provide more information, plus hints how to get started and who you can contact.

CPU: SMP (bring up secondary CPU cores), CPU frequency scaling, CPUidle
Storage: eMMC, SD cards, UFS, ...
Video: Hardware-accelerated video de/encoding
Modem: Calls, SMS, Internet
Suspend: Can reach (all) low-power modes of the SoC during suspend (either system suspend or cpuidle).
Unavailable (-) means that such a feature is not built directly into SoC. For example, WiFi/BT are also often used through SDIO/UART which can be chosen independently from the SoC.

Note: The statuses displayed below signify if a particular feature was tested on a device by someone in the postmarketOS community - not necessarily the components supported in the mainline kernel.

SoC	Arch	Year	UART	USB	Storage	Display	CPU	GPU	Pinctrl	I²C	Audio	Video	Thermal	WiFi	BT	Modem	GPS	Camera	Suspend
Allwinner A33	armv7	2014	Y	Y	Y	Y	P	P	Y	Y	P	Y	Y	-	-	-	-	P	P
Allwinner A64	aarch64	2015	Y	Y	Y	Y	P	P	Y	Y	Y		Y	-	-	-	-	Y	P
Allwinner D1-H	riscv64	2020	Y	Y	P	P	Y	Y	Y	Y	Y	P	Y	-	-	-	-	-
Allwinner H3	armv7	2014	Y	Y	Y	Y	P	P	Y	Y	P	Y	Y	-	-	-	-	P	P
Amlogic AML8726-MX	armv7f	2012	P	N	N	N	P	N	N	N	N	N	N	-	-	-	-	-
Amlogic S805	armv7f	2014	Y	Y	P	P	Y	Y	P	Y	N	P	Y	-	-	-	-	-
Amlogic S812/S812-B/S812-S/S802	armv7f	2014	Y	Y	P	P	Y	Y	P	Y	N	P	Y	-	-	-	-	-
Amlogic S905	aarch64	2015	Y	Y	P	Y	Y	Y	Y	Y	Y	P	Y	-	-	-	-	-	N
Amlogic S905D2/S905X2/S905Y2	aarch64	2019	Y	Y	P	Y	Y	Y	Y	Y	Y	P	Y	-	-	-	-	-
Amlogic S905D3/S905X3/S905Y3	aarch64	2019	Y	Y	P	Y	Y	Y	Y	Y	Y	P	Y	-	-	-	-	-
Amlogic S905W/S905D/S905X/S905L/S805X/S805Y/S905Z	aarch64	2016	Y	Y	P	Y	Y	Y	Y	Y	Y	P	Y	-	-	-	-	-
Amlogic S912	aarch64	2016	Y	Y	P	Y	Y	Y	Y	Y	Y	P	Y	-	-	-	-	-
Amlogic S922X/A311D	aarch64	2019	Y	Y	P	Y	Y	Y	Y	Y	Y	P	Y	-	-	-	-	-
Apple A7 (S5L8960X/S5L8965X)	aarch64	2013	Y	P		P	P		Y					-	-	-	-
Apple M1 (T8103/Tonga)	aarch64	2020	Y	Y	Y	Y	Y	Y			Y	Y		-	-	-	-	Y	P
Broadcom BCM21664	armv7	2012	Y	N	P		P		Y	Y				-	-	-	-
Broadcom BCM23550	armv7	2012	Y	N	P		P		Y	Y				-	-	-	-
Broadcom BCM2711	aarch64	2019	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	-	-	-	-
Broadcom BCM28155	armv7	2012												-	-	-	-
Broadcom BCM2835	armhf	2012	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	-	-	-	-	Y
Broadcom BCM2836	armv7	2015	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	-	-	-	-	Y
Broadcom BCM2837	aarch64	2016	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	-	-	-	-	Y
Freescale i.MX6 SLL	armv7	2011	Y	Y	Y	Y	Y	-	Y	Y	Y	-	Y	-	-	-	-		Y
Freescale i.MX6 SoloLite	armv7	2011	Y	Y	Y	Y	Y	-	Y	Y	Y	-	Y	-	-	-	-		Y
HiSilicon Kirin 620 (Hi6220)	aarch64	2015	Y	Y	Y	Y	Y	Y	Y	Y	P			-	-	-	-
Intel Bay Trail	x86_64	2013	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	-	-	-	-	N	Y
Intel Cherry Trail	x86_64	2013	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	-	-	-	-	N	Y
Intel Cloverview	x86	2013
Intel Medfield	x86_64	2013
Intel Menlow	x86	2008
Intel Merrifield	x86_64	2014
Intel Moorefield	x86_64	2014
Intel Moorestown	x86	2011
Intel SoFIA	x86_64	2015												-
Marvell Armada 510	armv7	2011	Y	Y	Y	Y	Y	-	Y	Y	Y	-	Y	-	-	-	-		Y
Marvell PXA1908	aarch64	2014/2015	Y	Y	Y	N	P	N	Y	Y	N	N	N	-	-	N	N	N	N
MediaTek Helio P10/P15/P18	aarch64	2014	P	N	N	P	P	N	N	N	N	N	N	N	N	N	N	N	N
MediaTek Helio P90	aarch64	2018	Y	N	Y	P	Y	N	Y	N	Y	N	N	N	N	N		N	N
MediaTek MT6580	armv7	2015	Y	N	N	N	P	N	N	N	N	N	N	N	N	N	N	N	N
MediaTek MT6582	armv7	2013	Y	N	N	N	P	N	N	N	N	N	N	N	N	N	N	N	N
MediaTek MT6589	armv7	2013	Y	N	N	N	P	N	N	N	N	N	N	-	-	N	-	N	N
MediaTek MT6735	aarch64	2014-2016	Y	P	P	Y	N	Y	Y	N	N	N	N	N	N	N	N	N	N
MediaTek MT6765	aarch64	2018/2020	Y	Y	Y	Y	Y	N	Y	Y	N	N	N	N	N	N	N	N	N
MediaTek MT6878 (Dimensity 7300)	aarch64	2024	Y	N	N	N	Y	N	N	N	N	N	N	N	N	N	N	N	N
MediaTek MT8173	aarch64	2015	Y	Y	Y	Y	Y	P	Y	Y	Y	Y	Y	N	N	-	-	N	Y
MediaTek MT8183	aarch64	2019	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	N	N	-	-	N	Y
MediaTek MT8186	aarch64	2022	Y	Y	Y	Y	Y	Y	Y	Y	N	Y	Y	-	-	-	-	N	N
Nvidia Tegra 2	armv7	2010	Y	Y	Y	Y	Y	P	Y	Y	Y	P	Y	-	-	-	-	N	Y
Nvidia Tegra 3	armv7	2011	Y	Y	Y	Y	Y	P	Y	Y	Y	P	Y	-	-	-	-	N	Y
Nvidia Tegra 4	armv7	2013	Y	P	Y	Y	P	P	Y	Y	Y	P	Y	-	-	-	-	N	Y
Nvidia Tegra K1 (T124)	armv7	2014	Y	Y	Y	Y	Y	P	Y	Y	Y		Y	-	-	-	-		P
Nvidia Tegra K1 (T132)	aarch64	2014	Y	Y	Y	Y	Y	P	Y	Y	Y		Y	-	-	-	-		P
Nvidia Tegra X1 (T210)	aarch64	2015	Y	Y	Y	Y	Y	P	Y	Y	Y	P	Y	-	-	-	-	P	P
Nvidia Tegra X1 (T214)	aarch64	2019	Y	Y	Y	Y	Y	P	Y	Y	Y	P	Y	-	-	-	-	P	P
Qualcomm APQ8064	armv7	2013	Y	Y	Y	Y	Y	N	Y	Y	N		Y	Y		N
Qualcomm MSM8212	armv7	2013	Y	Y	Y		P		Y	Y
Qualcomm MSM8226	armv7	2013	Y	Y	Y	Y	P	Y	Y	Y	N	N	Y	Y	Y	N	Y	P	N
Qualcomm MSM8909	armv7	2015	Y	Y	Y	P	Y		Y	Y	N			P	N	N		N
Qualcomm MSM8916	aarch64	2014	Y	Y	Y	Y	P	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	P
Qualcomm MSM8917	aarch64	2016	Y	Y	Y	Y	Y	Y	Y	Y	P	N	Y	Y	Y	N		P	N
Qualcomm MSM8929	aarch64	2014	Y	Y	Y	P	P	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	P
Qualcomm MSM8937	aarch64	2016	Y	Y	Y	Y	Y	Y	Y	Y	N	N	Y	Y	Y	N		N	N
Qualcomm MSM8939	aarch64	2014	Y	Y	Y	Y	P	Y	Y	Y	Y		Y	Y	Y	Y
Qualcomm MSM8940	aarch64	2016	Y	Y	Y	Y	Y	Y	Y	Y	N	N	Y	Y	Y	N		N	N
Qualcomm MSM8953	aarch64	2016	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	P	P	P	P
Qualcomm MSM8960	armv7	2012	Y	P	P		Y		Y	P
Qualcomm MSM8974	armv7	2013	Y	Y	Y	Y	P	P	Y	Y	N	N	Y	Y	Y	P	N	N	N
Qualcomm MSM8976	aarch64	2016	Y	P	Y	Y	P	Y	Y	Y	N	N	Y	Y	Y	P		N	N
Qualcomm MSM8992/MSM8994	aarch64	2015	Y	Y	P	P	Y	N	Y							B		B
Qualcomm MSM8996	aarch64	2016	Y	Y	P	Y	P	Y	Y	Y	Y	Y	Y	-	-	P		P	N
Qualcomm MSM8998	aarch64	2017	Y	Y	Y	P	P	N	Y	Y	Y		Y	Y	Y	P	Y
Qualcomm QM215	aarch64	2019
Qualcomm SC8280XP	aarch64	2021	Y	Y	Y	Y	Y	Y	Y	Y	P	P	Y	P	P	Y		P	P
Qualcomm SDM630	aarch64	2017	Y	P	Y	Y	P	P	Y	Y	N	Y	Y	Y	Y	Y		P
Qualcomm SDM636	aarch64	2017	Y	P	Y	Y	P	P	Y	Y	N	Y	Y	Y	Y	Y		P
Qualcomm SDM660	aarch64	2017	Y	P	Y	Y	P	P	Y	Y	N	Y	Y	Y	Y	Y		P
Qualcomm SDM670	aarch64	2018	N	P	P	Y	P	Y	Y	Y	P	P	Y	Y	Y	Y		Y
Qualcomm SDM845	aarch64	2018	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	P	Y	Y	Y
Qualcomm SM6115	aarch64	2020
Qualcomm SM6125	aarch64	2019		Y	Y	Y	P	N	Y	N	N	N	P	N	N	N		N
Qualcomm SM6225	aarch64	2021		N	N	P	P	N	N	N	N	N	N	N	N	N		N
Qualcomm SM6350	aarch64	2020	Y	Y	Y	Y	Y	Y	Y	Y	N	N	P	Y	-	Y		N	P
Qualcomm SM6375	aarch64	2021	Y	Y	P	N	Y	N	Y	Y	N	N	P	Y	-	Y		N
Qualcomm SM7125	aarch64	2020		Y	Y	Y	Y	Y	Y	Y		Y		Y	Y	P
Qualcomm SM7150	aarch64	2019/2020	Y	Y	Y	Y	Y	Y	Y	Y	P	N	Y	Y	Y	Y	P	P	Y
Qualcomm SM7325	aarch64	2021/2022	Y	Y	Y	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	N	N	N
Qualcomm SM8150	aarch64	2019	Y	Y	Y	Y	Y	Y	Y	Y	P		Y	Y	Y
Qualcomm SM8250	aarch64	2020/2021	Y	Y	Y	P	Y	Y	Y	Y	Y	Y		Y	Y
Qualcomm SM8350	aarch64	2021		P	Y		P		Y
Qualcomm SM8450	aarch64	2021	Y	Y	Y		Y	Y	Y	Y	Y			Y	Y
Qualcomm SM8475	aarch64	2022	Y	Y	Y		Y	Y	Y	Y
Qualcomm SM8550	aarch64	2022	Y	Y	Y	Y	Y		Y	Y			Y			Y
Qualcomm SM8650	aarch64	2023	Y	Y	Y	Y	Y	Y	Y	Y	Y	N	Y	Y	Y	Y	N	N
Qualcomm SM8750	aarch64	2024					Y		Y
Qualcomm X1E80100	aarch64	2023
RDA Microelectronics RDA8810PL	armv7	2014	Y				Y		Y					-	-		-
Rockchip RK3066	armv7	2012	Y	Y	Y	P	Y	Y	Y	Y	P			Y	Y		Y
Rockchip RK3126	armv7	2014	Y	Y	Y	Y	Y	Y	Y	Y	Y
Rockchip RK3128	armv7	2014	Y	Y	Y	Y	Y	Y	Y	Y	Y
Rockchip RK3188	armv7	2013	Y	Y	Y	Y	Y	Y	Y	Y	Y
Rockchip RK3288	armv7	2014	Y	Y	Y	P	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y
Rockchip RK3399	aarch64	2016	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	P	-	-	-	Y	Y
Samsung Exynos 3250	armv7	2014	Y	Y	Y	Y	Y	Y	Y	Y			Y						Y
Samsung Exynos 3475	armv7	2015	Y	N	Y	P	P	Y	Y	Y	N	N	N	-	-	N	N	N
Samsung Exynos 4210	armv7	2011	Y	Y	Y	Y	Y	Y	Y	Y			Y	-	-	-	-
Samsung Exynos 4212	armv7	2012	Y	Y	Y	Y	Y	Y	Y	Y	Y		Y	-	-	-	-		Y
Samsung Exynos 4412	armv7	2012	Y	Y	Y	Y	Y	Y	Y	Y	Y		Y	-	-	-	-	P	Y
Samsung Exynos 5250	armv7	2012	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	-	-	-	-		Y
Samsung Exynos 5420	armv7	2013	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	-	-	-	-		Y
Samsung Exynos 5422	armv7	2014	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	Y	-	-	-	-		Y
Samsung Exynos 5433	aarch64	2014	Y	Y	Y	Y	Y	Y	Y	Y	Y		Y						Y
Samsung Exynos 7870	aarch64	2016	Y	Y	Y	P	Y	Y	Y	Y	N	N	N	-	-	N	N	N
Samsung Exynos 8895	aarch64	2017	P			P	P		Y
Samsung Exynos 990	aarch64	2020	N	Y	Y	P	Y	Y	Y	N	N	N	N	N	N	N		N	N
SpacemiT Key Stone K1 / M1	riscv64	2024	Y	Y	P	Y	Y							-	-	-	-
ST-Ericsson NovaThor U8500	armv7	2011	Y	Y	Y	Y	Y	Y	Y	Y	N	N	Y	-	-	N	-	N	Y
Texas Instruments OMAP3xxx	armv7	2008	Y	Y	Y	Y	Y	N	Y	Y	Y	N	Y	-	-	-	-	Y	Y
Texas Instruments OMAP44xx	armv7	2011	Y	Y	Y	Y	Y	N	Y	Y	Y	N	Y	-	-	-	-	P	Y

Getting help

Mainlining related to postmarketOS is primarily discussed in the #postmarketOS-mainline channel. For one of the SoCs above there should be someone around who is able to help you to some extent. Note that you will be on your own for some of the problems - there are often device specific problems or components that no one has experience with yet.

Below are some more chat channels / mailing list that may be helpful for further help or if you want to work on a SoC that is not listed above (yet!):

In Matrix, you can browse most of Linux kernel development related channels in #linux-kernel-devspace

SoC	Where to ask for help
	#postmarketOS-mainline (SoC-specific questions might be limited to those listed above)
Allwinner	#linux-sunxi on OFTC, linux-sunxi@googlegroups.com
Amlogic	Chat room : 1 . #amlogic-postmarketos on OFTC (for postmarketOS amlogic porting) - Bridged Matrix Room : #oftc_#amlogic-postmarketos:pixie.town - Logs: https://irc.vitali64.duckdns.org/oftc/amlogic-postmarketos/ 2. #linux-amlogic on Libera.Chat - Log : https://libera.irclog.whitequark.org/linux-amlogic Mail List : 1. linux-amlogic@lists.infradead.org 2. u-boot-amlogic@groups.io Other Resources : 1. linux-meson.com.
Broadcom Kona	ask knuxify, bcm-kona-mainline on GitHub
Exynos	#replicant (with focus on older variants) or #linux-exynos on Libera.chat (ask forkbomb, Putti, krzk), linux-samsung-soc@vger.kernel.org
Samsung Exynos 4	Exynos entry + #exynos4xxx-mainline:matrix.org on Matrix
Samsung Exynos 5	Exynos entry + #exynos5-mainline:matrix.org on Matrix
Marvell PXA1908 (incl. other PXA and MMP)	#pxa1908-mainline on OFTC
MediaTek	#linux-mediatek on Libera.chat, linux-mediatek@lists.infradead.org
MediaTek MT6735	MediaTek entry + join #mt6735-foss:matrix.org
OMAP3 / OMAP4	ask @pavelm, linux-omap@vger.kernel.org
Qualcomm (msm...)	#linux-msm on OFTC, linux-arm-msm@vger.kernel.org
Qualcomm Snapdragon 400 (MSM8226)	Qualcomm entry + join #msm8226-mainline:postmarketos.org
Qualcomm Snapdragon 210 (MSM8909)	Qualcomm entry + join #msm8916:postmarketos.org
Qualcomm Snapdragon 410/412 (MSM8916)	Qualcomm entry + join #msm8916:postmarketos.org
Qualcomm Snapdragon 450/625/626/632 (MSM8953)	Qualcomm entry + join #msm8953-mainline:matrixorg
Qualcomm Snapdragon 600 (APQ8064) and Qualcomm Snapdragon S4 Plus (MSM8960)	Qualcomm entry + join #linux-apq8064:matrix.org
Qualcomm Snapdragon 615/616 (MSM8939)	Qualcomm entry + join #msm8916:postmarketos.org
Qualcomm Snapdragon 660/630/636 (SDM660/SDM630/SDM636)	Qualcomm entry + join #sdm660-mainline:matrix.org
Qualcomm Snapdragon 670 (SDM670)	Qualcomm entry + join #sargo/bonito porting:erebion.eu
Qualcomm Snapdragon 730/730G/732G (SM7150)	Qualcomm entry + join #sm7150-mainline:matrix.org
Qualcomm Snapdragon 820/821 (MSM8996)	Qualcomm entry + join #msm8996-mainline:matrix.org
Qualcomm Snapdragon 845 (SDM845)	Qualcomm entry + ask Caleb and join #sdm845:postmarketos.org
Qualcomm Snapdragon 865/865+/870 (SM8250)	Qualcomm entry + join #sm8250-mainline:matrix.org
Qualcomm Snapdragon 8 Gen 1/8+ Gen 1/7+ Gen 2 (SM8450/SM8475/SM7475)	Qualcomm entry + join #sm8450-mainline:matrix.org
Qualcomm Snapdragon 8 Gen 3 (SM8650)	#linux-msm and #sm8650-mainline on Matrix or OFTC
Nvidia Tegra	matrix.org:#postmarketOS-on-transformers:matrix.org (initially focused on the Asus Eee Pad Transformer family, now also offers help with other Tegra devices under PMOS), #tegra on Libera.chat, linux-tegra@vger.kernel.org

Getting Started

Collect as much information about your device as you can: downstream kernel sources, specifications, schematics (can be sometimes found with your favorite search engine) are all going to prove invaluable when attempting to port a device to mainline.
Serial debugging will be helpful especially in the beginning (before USB or network is working). Some devices expose it through the USB port or the audio jack; others have test points available on the mainboard.
Choose a kernel tree:
- Mainline: (the last kernel version released by Linus Torvalds) Usually a good compromise of being recent, yet quite stable. torvalds/linux.git
- linux-next: A collection of recently merged changes from many subsystems; use this if you are working on a very recent SoC. next/linux-next.git
- There may be more WIP changes available in Linux kernel trees specific to your SoC.
Compiling kernels with envkernel.sh is very useful to integrate kernel development into postmarketOS.
If you are attempting to port to a SoC listed above, check the SoC wiki page for further help and instructions.
Do not attempt to copy any code as-is from downstream. In general this won't work, and most importantly: it won't be accepted for inclusion into the mainline kernel upstream.
- Instead, try to understand what the downstream code does, and rewrite it from scratch for mainline by looking at similar code.

Materials to look into

This section include many useful sources to help you with mainlining as possible.

Note: Linux architecture regarding to embedded devices moving fast forward, so be aware that some specific information can get outdated in 2 – 3 years.

Books:

Mastering Linux Device Driver Development: Write custom device drivers to support computer peripherals in Linux operating systems (2021, John Madieu)
Linux Driver Development for Embedded Processors - Second Edition: Learn to develop Linux embedded drivers with kernel 4.9 LTS (2018, Alberto Liberal De Los Ríos)

Intro:

Porting mainline Linux to mobile phones, Luca Weiss, 2022 (40:00) FOSDEM
Running Mainline Linux on Snapdragon 410, Nikita Travkin, 2022 (40:00) FOSDEM
From Android to mainline on the Snapdragon 845, Caleb Connolly, 2022 (30:00) FOSDEM
Qualcomm SoC upstreaming adventures in 2020 Youtube Odysee
Porting Linux to your favorite obscure Arm SoC, 2020 media.ccc.de Youtube Odysee
Upstreaming a Qualcomm SoC, 2020 YouTube Slides Overview of porting 2020-era SoC
Introduction into how mainlining is done, 2017 (52:37) Youtube Odysee

Basics:

Introduction into ARM architecture, 2017 (46:33) Youtube Odysee Slides
ARM64 SoC Linux Support Check-List, 2017 (42:38) Youtube Odysee Slides Very useful step-by-step guide what should be done to bring in SoC support
Upstreaming ARM64 SoC's easier than before, 2019 (19:03) Youtube Odysee Slides
Mainlining blog by ichernev
How the ARM32 kernel starts by Linus Walleij

Writing device trees:

Device Tree 101, 2021 (1:54:58) Youtube
Device Tree (DT) introduction, 2013 (1:12:14) Youtube Odysee Slides
Device Tree: present, past, future, 2018 (37:28) Youtube Odysee Slides
Device tree bindings (i.e. documentation how to specify devices in your device tree) are mandatory for all mainline drivers. You can check those in the Documentation/devicetree/bindings directory to see which options are supported by a particular driver.
Device Tree Usage (beginner guide by elinux.org)
Device Tree Specification
DT Resources on elinux.org

Clocks:

Common Clock Framework: How To Use It, 2013 (44:50) Youtube OdyseeNot so useful, old and only a basic overview
What The Clock! - Linux Clock Subsystem Internals, 2020 (33:12) Odysee Youtube Slides
Clock sources, Clock events, sched_clock() and delay timers at kernel.org

Power Management

SAN19-421 Training: Device power management for idle Youtube

Camera

Camera⧸ISP Drivers Using V4L2 Media Controller Framework (35m) Youtube Odysee

Various topics:

Pin Control and GPIO, 2013 (48:03) Youtube Odysee Slides talk by Linus Walleij
Timekeeping in the Linux Kernel - Stephen Boyd, Qualcomm Innovation Center, 2017 (34:41) Youtube Odysee About how clocksource works.
How Dealing with Modern Interrupt Architectures can Affect Your Sanity Youtube Odysee Slides About interrupt controllers and their relationships
Linux Kernel Display Architecture, 2013 (56:53) Youtube Odysee DRM/KMS, MIPI DSI, panels,... slightly old, need something newer.
Demystifying Linux MIPI DSI Subsystem - Jagan Teki, 2019 (41:57) Youtube Odysee MIPI DSI, controllers, bridges, panels, DRM/KMS...
Power Management Integrated Circuits: Keep the Power in Your Hands, 2017 Youtube Odysee Slides About PMICs, regulators, power supply, fuelgauge, ...

Development tools

Using Visual Studio Code for Embedded Development - Michael Opdenacker, Bootlin, 2021 (31:37) Youtube

Note: for VSCodium, download extensions from https://marketplace.visualstudio.com/ manually.

Submitting patches:

Upstreaming

Upstreaming your mainline efforts is key to reduce the maintenance burden of rebasing your patches on newer version of the Linux kernel.

Upstreaming patches requires sending to the Linux Kernel Mailing List using git send-email. For a tutorial on git send-email have a look at https://git-send-email.io/.

See https://wiki.postmarketos.org/wiki/Submitting_Patches#Linux for a detailed overview for submitting patches upstream.